Local hash-based motion estimation for screen remoting scenarios

BACKGROUND

0001When video is streamed over the Internet and played back through a Web browser or media player, the video is delivered in digital form. Digital video is also used when video is delivered through many broadcast services, satellite services and cable television services. Real-time videoconferencing often uses digital video, and digital video is used during video capture with most smartphones, Web cameras and other video capture devices.

0002Digital video can consume an extremely high amount of bits. The number of bits that is used per second of represented video content is known as the bit rate. Engineers use compression (also called source coding or source encoding) to reduce the bit rate of digital video. Compression decreases the cost of storing and transmitting video information by converting the information into a lower bit rate form. Decompression (also called decoding) reconstructs a version of the original information from the compressed form. A "codec" is an encoder/decoder system.

0003Over the last 25 years, various video codec standards have been adopted, including the ITU-T H.261, H.262 (MPEG-2 or ISO/IEC 13818-2), H.263, H.264 (MPEG-4 AVC or ISO/IEC 14496-10), and H.265 (ISO/IEC 23008-2) standards, the MPEG-1 (ISO/IEC 11172-2) and MPEG-4 Visual (ISO/IEC 14496-2) standards, and the SMPTE 421M standard. A video codec standard typically defines options for the syntax of an encoded video bitstream, detailing parameters in the bitstream when particular features are used in encoding and decoding. In many cases, a video codec standard also provides details about the decoding operations a video decoder should perform to achieve conforming results in decoding. Aside from codec standards, various proprietary codec formats define options for the syntax of an encoded video bitstream and corresponding decoding operations.

0004In general, video compression techniques include "intra-picture" compression and "inter-picture" compression. Whereas intra-picture compression compresses a given picture using information within that picture, inter-picture compression compresses a given picture with reference to a preceding and/or following picture (often called a reference or anchor picture) or pictures.

0005Inter-picture compression techniques often use motion estimation and motion compensation to reduce bit rate by exploiting temporal redundancy in a video sequence. Motion estimation is a process for estimating motion between pictures. In one common technique, an encoder using motion estimation attempts to match a current block of sample values in a current picture with a candidate block of the same size in a search area in another picture, the reference picture. A reference picture is, in general, a picture that contains sample values that may be used for prediction in the encoding and decoding process of other pictures.

0006For a current block, when the video encoder finds an exact or "close enough" match in the search area in the reference picture, the video encoder parameterizes the change in position between the current and candidate blocks as motion data such as a motion vector ("MV"). An MV is conventionally a two-dimensional value, having a horizontal MV component that indicates left or right spatial displacement and a vertical MV component that indicates up or down spatial displacement. An MV can indicate a spatial displacement in terms of an integer number of samples starting from a co-located position in a reference picture for a current block. For example, for a current block at position (32, 16) in a current picture, the MV (-3, 1) indicates a block at position (29, 17) in the reference picture. In general, motion compensation is a process of reconstructing pictures from reference picture(s) using motion data.

0007When encoding a block using motion estimation and motion compensation, an encoder often computes the sample-by-sample differences (also called residual values or error values) between the sample values of the block and its motion-compensated prediction. The residual values may then be encoded. For the residual values, encoding efficiency depends on the complexity of the residual values and how much loss or distortion is introduced as part of the compression process. In general, a good motion-compensated prediction closely approximates a block, such that the residual values include few significant values, and the residual values can be efficiently encoded. On the other hand, a poor motion-compensated prediction often yields residual values that include many significant values, which are more difficult to encode efficiently.

0008Encoders typically spend a large proportion of encoding time performing motion estimation, attempting to find good matches and thereby improve rate-distortion performance. Encoder-side decisions about motion estimation are not made effectively, however, in certain encoding scenarios. In particular, motion estimation decisions are not made effectively in various situations when encoding screen capture content for remote screen presentation (also called "screen remoting"). For example, when screen capture video shows a user scrolling vertically or horizontally through a text document or Web page (with text, embedded images, and/or other content), conventional block-based motion estimation for 16x16 blocks, 8x8 blocks, 4x4 blocks, etc. is typically complex and time-consuming. Similarly, when screen capture video shows a user dragging a window (with text, embedded images, and/or other content) around a graphical user interface, conventional block-based motion estimation is typically complex and time-consuming. In addition to using a significant amount of processing resources, which is problematic for low-complexity devices, conventional block-based motion estimation can add delay, which is problematic for real-time screen remoting. Also, block-based motion estimation often fails to detect scrolling activity and window movement activity of large magnitude in screen capture video. Further, block-based motion estimation often fails to detect matches for new text (newly exposed text from scrolling, or newly typed text), even when that new text includes patterns (of letters) that match other patterns elsewhere on the screen. When scrolling activity and window movement activity are not efficiently encoded, or when new text is not efficiently encoded, overall compression efficiency suffers, which is especially problematic in low-bandwidth scenarios.

SUMMARY

0009In summary, the detailed description presents innovations in motion estimation that are adapted for screen remoting scenarios. Using the innovations, a video encoder can quickly perform local hash-based motion estimation ("HBME") to detect matches for new text that appears in screen capture video, while also detecting scrolling or window movement activity in screen capture video. Although particularly useful in screen remoting scenarios, the innovations can also be used in other video encoding scenarios.

0010According to various aspects of the innovations described herein, a video encoder receives one or more pictures in a video sequence and encodes the one or more pictures to produce encoded data. The encoding includes performing motion estimation for a current picture among the one or more pictures in the video sequence. The video encoder outputs the encoded data as part of a bitstream.

0011According to a first aspect of the innovations described herein, the video encoder calculates a hash value for a current block in the current picture. The video encoder searches, subject to a spatial constraint, for a matching block in a reference picture (<i>e.g.,</i> the previous picture in display order) based at least in part on the hash value for the current block. The spatial constraint defines a search area in the reference picture within which hash values for candidate blocks in the reference picture may be compared to the hash value for the current block. By using a spatial constraint to limit the range of local HBME, a video encoder can speed up the motion estimation process while still considering the candidate blocks in the reference picture that are most likely to match the current block.

0012For example, the reference picture is split into multiple grid areas. For a given candidate block among the candidate blocks in the reference picture, the video encoder has stored the hash value for the given candidate block and location of the given candidate block in a multi-level data structure for hashing. The multi-level data structure can organize the hash values for the candidate blocks by grid area and, within a grid area, by hash index. A given grid area among the multiple grid areas includes a block collocated with the current block. The search area (defined for the spatial constraint) can be the given grid area. Or, the search area can be the given grid area and multiple grid areas adjacent to the given grid area in the reference picture.

0013Before calculating the hash value for the current block and searching for a matching block in the reference picture, the video encoder can check one or more conditions that indicate whether local HBME is likely to be productive. For example, the video encoder can determine a changed region of the current picture, where the calculating the hash value for the current block and the searching are contingent on the current block being within the changed region. As another example, the video encoder can determine whether the current block contains textual content, where the calculating the hash value for the current block and the searching are contingent on the current block containing textual content. As another example, the video encoder can determine whether the current block contains a pivot point, wherein the calculating the hash value for the current block and the searching are contingent on the current block containing a pivot point.

0014According to a second aspect of the innovations described herein, for the current picture, the video encoder determines one or more changed regions of the current picture relative to a reference picture (<i>e.g.,</i> the previous picture in display order). Within the changed region(s), the video encoder performs global HBME to identify any blocks that have uniform motion relative to the reference picture. (The video encoder can also check one or more other conditions for the global HBME, such as whether the identified block(s) that have uniform motion also have a size larger than a threshold size.) Within any remaining portions of the changed region(s) after the global HBME, the video encoder performs local HBME for any blocks that have distinct patterns (<i>e.g</i>., text, pivot points). For example, the video encoder performs the local HBME as described above for the first aspect of the innovations. Within any remaining portions of the changed region(s) after the local HBME, the video encoder performs full block-matching motion estimation. In this way, the video encoder can effectively detect motion while avoiding expensive full block-matching motion estimation for most blocks of typical video sequences in screen remoting scenarios.

0015According to a third aspect of the innovations described herein, for a current block of the current picture, the video encoder determines whether the current block contains textual content and contains a pivot point. If so, the video encoder calculates a hash value for the current block and searches, subject to a spatial constraint, for a matching block in a reference picture based at least in part on the hash value for the current block (<i>e.g.,</i> as described above for the first aspect of the innovations). Otherwise (current block does not contain textual content and/or does not contain a pivot point), the video encoder performs full block-matching motion estimation for the current block. In this way, the video encoder can selectively perform local HBME when it is likely to be productive for the current block.

0016The innovations can be implemented as part of a method, as part of a computing system configured to perform operations for the method, or as part of one or more computer-readable media storing computer-executable instructions for causing a computing system to perform the operations for the method. The various innovations can be used in combination or separately. This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. The foregoing and other objects, features, and advantages of the invention will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

0017<ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">FIG. 1</figref> is a diagram illustrating an example computing system in which some described embodiments can be implemented.</li><li><figref idref="f0001">FIGS. 2a and 2b</figref> are diagrams illustrating example network environments in which some described embodiments can be implemented.</li><li><figref idref="f0002">FIG. 3</figref> is a diagram illustrating an example video encoder system, and <figref idref="f0003">FIGS. 4a</figref> and <figref idref="f0004">4b</figref> are diagrams illustrating an example video encoder, in conjunction with which some described embodiments can be implemented.</li><li><figref idref="f0005">FIG. 5</figref> is a diagram illustrating an example video decoder system, and <figref idref="f0006">FIG. 6</figref> is a diagram illustrating an example video decoder, in conjunction with which some described embodiments can be implemented.</li><li><figref idref="f0007">FIGS. 7a and 7b</figref> are flowcharts illustrating a generalized technique for video encoding with motion estimation in screen remoting scenarios.</li><li><figref idref="f0008">FIG. 8</figref> is a diagram illustrating an example of hashing of sample values for pivot points in global HBME.</li><li><figref idref="f0009">FIGS. 9</figref> and <figref idref="f0010">10</figref> are diagrams illustrating example data structures used in global HBME.</li><li><figref idref="f0011">FIGS. 11a, 11b, and 11c</figref> are diagrams illustrating example patterns for pivot points in global HBME.</li><li><figref idref="f0012">FIG. 12</figref> is a diagram illustrating an example of changed regions in which global HBME is performed.</li><li><figref idref="f0013">FIGS. 13a</figref> and <figref idref="f0014">13b</figref> are flowcharts illustrating an example technique for video encoding that includes global HBME for changed regions of a current picture.</li><li><figref idref="f0015">FIGS. 14a</figref> and <figref idref="f0016">14b</figref> are flowcharts illustrating an example technique for searching for a matching area in a previous picture based at least in part on a hash value for a pivot point in a changed region of a current picture during global HBME.</li><li><figref idref="f0017">FIGS. 15a and 15b</figref> are diagrams illustrating examples of spatial constraints along lines of grid areas of a reference picture in local HBME.</li><li><figref idref="f0017">FIG. 16</figref> is a diagram illustrating an example pattern for pivot points in local HBME.</li><li><figref idref="f0018">FIG. 17</figref> is a diagram illustrating example data structures used in local HBME.</li><li><figref idref="f0019">FIGS. 18a and 18b</figref> are flowcharts illustrating a generalized technique for local HBME.</li><li><figref idref="f0020">FIGS. 19a</figref> and <figref idref="f0021">19b</figref> are flowcharts illustrating an example technique for video encoding that includes local HBME for changed regions of a current picture.</li><li><figref idref="f0022">FIG. 20</figref> is a flowchart illustrating an example technique for searching for a matching block in a previous picture based at least in part on a hash value for a current block in a changed region of a current picture during local HBME.</li></ul>

DETAILED DESCRIPTION

0018The detailed description presents innovations in motion estimation adapted for screen remoting scenarios. Using the innovations, a video encoder can quickly perform local hash-based motion estimation ("HBME") to detect matches for new text that appears in screen capture video, while also detecting scrolling or window movement activity in the screen capture video. For example, a video encoder calculates a hash value for a current block in a current picture. The video encoder searches, subject to a spatial constraint, for a matching block in a reference picture (<i>e.g.,</i> the previous picture in display order) based at least in part on the hash value for the current block. The spatial constraint defines a search area in the reference picture within which hash values for candidate blocks in the reference picture may be compared to the hash value for the current block. By using a spatial constraint to limit the range of the local HBME, the video encoder can speed up the motion estimation process while still considering the candidate blocks in the reference picture that are most likely to match the current block.

0019Some of the innovations described herein are illustrated with reference to screen remoting scenarios. Using innovations described herein, a video encoder can encode screen capture video with very low encoding latency. Such video is common for remote desktop presentation scenarios. More generally, the innovations described herein can be used when encoding other types of video (<i>e.g.,</i> "natural" video captured with a camera).

0020Some of the innovations described herein are illustrated with reference to terms specific to the H.264 standard or H.265 standard, or extensions or variations thereof. The innovations described herein can also be implemented for other video codec standards or formats (e.g., the VP8 format or VP9 format), or extensions or variations thereof.

0021In the examples described herein, identical reference numbers in different figures indicate an identical component, module, or operation. Depending on context, a given component or module may accept a different type of information as input and/or produce a different type of information as output.

0022More generally, various alternatives to the examples described herein are possible. For example, some of the methods described herein can be altered by changing the ordering of the method acts described, by splitting, repeating, or omitting certain method acts, etc. The various aspects of the disclosed technology can be used in combination or separately. Some of the innovations described herein address one or more of the problems noted in the background. Typically, a given technique/tool does not solve all such problems.

I. Example Computer Systems.

0023<figref idref="f0001">FIG. 1</figref> illustrates a generalized example of a suitable computer system (100) in which several of the described innovations may be implemented. The computer system (100) is not intended to suggest any limitation as to scope of use or functionality, as the innovations may be implemented in diverse general-purpose or special-purpose computer systems.

0024With reference to <figref idref="f0001">FIG. 1</figref>, the computer system (100) includes one or more processing units (110, 115) and memory (120, 125). The processing units (110, 115) execute computer-executable instructions. A processing unit can be a general-purpose central processing unit ("CPU"), processor in an application-specific integrated circuit ("ASIC") or any other type of processor. In a multi-processing system, multiple processing units execute computer-executable instructions to increase processing power. For example, <figref idref="f0001">FIG. 1</figref> shows a CPU (110) as well as a GPU (115). In general, the GPU (115) is any specialized circuit, different from the CPU (110), that accelerates creation and/or manipulation of image data in a graphics pipeline. The GPU (115) can be implemented as part of a dedicated graphics card (video card), as part of a motherboard, as part of a system on a chip ("SoC"), or in some other way (even on the same die as the CPU (110)).

0025The tangible memory (120, 125) may be volatile memory (<i>e.g.,</i> registers, cache, RAM), non-volatile memory (<i>e.g.,</i> ROM, EEPROM, flash memory, etc.), or some combination of the two, accessible by the processing unit(s). In <figref idref="f0001">FIG. 1</figref>, the memory (120) is CPU memory, accessible to the CPU (110), and the memory (125) is GPU memory, accessible to the GPU (115). Depending on architecture (<i>e.g.,</i> whether the GPU (115) is part of a video card, motherboard, or SoC), the CPU memory can be completely separate from the GPU memory, or the CPU memory and GPU memory can, at least in part, be shared memory or drawn from the same source (<i>e.g.,</i> RAM). The memory (120, 125) stores software (180) implementing one or more innovations for local HBME for screen remoting scenarios, in the form of computer-executable instructions suitable for execution by the processing unit(s).

0026A computer system may have additional features. For example, the computer system (100) includes storage (140), one or more input devices (150), one or more output devices (160), and one or more communication connections (170). An interconnection mechanism (not shown) such as a bus, controller, or network interconnects the components of the computer system (100). Typically, operating system ("OS") software (not shown) provides an operating environment for other software executing in the computer system (100), and coordinates activities of the components of the computer system (100).

0027The tangible storage (140) may be removable or non-removable, and includes magnetic storage media such as magnetic disks, magnetic tapes or cassettes, optical storage media such as CD-ROMs or DVDs, or any other medium which can be used to store information and which can be accessed within the computer system (100). The storage (140) can store instructions for the software (180) implementing one or more innovations for local HBME for screen remoting scenarios.

0028The input device(s) (150) may be a touch input device such as a keyboard, mouse, pen, or trackball, a voice input device, a scanning device, or another device that provides input to the computer system (100). For video, the input device(s) (150) may be a camera, video card, screen capture module, TV tuner card, or similar device that accepts video input in analog or digital form, or a CD-ROM or CD-RW that reads video input into the computer system (100). The output device(s) (160) may be a display, printer, speaker, CD-writer, or another device that provides output from the computer system (100).

0029The communication connection(s) (170) enable communication over a communication medium to another computing entity. The communication medium conveys information such as computer-executable instructions, audio or video input or output, or other data in a modulated data signal. A modulated data signal is a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media can use an electrical, optical, RF, or other carrier.

0030The innovations can be described in the general context of computer-readable media. Computer-readable media are any available tangible media that can be accessed within a computing environment. By way of example, and not limitation, with the computer system (100), computer-readable media include memory (120, 125), storage (140), and combinations thereof. As used herein, the term computer-readable media does not include transitory signals or propagating carrier waves.

0031The innovations can be described in the general context of computer-executable instructions, such as those included in program modules, being executed in a computer system on a target real or virtual processor. Generally, program modules include routines, programs, libraries, objects, classes, components, data structures, etc. that perform particular tasks or implement particular abstract data types. The functionality of the program modules may be combined or split between program modules as desired in various embodiments. Computer-executable instructions for program modules may be executed within a local or distributed computer system.

0032The terms "system" and "device" are used interchangeably herein. Unless the context clearly indicates otherwise, neither term implies any limitation on a type of computer system or computer device. In general, a computer system or computer device can be local or distributed, and can include any combination of special-purpose hardware and/or general-purpose hardware with software implementing the functionality described herein.

0033For the sake of presentation, the detailed description uses terms like "determine," "find," "receive," and "search" to describe computer operations in a computer system. These terms are high-level abstractions for operations performed by a computer, and should not be confused with acts performed by a human being. The actual computer operations corresponding to these terms vary depending on implementation.

II. Example Network Environments.

0034<figref idref="f0001">FIGS. 2a and 2b</figref> show example network environments (201, 202) that include video encoders (220) and video decoders (270). The encoders (220) and decoders (270) are connected over a network (250) using an appropriate communication protocol. The network (250) can include the Internet or another computer network.

0035In the network environment (201) shown in <figref idref="f0001">FIG. 2a</figref>, each real-time communication ("RTC") tool (210) includes both an encoder (220) and a decoder (270) for bidirectional communication. A given encoder (220) can produce output compliant with the H.265/HEVC standard, SMPTE 421M standard, ISO/IEC 14496-10 standard (also known as H.264/AVC), another standard, or a proprietary format such as VP8 or VP9, or a variation or extension thereof, with a corresponding decoder (270) accepting encoded data from the encoder (220). The bidirectional communication can be part of a video conference, video telephone call, or other two-party or multi-party communication scenario. Although the network environment (201) in <figref idref="f0001">FIG. 2a</figref> includes two RTC tools (210), the network environment (201) can instead include three or more RTC tools (210) that participate in multi-party communication.

0036An RTC tool (210) manages encoding by an encoder (220) and also manages decoding by a decoder (270). <figref idref="f0002">FIG. 3</figref> shows an example video encoder system (300) that can be included in the RTC tool (210). Alternatively, the RTC tool (210) uses another encoder system. <figref idref="f0005">FIG. 5</figref> shows an example video decoder system (500) that can be included in the RTC tool (210). Alternatively, the RTC tool (210) uses another decoder system.

0037In the network environment (202) shown in <figref idref="f0001">FIG. 2b</figref>, an encoding tool (212) includes an encoder (220) that encodes video for delivery to multiple playback tools (214), which include decoders (270). The unidirectional communication can be provided for a video surveillance system, web camera monitoring system, remote desktop conferencing presentation or sharing, wireless screen casting, cloud computing or gaming, or other scenario in which video is encoded and sent from one location to one or more other locations. Although the network environment (202) in <figref idref="f0001">FIG. 2b</figref> includes two playback tools (214), the network environment (202) can include more or fewer playback tools (214). In general, a playback tool (214) communicates with the encoding tool (212) to determine a stream of video for the playback tool (214) to receive. The playback tool (214) receives the stream, buffers the received encoded data for an appropriate period, and begins decoding and playback.

0038The encoding tool (212) can include server-side controller logic for managing connections with one or more playback tools (214). <figref idref="f0002">FIG. 3</figref> shows an example video encoder system (300) that can be included in the encoding tool (214). Alternatively, the encoding tool (214) uses another encoder system. A playback tool (214) can include client-side controller logic for managing connections with the encoding tool (212). <figref idref="f0005">FIG. 5</figref> shows an example video decoder system (500) that can be included in the playback tool (214). Alternatively, the playback tool (214) uses another decoder system.

III. Example Video Encoder Systems.

0039<figref idref="f0002">FIG. 3</figref> shows an example video encoder system (300) in conjunction with which some described embodiments may be implemented. The video encoder system (300) includes a video encoder (340) the implements motion estimation with one or more of the innovations described herein. The video encoder (340) is further detailed in <figref idref="f0003">FIGS. 4a</figref> and <figref idref="f0004">4b</figref>.

0040The video encoder system (300) can be a general-purpose encoding tool capable of operating in any of multiple encoding modes such as a low-latency encoding mode for real-time communication, a transcoding mode, and a higher-latency encoding mode for producing media for playback from a file or stream, or it can be a special-purpose encoding tool adapted for one such encoding mode. The video encoder system (300) can be adapted for encoding of a particular type of content (<i>e.g</i>., screen capture video). The video encoder system (300) can be implemented as part of an operating system module, as part of an application library, as part of a standalone application, or using special-purpose hardware. Overall, the video encoder system (300) receives a sequence of source video pictures (311) from a video source (310) and produces encoded data as output to a channel (390). The encoded data output to the channel can include content encoded using one or more of the innovations described herein.

0041The video source (310) can be a camera, tuner card, storage media, screen capture module, or other digital video source. The video source (310) produces a sequence of video pictures at a frame rate of, for example, 30 frames per second. As used herein, the term "picture" generally refers to source, coded or reconstructed image data. For progressive-scan video, a picture is a progressive-scan video frame. For interlaced video, an interlaced video frame might be de-interlaced prior to encoding. Alternatively, two complementary interlaced video fields are encoded together as a single video frame or encoded as two separately-encoded fields. Aside from indicating a progressive-scan video frame or interlaced-scan video frame, the term "picture" can indicate a single non-paired video field, a complementary pair of video fields, a video object plane that represents a video object at a given time, or a region of interest in a larger image. The video object plane or region can be part of a larger image that includes multiple objects or regions of a scene.

0042An arriving source picture (311) is stored in a source picture temporary memory storage area (320) that includes multiple picture buffer storage areas (321, 322, ... , 32n). A picture buffer (321, 322, etc.) holds one source picture in the source picture storage area (320). After one or more of the source pictures (311) have been stored in picture buffers (321, 322, etc.), a picture selector (330) selects an individual source picture from the source picture storage area (320) to encode as the current picture (331). The order in which pictures are selected by the picture selector (330) for input to the video encoder (340) may differ from the order in which the pictures are produced by the video source (310), <i>e.g.,</i> the encoding of some pictures may be delayed in order, so as to allow some later pictures to be encoded first and to thus facilitate temporally backward prediction. Before the video encoder (340), the video encoder system (300) can include a preprocessor (not shown) that performs pre-processing of the current picture (331) before encoding. The pre-processing can include color space conversion and resampling processing (<i>e.g.,</i> to reduce the spatial resolution of chroma components) for encoding.

0043In general, a pixel is the set of one or more collocated sample values for a location in a picture, which may be arranged in different ways for different chroma sampling formats. Typically, before encoding, the sample values of video are converted to a color space such as YUV, in which sample values of a luma (Y) component represent brightness or intensity values, and sample values of chroma (U, V) components represent color-difference values. The precise definitions of the color-difference values (and conversion operations between YUV color space and another color space such as RGB) depend on implementation. In general, as used herein, the term YUV indicates any color space with a luma (or luminance) component and one or more chroma (or chrominance) components, including Y'UV, YIQ, Y'IQ and YDbDr as well as variations such as YCbCr and YCoCg. Chroma sample values may be sub-sampled to a lower chroma sampling rate (<i>e.g.,</i> for a YUV 4:2:0 format) in order to reduce the spatial resolution of chroma sample values, or the chroma sample values may have the same resolution as the luma sample values (<i>e.g.,</i> for a YUV 4:4:4 format).

0044The video encoder (340) encodes the current picture (331) to produce a coded picture (341). As shown in <figref idref="f0003">FIGS. 4a</figref> and <figref idref="f0004">4b</figref>, the video encoder (340) receives the current picture (331) as an input video signal (405) and produces encoded data for the coded picture (341) in a coded video bitstream (495) as output. As part of the encoding, the video encoder (340) in some cases uses one or more of the innovations for motion estimation as described herein.

0045Generally, the video encoder (340) includes multiple encoding modules that perform encoding tasks such as splitting into tiles, intra-picture prediction estimation and prediction, motion estimation and compensation, frequency transforms, quantization, and entropy coding. Many of the components of the video encoder (340) are used for both intra-picture coding and inter-picture coding. The exact operations performed by the video encoder (340) can vary depending on compression format and can also vary depending on encoder-optional implementation decisions.

0046As shown in <figref idref="f0003">FIG. 4a</figref>, the video encoder (340) can include a tiling module (410). With the tiling module (410), the video encoder (340) can split a picture into multiple tiles of the same size or different sizes. For example, the tiling module (410) splits the picture along tile rows and tile columns that, with picture boundaries, define horizontal and vertical boundaries of tiles within the picture, where each tile is a rectangular region. Tiles are often used to provide options for parallel processing. A picture can also be organized as one or more slices, where a slice can be an entire picture or section of the picture. A slice can be decoded independently of other slices in a picture, which improves error resilience. The content of a slice or tile is further split into blocks or other sets of sample values for purposes of encoding and decoding. Blocks may be further sub-divided at different stages, <i>e.g.,</i> at the prediction, frequency transform and/or entropy encoding stages. For example, a picture can be divided into 64x64 blocks, 32x32 blocks, or 16x16 blocks, which can in turn be divided into smaller blocks of sample values for coding and decoding.

0047For syntax according to the H.264/AVC standard, the video encoder (340) can split a picture into one or more slices of the same size or different sizes. The video encoder (340) splits the content of a picture (or slice) into 16x16 macroblocks. A macroblock ("MB") includes luma sample values organized as four 8x8 luma blocks and corresponding chroma sample values organized as 8x8 chroma blocks. Generally, a MB has a prediction mode such as inter or intra. A MB includes one or more prediction units (<i>e.g.,</i> 8x8 blocks, 4x4 blocks, which may be called partitions for inter-picture prediction) for purposes of signaling of prediction information (such as prediction mode details, MV information, etc.) and/or prediction processing. A MB also has one or more residual data units for purposes of residual coding/decoding.

0048For syntax according to the H.265/HEVC standard, the video encoder (340) splits the content of a picture (or slice or tile) into coding tree units. A coding tree unit ("CTU") includes luma sample values organized as a luma coding tree block ("CTB") and corresponding chroma sample values organized as two chroma CTBs. The size of a CTU (and its CTBs) is selected by the video encoder. A luma CTB can contain, for example, 64x64, 32x32, or 16x16 luma sample values. A CTU includes one or more coding units. A coding unit ("CU") has a luma coding block ("CB") and two corresponding chroma CBs. For example, according to quadtree syntax, a CTU with a 64x64 luma CTB and two 64x64 chroma CTBs (YUV 4:4:4 format) can be split into four CUs, with each CU including a 32x32 luma CB and two 32x32 chroma CBs, and with each CU possibly being split further into smaller CUs according to quadtree syntax. Or, as another example, according to quadtree syntax, a CTU with a 64x64 luma CTB and two 32x32 chroma CTBs (YUV 4:2:0 format) can be split into four CUs, with each CU including a 32x32 luma CB and two 16x16 chroma CBs, and with each CU possibly being split further into smaller CUs according to quadtree syntax.

0049In H.265/HEVC implementations, a CU has a prediction mode such as inter or intra. A CU typically includes one or more prediction units for purposes of signaling of prediction information (such as prediction mode details, displacement values, etc.) and/or prediction processing. A prediction unit ("PU") has a luma prediction block ("PB") and two chroma PBs. For an inter-picture-predicted CU, the CU can have one, two, or four PUs, where splitting into four PUs is allowed only if the CU has the smallest allowable size.

0050In H.265/HEVC implementations, a CU also typically has one or more transform units for purposes of residual coding/decoding, where a transform unit ("TU") has a luma transform block ("TB") and two chroma TBs. A CU may contain a single TU (equal in size to the CU) or multiple TUs. According to quadtree syntax, a TU can be split into four smaller TUs, which may in turn be split into smaller TUs according to quadtree syntax. The video encoder decides how to split video into CTUs (CTBs), CUs (CBs), PUs (PBs) and TUs (TBs).

0051As used herein, the term "block" can indicate a MB, residual data unit, CTB, CB, PB or TB, or some other set of sample values, depending on context. The term "unit" can indicate a MB, CTU, CU, PU, TU or some other set of blocks, or it can indicate a single block, depending on context. The term "partition" can indicate a PU or other unit used in prediction operations, or PB or other block used in prediction operations, depending on context.

0052As shown in <figref idref="f0003">FIG. 4a</figref>, the video encoder (340) includes a general encoding control (420), which receives the input video signal (405) for the current picture (331) as well as feedback (not shown) from various modules of the video encoder (340). Overall, the general encoding control (420) provides control signals (not shown) to other modules, such as the tiling module (410), transformer/scaler/quantizer (430), scaler/inverse transformer (435), intra-picture prediction estimator (440), motion estimator (450), and intra/inter switch, to set and change coding parameters during encoding. The general encoding control (420) can evaluate intermediate results during encoding, typically considering bit rate costs and/or distortion costs for different options. In particular, the general encoding control (420) decides whether to use intra-picture prediction or inter-picture prediction for the units of the current picture (331). If inter-picture prediction is used for a unit, in conjunction with the motion estimator (450), the general encoding control (420) decides which reference picture(s) to use for the inter-picture prediction. The general encoding control (420) determines which reference pictures to retain in a decoded picture buffer ("DPB") or other buffer. The general encoding control (420) produces general control data (422) that indicates decisions made during encoding, so that a corresponding decoder can make consistent decisions. The general control data (422) is provided to the header formatter/entropy coder (490).

0053With reference to <figref idref="f0004">FIG. 4b</figref>, if a unit of the current picture (331) is predicted using inter-picture prediction, a motion estimator (450) estimates the motion of blocks of sample values of the unit with respect to one or more reference pictures. The current picture (331) can be entirely or partially coded using inter-picture prediction. When multiple reference pictures are used, the multiple reference pictures can be from different temporal directions or the same temporal direction. The motion estimator (450) evaluates candidate MVs. The motion estimator (450) can evaluate different partition patterns for motion compensation for partitions of a given unit of the current picture (331) (<i>e.g.,</i> 2Nx2N, 2NxN, Nx2N, or NxN partitions for PUs of a CU in the H.265/HEVC standard). The motion estimator (450) can use one or more of the features of motion estimation described below. For example, the motion estimator (450) performs global HBME for one or more blocks of a current picture and/or performs local HBME for one or more blocks of the current picture. For use in hashing operations, the motion estimator (450) can create and update data structures that track hash values and locations of pivot points, as described below. For global HBME, the motion estimator (450) can determine global motion metadata, which can be used to guide or skip later block-based motion estimation decisions or can be signaled along with encoded data in the bitstream (495). The motion estimator (450) can identify regions, if any, that have changed between pictures and limits motion estimation operations to changed regions. These features of motion estimation can be used in combination or separately.

0054The DPB (470), which is an example of decoded picture temporary memory storage area (360) as shown in <figref idref="f0002">FIG. 3</figref>, buffers one or more reconstructed previously coded pictures for use as reference pictures.

0055The motion estimator (450) produces motion data (452) as side information. In particular, the motion data (452) can include information that indicates whether contextual motion mode (<i>e.g.,</i> merge mode in the H.265/HEVC standard) is used and, if so, the candidate MV for contextual motion mode (<i>e.g.,</i> merge mode index value in the H.265/HEVC standard). More generally, the motion data (452) can include MV data and reference picture selection data. The motion estimator (450) can also produce global motion metadata (457), which is provided to the header formatter/entropy coder (490), for implementations in which global motion metadata (457) is signaled as part of the bitstream (495). The motion data (452) is provided to the header formatter/entropy coder (490) as well as the motion compensator (455). The motion compensator (455) applies MV(s) for a block to the reconstructed reference picture(s) from the DPB (470) or other buffer. For the block, the motion compensator (455) produces a motion-compensated prediction, which is an area of sample values in the reference picture(s) that are used to generate motion-compensated prediction values for the block.

0056With reference to <figref idref="f0004">FIG. 4b</figref>, if a unit of the current picture (331) is predicted using intra-picture prediction, an intra-picture prediction estimator (440) determines how to perform intra-picture prediction for blocks of sample values of the unit. The current picture (331) can be entirely or partially coded using intra-picture prediction. If the current picture (331) is entirely coded using intra-picture prediction, it is termed a "key" picture. Otherwise (the current picture (331) is at least partially coded using inter-picture prediction), the current picture (331) is termed a "non-key" picture. Using values of a reconstruction (438) of the current picture (331), for intra spatial prediction, the intra-picture prediction estimator (440) determines how to spatially predict sample values of a block of the current picture (331) from previously reconstructed sample values of the current picture (331), <i>e.g.,</i> selecting an intra-picture prediction mode. Or, for intra block copy mode, the intra-picture prediction estimator (440) determines how to predict sample values of a block of the current picture (331) using an offset (sometimes called a block vector) that indicates a previously encoded/decoded portion of the current picture (331). Intra block copy mode can be implemented as a special case of inter-picture prediction in which the reference picture is the current picture (331), and only previously encoded/decoded sample values of the current picture (331) can be used for prediction. As side information, the intra-picture prediction estimator (440) produces intra prediction data (442), such as the prediction mode/direction used. The intra prediction data (442) is provided to the header formatter/entropy coder (490) as well as the intra-picture predictor (445).

0057According to the intra prediction data (442), the intra-picture predictor (445) spatially predicts sample values of a block of the current picture (331) from previously reconstructed sample values of the current picture (331), producing intra-picture predicted sample values for the block. Or, the intra-picture predictor (445) predicts sample values of the block using intra block copy prediction, using an offset (block vector) for the block.

0058As shown in <figref idref="f0004">FIG. 4b</figref>, the intra/inter switch selects whether the predictions (458) for a given unit will be motion-compensated predictions or intra-picture predictions. Intra/inter switch decisions for units of the current picture (331) can be made using various criteria.

0059The video encoder (340) can determine whether or not to encode and transmit the differences (if any) between a block's prediction values (intra or inter) and corresponding original values. The differences (if any) between a block of the prediction (458) and a corresponding part of the original current picture (331) of the input video signal (405) provide values of the residual (418). If encoded/transmitted, the values of the residual (418) are encoded using a frequency transform (if the frequency transform is not skipped), quantization, and entropy encoding. In some cases, no residual is calculated for a unit. Instead, residual coding is skipped, and the predicted sample values are used as the reconstructed sample values.

0060With reference to <figref idref="f0003">FIG. 4a</figref>, when values of the residual (418) are encoded, in the transformer/scaler/quantizer (430), a frequency transformer converts spatial-domain video information into frequency-domain (<i>i.e.,</i> spectral, transform) data. For block-based video coding, the frequency transformer applies a discrete cosine transform ("DCT"), an integer approximation thereof, or another type of forward block transform (e.g., a discrete sine transform or an integer approximation thereof) to blocks of values of the residual (418) (or sample value data if the prediction (458) is null), producing blocks of frequency transform coefficients. The transformer/scaler/quantizer (430) can apply a transform with variable block sizes. In this case, the transformer/scaler/quantizer (430) can determine which block sizes of transforms to use for the residual values for a current block. For example, in H.265/HEVC implementations, the transformer/scaler/quantizer (430) can split a TU by quadtree decomposition into four smaller TUs, each of which may in turn be split into four smaller TUs, down to a minimum TU size. In H.265/HEVC implementations, the frequency transform can be skipped. In this case, values of the residual (418) can be quantized and entropy coded.

0061With reference to <figref idref="f0003">FIG. 4a</figref>, in the transformer/scaler/quantizer (430), a scaler/quantizer scales and quantizes the transform coefficients. For example, the quantizer applies dead-zone scalar quantization to the frequency-domain data with a quantization step size that varies on a picture-by-picture basis, tile-by-tile basis, slice-by-slice basis, block-by-block basis, frequency-specific basis, or other basis. The quantization step size can depend on a quantization parameter ("QP"), whose value is set for a picture, tile, slice, and/or other portion of video. The quantized transform coefficient data (432) is provided to the header formatter/entropy coder (490). If the frequency transform is skipped, the scaler/quantizer can scale and quantize the blocks of prediction residual data (or sample value data if the prediction (458) is null), producing quantized values that are provided to the header formatter/entropy coder (490).

0062As shown in <figref idref="f0003">FIGS. 4a</figref> and <figref idref="f0004">4b</figref>, the header formatter/entropy coder (490) formats and/or entropy codes the general control data (422), quantized transform coefficient data (432), intra prediction data (442), motion data (452), global motion metadata (457) (if provided), and filter control data (462). The entropy coder of the video encoder (340) compresses quantized transform coefficient values as well as certain side information (e.g., MV information, QP values, mode decisions, parameter choices, filter parameters). Typical entropy coding techniques include Exponential-Golomb coding, Golomb-Rice coding, context-adaptive binary arithmetic coding ("CABAC"), differential coding, Huffman coding, run length coding, variable-length-to-variable-length ("V2V") coding, variable-length-to-fixed-length ("V2F") coding, Lempel-Ziv ("LZ") coding, dictionary coding, and combinations of the above. The entropy coder can use different coding techniques for different kinds of information, can apply multiple techniques in combination (<i>e.g.,</i> by applying Exponential-Golomb coding or Golomb-Rice coding as binarization for CABAC), and can choose from among multiple code tables within a particular coding technique.

0063The video encoder (340) produces encoded data for the coded picture (341) in an elementary bitstream, such as the coded video bitstream (495) shown in <figref idref="f0003">FIG. 4a</figref>. In <figref idref="f0003">FIG. 4a</figref>, the header formatter/entropy coder (490) provides the encoded data in the coded video bitstream (495). The syntax of the elementary bitstream is typically defined in a codec standard or format, or extension or variation thereof. For example, the format of the coded video bitstream (495) can be a Windows Media Video format, SMPTE 421M format, MPEG-x format (<i>e.g.,</i> MPEG-1, MPEG-2, or MPEG-4), H.26x format (<i>e.g.,</i> H.261, H.262, H.263, H.264, H.265), VPx format, or another format, or a variation or extension thereof. After output from the video encoder (340), the elementary bitstream is typically packetized or organized in a container format, as explained below.

0064The encoded data in the elementary bitstream includes syntax elements organized as syntax structures. In general, a syntax element can be any element of data, and a syntax structure is zero or more syntax elements in the elementary bitstream in a specified order.

0065As shown in <figref idref="f0002">FIG. 3</figref>, the video encoder (340) also produces memory management control operation ("MMCO") signals (342) or reference picture set ("RPS") information. The RPS is the set of pictures that may be used for reference in motion compensation for a current picture or any subsequent picture. If the current picture (331) is not the first picture that has been encoded, when performing its encoding process, the video encoder (340) may use one or more previously encoded/decoded pictures (369) that have been stored in a decoded picture temporary memory storage area (360). Such stored decoded pictures (369) are used as reference pictures for inter-picture prediction of the content of the current picture (331). The MMCO/RPS information (342) indicates to a video decoder which reconstructed pictures may be used as reference pictures, and hence should be stored in a picture storage area such as the DPB (470) in <figref idref="f0003">FIGS. 4a</figref> and <figref idref="f0004">4b</figref>.

0066The decoding process emulator (350) implements some of the functionality of a video decoder, for example, decoding tasks to reconstruct reference pictures. In a manner consistent with the MMCO/RPS information (342), the decoding process emulator (350) determines whether a given coded picture (341) needs to be reconstructed and stored for use as a reference picture in inter-picture prediction of subsequent pictures to be encoded. If a coded picture (341) needs to be stored (and possibly modified), the decoding process emulator (350) models the decoding process that would be conducted by a video decoder that receives the coded picture (341) and produces a corresponding decoded picture (351).

0067The decoding process emulator (350) may be implemented as part of the video encoder (340). For example, the decoding process emulator (350) includes certain modules and logic as shown in <figref idref="f0003">FIGS. 4a</figref> and <figref idref="f0004">4b</figref>. During reconstruction of the current picture (331), when values of the residual (418) have been encoded/signaled, reconstructed residual values are combined with the prediction (458) to produce an approximate or exact reconstruction (438) of the original content from the video signal (405) for the current picture (331). (In lossy compression, some information is lost from the video signal (405).)

0068With reference to <figref idref="f0003">FIG. 4a</figref>, to reconstruct residual values, in the scaler/inverse transformer (435), a scaler/inverse quantizer performs inverse scaling and inverse quantization on the quantized transform coefficients. When the transform stage has not been skipped, an inverse frequency transformer performs an inverse frequency transform, producing blocks of reconstructed prediction residual values or sample values. If the transform stage has been skipped, the inverse frequency transform is also skipped. In this case, the scaler/inverse quantizer can perform inverse scaling and inverse quantization on blocks of prediction residual data (or sample value data), producing reconstructed values. When residual values have been encoded/signaled, the video encoder (340) combines reconstructed residual values with values of the prediction (458) (<i>e.g.,</i> motion-compensated prediction values, intra-picture prediction values) to form the reconstruction (438). When residual values have not been encoded/signaled, the video encoder (340) uses the values of the prediction (458) as the reconstruction (438).

0069With reference to <figref idref="f0003">FIGS. 4a</figref> and <figref idref="f0004">4b</figref>, for intra-picture prediction, the values of the reconstruction (438) can be fed back to the intra-picture prediction estimator (440) and intra-picture predictor (445). The values of the reconstruction (438) can be used for motion-compensated prediction of subsequent pictures. The values of the reconstruction (438) can be further filtered. A filtering control (460) determines how to perform deblock filtering and sample adaptive offset ("SAO") filtering on values of the reconstruction (438), for the current picture (331). The filtering control (460) produces filter control data (462), which is provided to the header formatter/entropy coder (490) and merger/filter(s) (465).

0070In the merger/filter(s) (465), the video encoder (340) merges content from different tiles into a reconstructed version of the current picture. The video encoder (340) selectively performs deblock filtering and SAO filtering according to the filter control data (462) and rules for filter adaptation, so as to adaptively smooth discontinuities across boundaries in the current picture (331). Other filtering (such as de-ringing filtering or adaptive loop filtering ("ALF"); not shown) can alternatively or additionally be applied. Tile boundaries can be selectively filtered or not filtered at all, depending on settings of the video encoder (340), and the video encoder (340) may provide syntax elements within the coded bitstream to indicate whether or not such filtering was applied.

0071In <figref idref="f0003">FIGS. 4a</figref> and <figref idref="f0004">4b</figref>, the DPB (470) buffers the reconstructed current picture for use in subsequent motion-compensated prediction. More generally, as shown in <figref idref="f0002">FIG. 3</figref>, the decoded picture temporary memory storage area (360) includes multiple picture buffer storage areas (361, 362, ..., 36<i>n</i>). In a manner consistent with the MMCO/RPS information (342), the decoding process emulator (350) manages the contents of the storage area (360) in order to identify any picture buffers (361, 362, etc.) with pictures that are no longer needed by the video encoder (340) for use as reference pictures. After modeling the decoding process, the decoding process emulator (350) stores a newly decoded picture (351) in a picture buffer (361, 362, etc.) that has been identified in this manner.

0072As shown in <figref idref="f0002">FIG. 3</figref>, the coded picture (341) and MMCO/RPS information (342) are buffered in a temporary coded data area (370). The coded data that is aggregated in the coded data area (370) contains, as part of the syntax of the elementary bitstream, encoded data for one or more pictures. The coded data that is aggregated in the coded data area (370) can also include media metadata relating to the coded video data (<i>e.g.,</i> as one or more parameters in one or more supplemental enhancement information ("SEI") messages or video usability information ("VUI") messages). The media metadata can include global motion metadata (457) when signaled as part of the bitstream (495).

0073The aggregated data (371) from the temporary coded data area (370) is processed by a channel encoder (380). The channel encoder (380) can packetize and/or multiplex the aggregated data for transmission or storage as a media stream or file. Or, more generally, the channel encoder (380) can implement one or more media system multiplexing protocols or transport protocols. The channel encoder (380) provides output to a channel (390), which represents storage, a communications connection, or another channel for the output. The channel encoder (380) or channel (390) may also include other elements (not shown), <i>e.g</i>., for forward-error correction ("FEC") encoding and analog signal modulation.

0074Depending on implementation and the type of compression desired, modules of the video encoder system (300) and/or video encoder (340) can be added, omitted, split into multiple modules, combined with other modules, and/or replaced with like modules. In alternative embodiments, encoder systems or encoders with different modules and/or other configurations of modules perform one or more of the described techniques. Specific embodiments of encoder systems typically use a variation or supplemented version of the video encoder system (300). Specific embodiments of video encoders typically use a variation or supplemented version of the video encoder (340). The relationships shown between modules within the video encoder system (300) and video encoder (340) indicate general flows of information in the video encoder system (300) and video encoder (340), respectively; other relationships are not shown for the sake of simplicity. In general, a given module of the video encoder system (300) or video encoder (340) can be implemented by software executable on a CPU, by software controlling special-purpose hardware (<i>e.g.,</i> graphics hardware for video acceleration), or by special-purpose hardware (<i>e.g.,</i> in an ASIC).

IV. Example Video Decoder Systems

.

0075<figref idref="f0005">FIG. 5</figref> is a block diagram of an example video decoder system (500) in conjunction with which some described embodiments may be implemented. The video decoder system (500) includes a video decoder (550), which is further detailed in <figref idref="f0006">FIG. 6</figref>.

0076The video decoder system (500) can be a general-purpose decoding tool capable of operating in any of multiple decoding modes such as a low-latency decoding mode for real-time communication, a transcoding mode, and a higher-latency decoding mode for media playback from a file or stream, or it can be a special-purpose decoding tool adapted for one such decoding mode. The video decoder system (500) can be implemented as part of an operating system module, as part of an application library, as part of a standalone application or using special-purpose hardware. Overall, the video decoder system (500) receives coded data from a channel (510) and produces reconstructed pictures as output for an output destination (590). The received encoded data can include content encoded using one or more of the innovations described herein.

0077The decoder system (500) includes a channel (510), which can represent storage, a communications connection, or another channel for coded data as input. The channel (510) produces coded data that has been channel coded. A channel decoder (520) can process the coded data. For example, the channel decoder (520) de-packetizes and/or demultiplexes data that has been organized for transmission or storage as a media stream or file. Or, more generally, the channel decoder (520) can implement one or more media system demultiplexing protocols or transport protocols. The channel (510) or channel decoder (520) may also include other elements (not shown), <i>e.g</i>., for FEC decoding and analog signal demodulation.

0078The coded data (521) that is output from the channel decoder (520) is stored in a temporary coded data area (530) until a sufficient quantity of such data has been received. The coded data (521) includes coded pictures (531) and MMCO/RPS information (532). The coded data (521) in the coded data area (530) contain, as part of the syntax of an elementary coded video bitstream, coded data for one or more pictures. The coded data (521) in the coded data area (530) can also include media metadata relating to the encoded video data (<i>e.g.,</i> as one or more parameters in one or more SEI messages or VUI messages). The media metadata can include global motion metadata (657) when signaled as part of the bitstream (605), as explained with reference to <figref idref="f0006">FIG. 6</figref>.

0079In general, the coded data area (530) temporarily stores coded data (521) until such coded data (521) is used by the video decoder (550). At that point, coded data for a coded picture (531) and MMCO/RPS information (532) are transferred from the coded data area (530) to the video decoder (550). As decoding continues, new coded data is added to the coded data area (530) and the oldest coded data remaining in the coded data area (530) is transferred to the video decoder (550).

0080The video decoder (550) decodes a coded picture (531) to produce a corresponding decoded picture (551). As shown in <figref idref="f0006">FIG. 6</figref>, the video decoder (550) receives the coded picture (531) as input as part of a coded video bitstream (605), and the video decoder (550) produces the corresponding decoded picture (551) as output as reconstructed video (695).

0081Generally, the video decoder (550) includes multiple decoding modules that perform decoding tasks such as entropy decoding, inverse quantization, inverse frequency transforms, motion compensation, intra-picture prediction, and filtering. Many of the components of the decoder (550) are used for both intra-picture decoding and inter-picture decoding. The exact operations performed by those components can vary depending on the type of information being decompressed. The format of the coded video bitstream (605) can be a Windows Media Video format, SMPTE 421M format, MPEG-x format (<i>e.g.,</i> MPEG-1, MPEG-2, or MPEG-4), H.26x format (<i>e.g.,</i> H.261, H.262, H.263, H.264, H.265), or VPx format, or another format, or variation or extension thereof.

0082A picture can be organized into multiple tiles of the same size or different sizes. A picture can also be organized as one or more slices. The content of a slice or tile can be further organized as blocks or other sets of sample values. Blocks may be further sub-divided at different stages. For example, a picture can be divided into 64x64 blocks, 32x32 blocks or 16x16 blocks, which can in turn be divided into smaller blocks of sample values. In implementations of decoding for the H.264/AVC standard, for example, a picture is divided into MBs and blocks. In implementations of decoding for the H.265/HEVC standard, for example, a picture is split into CTUs (CTBs), CUs (CBs), PUs (PBs) and TUs (TBs).

0083With reference to <figref idref="f0006">FIG. 6</figref>, a buffer receives encoded data in the coded video bitstream (605) and makes the received encoded data available to the parser/entropy decoder (610). The parser/entropy decoder (610) entropy decodes entropy-coded data, typically applying the inverse of entropy coding performed in the video encoder (340) (<i>e.g.,</i> context-adaptive binary arithmetic decoding with binarization using Exponential-Golomb or Golomb-Rice). As a result of parsing and entropy decoding, the parser/entropy decoder (610) produces general control data (622), quantized transform coefficient data (632), intra prediction data (642) (<i>e.g.,</i> intra-picture prediction modes), motion data (652), global motion metadata (657) (if provided), and filter control data (662).

0084The general decoding control (620) receives the general control data (622). The general decoding control (620) provides control signals (not shown) to other modules (such as the scaler/inverse transformer (635), intra-picture predictor (645), motion compensator (655), and intra/inter switch) to set and change decoding parameters during decoding.

0085With reference to <figref idref="f0005">FIG. 5</figref>, as appropriate, when performing its decoding process, the video decoder (550) may use one or more previously decoded pictures (569) as reference pictures for inter-picture prediction. The video decoder (550) reads such previously decoded pictures (569) from a decoded picture temporary memory storage area (560), which is, for example, DPB (670).

0086With reference to <figref idref="f0006">FIG. 6</figref>, if the current picture is predicted using inter-picture prediction, a motion compensator (655) receives the motion data (652), such as MV data, reference picture selection data and merge mode index values. The motion compensator (655) applies MVs to the reconstructed reference picture(s) from the DPB (670). The motion compensator (655) produces motion-compensated predictions for inter-coded blocks of the current picture.

0087If global motion data (657) has been provided, a global motion processor (656) can interpret the global motion data (657) and assign motion data for partitions covered by the global motion data (657). For example, for partitions in a global motion area, the global motion processor (656) assigns MV data based on the global motion metadata (657). The motion data is passed to the motion compensator (655) for normal processing.

0088In a separate path within the video decoder (550), the intra-picture predictor (645) receives the intra prediction data (642), such as information indicating the prediction mode/direction used. For intra spatial prediction, using values of a reconstruction (638) of the current picture, according to the prediction mode/direction, the intra-picture predictor (645) spatially predicts sample values of a current block of the current picture from previously reconstructed sample values of the current picture. Or, for intra block copy mode, the intra-picture predictor (645) predicts the sample values of a current block using previously reconstructed sample values of a reference block, which is indicated by an offset (block vector) for the current block.

0089The intra/inter switch selects values of a motion-compensated prediction or intra-picture prediction for use as the prediction (658) for a given block. For example, when H.265/HEVC syntax is followed, the intra/inter switch can be controlled based on a syntax element encoded for a CU of a picture that can contain intra-predicted CUs and inter-predicted CUs. When residual values have been encoded/signaled, the video decoder (550) combines the prediction (658) with reconstructed residual values to produce the reconstruction (638) of the content from the video signal. When residual values have not been encoded/signaled, the video decoder (550) uses the values of the prediction (658) as the reconstruction (638).

0090The video decoder (550) also reconstructs prediction residual values. To reconstruct the residual when residual values have been encoded/signaled, the scaler/inverse transformer (635) receives and processes the quantized transform coefficient data (632). In the scaler/inverse transformer (635), a scaler/inverse quantizer performs inverse scaling and inverse quantization on the quantized transform coefficients. The scaler/inverse transformer (635) sets values for QP for a picture, tile, slice and/or other portion of video based on syntax elements in the bitstream. An inverse frequency transformer performs an inverse frequency transform, producing blocks of reconstructed prediction residual values or sample values. For example, the inverse frequency transformer applies an inverse block transform to frequency transform coefficients, producing sample value data or prediction residual data. The inverse frequency transform can be an inverse DCT, an integer approximation thereof, or another type of inverse frequency transform (<i>e.g.,</i> an inverse discrete sine transform or an integer approximation thereof). If the frequency transform was skipped during encoding, the inverse frequency transform is also skipped. In this case, the scaler/inverse quantizer can perform inverse scaling and inverse quantization on blocks of prediction residual data (or sample value data), producing reconstructed values. The video decoder (550) combines reconstructed prediction residual values with prediction values of the prediction (658), producing values of the reconstruction (638).

0091For intra-picture prediction, the values of the reconstruction (638) can be fed back to the intra-picture predictor (645). For inter-picture prediction, the values of the reconstruction (638) can be further filtered. In the merger/filter(s) (665), the video decoder (550) merges content from different tiles into a reconstructed version of the picture. The video decoder (550) selectively performs deblock filtering and SAO filtering according to the filter control data (662) and rules for filter adaptation, so as to adaptively smooth discontinuities across boundaries in the pictures. Other filtering (such as de-ringing filtering or ALF; not shown) can alternatively or additionally be applied. Tile boundaries can be selectively filtered or not filtered at all, depending on settings of the video decoder (550) or a syntax element within the encoded bitstream data. The DPB (670) buffers the reconstructed current picture for use as a reference picture in subsequent motion-compensated prediction.

0092The video decoder (550) can also include a post-processing filter. The post-processing filter can include deblock filtering, de-ringing filtering, adaptive Wiener filtering, film-grain reproduction filtering, SAO filtering or another kind of filtering. Whereas "in-loop" filtering is performed on reconstructed sample values of pictures in a motion compensation loop, and hence affects sample values of reference pictures, the post-processing filter is applied to reconstructed sample values outside of the motion compensation loop, before output for display.

0093With reference to <figref idref="f0005">FIG. 5</figref>, the decoded picture temporary memory storage area (560) includes multiple picture buffer storage areas (561, 562, ..., 56<i>n</i>). The decoded picture storage area (560) is, for example, the DPB (670). The decoder (550) uses the MMCO/RPS information (532) to identify a picture buffer (561, 562, etc.) in which it can store a decoded picture (551). The decoder (550) stores the decoded picture (551) in that picture buffer. The decoder (550) also determines whether to remove any reference pictures from the multiple picture buffer storage areas (561, 562, ..., 56<i>n</i>).

0094An output sequencer (580) identifies when the next picture to be produced in display order (also called output order) is available in the decoded picture storage area (560). When the next picture (581) to be produced in display order is available in the decoded picture storage area (560), it is read by the output sequencer (580) and output to the output destination (590) (<i>e.g.,</i> display). In general, the order in which pictures are output from the decoded picture storage area (560) by the output sequencer (580) (display order) may differ from the order in which the pictures are decoded by the decoder (550) (bitstream order).

0095Depending on implementation and the type of decompression desired, modules of the video decoder system (500) and/or video decoder (550) can be added, omitted, split into multiple modules, combined with other modules, and/or replaced with like modules. In alternative embodiments, decoder systems or decoders with different modules and/or other configurations of modules perform one or more of the described techniques. Specific embodiments of decoder systems typically use a variation or supplemented version of the video decoder system (500). Specific embodiments of video decoders typically use a variation or supplemented version of the video decoder (550). The relationships shown between modules within the video decoder system (500) and video decoder (550) indicate general flows of information in the video decoder system (500) and video decoder (550), respectively; other relationships are not shown for the sake of simplicity. In general, a given module of the video decoder system (500) or video decoder (550) can be implemented by software executable on a CPU, by software controlling special-purpose hardware (<i>e.g.</i>, graphics hardware for video acceleration), or by special-purpose hardware (<i>e.g</i>., in an ASIC).

V. Motion Estimation Adapted for Screen Remoting Scenarios.

0096In screen remoting scenarios, screen capture video shows a screen or window of a graphical user interface as it changes over time. In typical screen capture video, text and embedded images are scrolled horizontally or vertically from time to time as a user navigates through content. As a user adds text, previous content can be shifted horizontally or vertically. A user can also move a window including text and other content around a screen. In many cases, previous approaches to motion estimation for screen remoting scenarios fail to detect and encode such motion effectively. Further, previous approaches to motion estimation often fail to detect matches for new text (newly exposed text from scrolling, or newly typed text), even when that new text includes patterns (of letters) that match other patterns elsewhere on the screen.

0097This section describes various innovations for global HBME and various innovations for local HBME, as well as approaches to overall encoding with HBME in screen remoting scenarios.

A. Example Approaches for Video Encoding with Hash-Based Motion Estimation in Screen Remoting Scenarios.

0098A video encoder can use a combination of global HBME, local HBME, full block-matching motion estimation, and other compression during video encoding for screen remoting scenarios. Different modes of motion estimation tend to have different benefits in screen remoting scenarios.

0099In general, global HBME is adapted to detect and represent uniform motion of a large area. For example, global HBME may detect uniform motion due to movement of a window or scrolling of content within a window. The entire window, or content within the window, may be exactly and efficiently represented as a large area of uniform motion, with no residuals coded for blocks in the large area. Global HBME can quickly detect motion for a large number of blocks. Global HBME may fail to detect and represent motion effectively, however, for areas with complex motion or new content. Section V.B describes variations of global HBME.

0100Local HBME can be performed when global HBME has failed. In general, local HBME is adapted to detect whether new content in a current picture matches content in a previous picture. For example, when blocks of new content appear in the current picture due to scrolling or typing of new text, local HBME can identify blocks of matching content (<i>e.g.,</i> a matching letter, matching pattern) in the previous picture. In some example implementations, local HBME is adapted to detect repeated individual letters from nearby in the same line or an adjacent line of text. For local HBME, blocks can be 4x4, 8x8, or 16x16 or another size likely to contain one character. Residual values can be encoded if the blocks in the previous picture do not exactly match the blocks of new content in the current picture. Like global HBME, local HBME can be performed relatively quickly, compared to full block-matching motion estimation. Section V.C describes variations of local HBME.

0101If global HBME and local HBME do not succeed, the video encoder can perform full block-matching motion estimation. Although it tends to be slow compared to HBME, full block-matching motion estimation may detect motion that HBME fails to detect.

0102<figref idref="f0007">FIGS. 7a and 7b</figref> show a generalized technique (700) for video encoding with various modes of motion estimation in screen remoting scenarios. A video encoder such as the video encoder shown in <figref idref="f0002">FIGS. 3</figref>, <figref idref="f0003">4a</figref>, and <figref idref="f0004">4b</figref>, or other video encoder, can perform the technique (700).

0103With reference to <figref idref="f0007">FIG. 7a</figref>, the video encoder receives (710) a current picture in a video sequence. An input buffer can be configured to receive one or more pictures for encoding. The video encoder encodes (720) the current picture to produce encoded data. An output buffer can be configured to store the encoded data for output. The video encoder outputs (730) the encoded data as part of a bitstream. When encoding (720) the current picture, the video encoder can use intra-picture prediction or inter-picture prediction for different units of the current picture.

0104<figref idref="f0007">FIG. 7b</figref> shows one example (721) of the encoding (720) shown in <figref idref="f0007">FIG. 7a</figref> for the current picture. The video encoder determines (722) one or more changed regions of the current picture relative to a reference picture. For example, the reference picture is the previous picture in display order. Section V.B.7 describes ways for a video encoder to determine changed region(s) in a current picture relative to a reference picture. Alternatively, the video encoder determines the changed region(s) in some other way. In typical screen remoting scenarios, by limiting motion estimation operations to changed regions, the video encoder can speed up the overall encoding process.

0105Within the changed region(s), the video encoder performs (724) global HBME to identify any blocks that have uniform motion relative to the reference picture. The video encoder can also check one or more other conditions, such as whether the identified block(s) with uniform motion also have a size larger than a threshold size. Section V.B describes examples of global HBME. Alternatively, the video encoder performs global HBME in some other way. With global HBME, in typical screen remoting scenarios, the video encoder can identify large areas that have uniform motion, which leaves fewer blocks to encode with other, more complex operations.

0106Within any remaining portions of the changed region(s) after the global HBME, the video encoder performs (726) local HBME for any blocks that have distinct patterns. The distinct patterns can be, for example, text content or pivot points, as described below. Section V.C describes examples of local HBME. Alternatively, the video encoder performs local HBME in some other way. With local HBME, in typical screen remoting scenarios, the video encoder can identify blocks that have distinct content such as text, which leaves fewer blocks to encode with complex operations such as full block-matching motion estimation.

0107In some example implementations, local HBME can be selectively disabled in various scenarios. For example, the video encoder selectively disables (skips) local HBME depending on the results of content classification for a picture (e.g., determining that a picture includes only natural image content). Or, the video encoder selectively disables (skips) local HBME depending on available processing resources (e.g., determining insufficient processing resources are available to perform local HBME). Or, the video encoder selectively disables (skips) local HBME depending on an application setting or user setting.

0108Within any remaining portions of the changed region(s) after the local HBME, the video encoder performs (728) full block-matching motion estimation. The full block-matching motion estimation can use a motion estimation search pattern that is relatively exhaustive. In some example implementations, the full block-matching motion estimation can be selectively disabled in various scenarios. For example, the video encoder selectively disables (skips) full block-matching motion estimation depending on available processing resources (e.g., determining insufficient processing resources are available to perform full block-matching motion estimation), or depending on an application setting or user setting.

0109With reference to <figref idref="f0007">FIG. 7a</figref>, the video encoder checks (740) whether to continue with the next picture and, if so, receives (710) the next picture in the video sequence. Thus, the video encoder can repeat the receiving (710), encoding (720), and outputting (730) operations for one or more other pictures in the video sequence. In this way, the video encoder receives one or more pictures in a video sequence, encodes the one or more pictures, and outputs the encoded data for the one or more pictures as part of a bitstream.

B. Global Hash-Based Motion Estimation.

0110This section describes various features of global hash-based motion estimation ("HBME") adapted for screen remoting scenarios. Some of the features described in this section (e.g., examples of hashing functions, example definitions of pivot points) can also be used in local HBME.

0111In some example implementations, various features of global HBME allow a video encoder to efficiently detect uniform motion in large, rectangular areas of content in screen capture video. For example, the video encoder detects distinctive patterns of sample values, called pivot points, in pictures of a video sequence. For a pivot point in a current picture, the video encoder detects one or more matching pivot points in a previous picture. The video encoder can use a hashing function and data structure that tracks hash values to speed up the matching process for pivot points. Then, for matched pivot points, the video encoder finds a matching area around the respective pivot points. A matching area can be very large, indicating global motion for many blocks in the matching area. Based on the motion of a matching area between the current picture and previous picture, blocks in the matching area are assigned MV values. In this way, the video encoder can quickly detect and encode large areas of exact-match blocks having uniform motion, which reduces overall latency and improves compression efficiency. For many cases, global HBME avoids time-consuming comparisons between sample values of a current block and candidate blocks, which characterize full block-matching motion estimation.

0112New features of global HBME described herein include, but are not limited to, calculating derivative sample values for global HBME operations, hashing of sample values for pivot points, creating and updating a multi-level data structure for use in hashing operations, detecting changed regions in which global HBME operations are performed, and detection and use of global motion metadata. These different features can be used in combination or separately.

1. Using Derivative Sample Values in Operations for Global Hash-Based Motion Estimation.

0113As part of global HBME, a video encoder can calculate derivative sample values to use in motion estimation operations. In this case, when performing global HBME operations for a current picture, the video encoder calculates multiple derivative sample values for the current picture based on base sample values for the current picture. When global HBME includes hashing of sample values for pivot points, the video encoder can use the derivative sample values to find pivot points and calculate hash values for the pivot points. Thus, derivative sample values can be used to find a pivot point in the current picture and calculate a hash value for the pivot point in the current picture, which is compared to hash values for pivot points in a previous picture (also calculated from derivative sample values).

0114The way that derivative sample values are calculated depends on implementation. In some approaches, the derivative sample values are Y_deriv sample values computed with a "data hiding" mechanism from base luma (Y) sample values and chroma (U, V) sample values. A given Y_deriv sample value is calculated by combining multiple bits of a Y sample value with at least one bit of a U sample value and at least one bit of a V sample value. For example, from 8-bit Y, U, and V sample values, an 8-bit Y_deriv sample value is computed as: <maths id="math0001"><math display="block"><msub><mi mathvariant="normal">Y</mi><mi>deriv</mi></msub><mo>=</mo><mfenced><mi mathvariant="normal">Y</mi><mspace width="1ex" /><mo>&amp;</mo><mspace width="1ex" /><mn>0</mn><mi>xFE</mi></mfenced><mo>+</mo><mfenced><mi mathvariant="normal">U</mi><mspace width="1ex" /><mo>&amp;</mo><mspace width="1ex" /><mn>0</mn><mi mathvariant="normal">x</mi><mn>01</mn></mfenced><mo>+</mo><mfenced><mi mathvariant="normal">V</mi><mspace width="1ex" /><mo>&amp;</mo><mspace width="1ex" /><mn>0</mn><mi mathvariant="normal">x</mi><mn>02</mn></mfenced><mo>.</mo></math><img file="EP4583510A2_D0001.tif" /></maths> That is, the 6 most significant bits of the Y sample value are combined with the least significant bit of the U sample value and the second-least significant bit of the V sample value. Alternatively, a Y_deriv sample values can have some other bit depth (<i>e.g.,</i> 10 bits, 12 bits, or more bits per sample value). Also, a Y_deriv sample values can be calculated from base YUV sample values having some other bit depth (<i>e.g.,</i> 10 bits, 12 bits, or more bits per sample value).

0115Calculation of Y_deriv sample values can be performed concurrently with conversion of sample values from an RGB color space to a YUV color space. For a given combination of RGB sample values, the corresponding Y_deriv sample value provides a distinctive, representative value for motion estimation operations. Alternatively, derivative sample values can be computed directly from base sample values in an RGB color space or other color space.

0116By using only derivative sample values (and not base sample values), a video encoder can perform certain global HBME operations more quickly while still detecting motion accurately. For example, a video encoder can compute pivot points and hash values using only Y_deriv sample values (rather than YUV sample values), which provides a quick and accurate way to detect motion in screen capture video. Later motion estimation operations (<i>e.g.,</i> sample-by-sample comparisons when hash values match) can be performed using Y_deriv sample values or YUV sample values. In general, using YUV sample values for motion estimation operations requires more comparisons but is more accurate. Using Y_deriv sample values is faster but potentially not as accurate.

0117Alternatively, a video encoder skips conversion of base sample values to derivative sample values, and instead performs global HBME operations with the base sample values or a subset of the base sample values. For example, the video encoder performs motion estimation operations using Y, U, and V sample values when finding motion data for partitions of a current picture. Or, the video encoder performs global HBME operations using only Y sample values when finding motion data for partitions of a current picture. Or, the video encoder finds different motion data for different color components of a current picture, using sample values of the respective color components.

2. Example of Hashing of Sample Values for Pivot Points in Global HBME.

0118As part of global HBME, a video encoder can calculate hash values for pivot points in pictures in order to speed up the motion estimation process, while still detecting motion effectively. In particular, by using hash values for pivot points, a video encoder can quickly detect scrolling motion and window motion in screen capture video in many cases, even when the motion has a large magnitude.

0119<figref idref="f0008">FIG. 8</figref> shows a simplified example (800) of global HBME with hashing of sample values for pivot points. A previous picture (810) of screen capture video includes multiple pivot points spread throughout the previous picture (810). For a pivot point, a hash value for the pivot point and the location of the pivot point in the previous picture are stored. (A video encoder finds the pivot points in the previous picture (810) when encoding that picture.)

0120When encoding a current picture (820) of screen capture video, the video encoder finds pivot points in the current picture (820) and calculates hash values for the pivot points. For a given pivot point in the current picture (820), the video encoder checks one or more of the pivot points in the previous picture (810) for a matching hash value. In the example (800) of <figref idref="f0008">FIG. 8</figref>, the video encoder finds a pivot point with the same hash value in the previous picture (810). (These matching pivot points are shown shaded.) Then, the video encoder finds a matching area (812) around the matching pivot points in the current picture (820) and previous picture (810) by comparing sample values within the matching area (812). In <figref idref="f0008">FIG. 8</figref>, the matching area (812) exhibits global motion between the previous picture (810) and the current picture (820), <i>e.g.,</i> due to scrolling of content within a Web browser, word processor, etc., or due to window movement. The video encoder can similarly try to find matches for other pivot points in the current picture (820). By limiting comparison operations to pivot points and, for matching pivot points, their surrounding areas, the video encoder can greatly simplify motion estimation.

0121In more detail, as part of global HBME for a current, non-key picture, the video encoder finds one or more pivot points in the current picture. In general, a pivot point is a distinctive pattern of sample values in a picture. Example patterns for pivot points are described in section V.B.4. Alternatively, the video encoder finds pivot points having other and/or additional patterns.

0122For a given pivot point in the current picture, the video encoder calculates a hash value. The hash value can be computed using sample values in and around the pattern for the given pivot point. After that, the given pivot point can be represented using the hash value and the location of the pivot point in the picture (<i>e.g.,</i> x, y coordinates). Example hashing functions are described in section V.B.5. Alternatively, the video encoder uses another hashing function to calculate hash values for pivot points.

0123For the given pivot point in the current picture, the video encoder searches for a matching area in a previous picture based at least in part on the hash value for the pivot point in the current picture. For example, the video encoder calculates a hash index from the hash value for the pivot point in the current picture. The video encoder can calculate the hash index from the hash value for the pivot point and a bit mask. In some example implementations, an 8-bit hash index <i>hash_index</i> is calculated as: <maths id="math0002"><math display="block"><msub><mi mathvariant="italic">hash</mi><mi mathvariant="italic">index</mi></msub><mo>=</mo><msub><mi mathvariant="italic">hash</mi><mi mathvariant="italic">value</mi></msub><mspace width="1ex" /><mo>&amp;</mo><mspace width="1ex" /><mn>0</mn><mi>xFF</mi><mo>,</mo></math><img file="EP4583510A2_D0002.tif" /></maths> where <i>hash_value</i> represents a full hash value having 32 bits. Alternatively, the hash index is calculated in some other way. For example, an 8-bit hash index <i>hash_index</i> is calculated as: <maths id="math0003"><math display="block"><msub><mi mathvariant="italic">hash</mi><mi mathvariant="italic">index</mi></msub><mo>=</mo><mfenced><msup><mfenced><msub><mi mathvariant="italic">hash</mi><mi mathvariant="italic">value</mi></msub><mo>≫</mo><mn>23</mn></mfenced><mo>∧</mo></msup><msub><mi mathvariant="italic">hash</mi><mi mathvariant="italic">value</mi></msub></mfenced><mspace width="1ex" /><mo>&amp;</mo><mspace width="1ex" /><mi>MAX_HASH_INDEX</mi><mo>,</mo></math><img file="EP4583510A2_D0003.tif" /></maths> where MAX_HASH_INDEX is, for example, 0xFF. The hash index can be calculated with a shorter or longer bit mask for hash index values having a different number of bits.

0124The video encoder looks up the hash index in a data structure to find a list of one or more candidate pivot points in the previous picture. In general, the data structure stores hash values for pivot points detected during encoding of one or more earlier pictures in the video sequence. For each possible value of the hash index, the data structure may include a list of candidate pivot points in the previous picture that are associated with that value of the hash index, or may include no associated list of candidate pivot points (if no candidate pivot points in the previous picture are associated with that value of the hash index). In some example implementations, for a given candidate pivot point, the data structure stores the full hash value and the location of the candidate pivot point in the previous picture (<i>e.g.,</i> x, y coordinates). Different pivot points in a list have different locations and can have different hash values. Example data structures used to manage hash values for pivot points in global HBME are described in section V.B.3. Alternatively, the video encoder uses another data structure to manage hash values for pivot points in global HBME.

0125When a list is found for the hash index calculated for the pivot point in the current picture, for each of at least one of the candidate pivot point(s) in the list, the video encoder compares the hash value for the pivot point in the current picture to the hash value for the candidate pivot point. In other words, the full hash values of the pivot point in the current picture and candidate pivot point in the previous picture are compared. If the hash value for the pivot point in the current picture does not match the hash value for a given candidate pivot point among the candidate pivot point(s) in the list, the video encoder checks the next candidate pivot point, if any, in the list.

0126On the other hand, if the hash value for the pivot point in the current picture matches the hash value for a given candidate pivot point among the candidate pivot point(s) in the list, the video encoder can compare sample values in an area around the pivot point in the current picture with corresponding sample values around the given candidate pivot point in the previous picture. For example, the video encoder checks sample values in an <i>m</i> x <i>n</i> area centered on the respective pivot points. The <i>m</i> x <i>n</i> area can be a 4 x 4 area, 8 x 8 area, or some other size of rectangular area. The video encoder can check all sample values in the area or a subset (<i>e.g.,</i> random sampling) of the sample values in the area. The sample values that are compared can be derivative sample values (as described in section V.B.1) or base sample values (<i>e.g.,</i> YUV sample values).

0127If the sample values in the <i>m</i> x <i>n</i> area around the pivot point in the current picture match the corresponding sample values around the given candidate pivot point in the previous picture, the video encoder can enlarge the area of comparison. For example, the video encoder doubles the size of the <i>m</i> x <i>n</i> area or otherwise increases the size of the <i>m</i> x <i>n</i> area. The increase can be uniform <i>(e.g.,</i> plus <i>i</i> rows or columns in each direction) or non-uniform (plus <i>i</i> rows/columns in first direction, plus <i>j</i> rows/columns in second direction, and so on). For example, the increase can be non-uniform after the edge of the current picture or a changed region (<i>see</i> section V.B.6) is reached. The video encoder then compares multiple sample values in the enlarged area around the pivot point in the current picture with corresponding sample values around the given candidate pivot point in the previous picture. In this way, the video encoder can enlarge the area, by successively evaluating areas with increased sizes, until a stop condition occurs. For example, the stop condition is a failure to match between the sample values in the (enlarged) area around the pivot point in the current picture and the corresponding sample values around the given candidate pivot point in the previous picture. Or, the stop condition is reaching edges of a changed region (<i>see</i> section V.B.6) in all directions.

0128When evaluating the pivot point in the current picture, the matching area around the pivot point (first pivot point) in the current picture may expand so that it covers another pivot point (second pivot point) in the current picture. As such, when searching for a matching area, the video encoder can check whether the area around the first pivot point overlaps another (second) pivot point in the current picture. If so, the video encoder can discard the first pivot point or the second pivot point in the current picture. For example, the video encoder discards the second pivot point and continues expanding the matching area around the first pivot point. The discarded pivot point is not further considered (that is, it is not considered in later motion estimation operations for the current picture, nor is it tracked as a candidate pivot point for motion estimation operations for subsequent pictures). In this way, the video encoder can weed out pivot points that are redundant or unhelpful, so as to further speed up the global HBME process.

0129When a matching area has been found (whether or not enlargement of the area has succeeded), the video encoder can check whether the matching area satisfies a threshold size, which depends on implementation. For example, the threshold size is 32x32 or some other size. If the matching area is at least as large as the threshold size, the matching area is retained as a matching area for the pivot point in the current picture. In this case, the motion associated with the matching area from the previous picture to the current picture can be used to encode partitions within the matching area. Otherwise (matching area does not satisfy threshold size), the video encoder can discard the matching area.

0130When a candidate pivot point does not provide a sufficient matching area (<i>e.g.,</i> because hash values do not match, or sample values do not match, or a matching area does not satisfy the threshold size), the video encoder checks the next candidate pivot point, if any, in the list associated with the value of hash index.

0131This process continues until a candidate pivot point provides a sufficient matching area or the last candidate pivot point in the list is evaluated. If none of the candidate pivot point(s) in the list provides a sufficient matching area, the video encoder can perform other encoding processes for blocks in the affected section of the current picture. Such other encoding processes can include local HBME (<i>see</i> section V.C), full block-matching motion estimation, and/or intra-picture compression processes.

0132After completing the process of searching for a matching area in the previous picture for a given pivot point in the current picture, the video encoder checks whether to continue for the next pivot point in the current picture. If so, the video encoder calculates a hash value for the next pivot point in the current picture and searches for a matching area in the previous picture. In this way, the video encoder performs global HBME operations for the respective pivot points in the current picture.

3. Example Data Structures Tracking Hash Values for Global HBME.

0133When using hash values for pivot points to speed up global HBME, a video encoder can use data structures to track the hash values for the pivot points. <figref idref="f0009">FIGS. 9</figref> and <figref idref="f0010">10</figref> show example data structures (900, 1000) used in global HBME with hashing of sample values for pivot points. Each of the data structures (900, 1000) uses a multi-level, dynamic array scheme to store hash values for pivot points. This approach enables fast, accurate global HBME by searching hash values of pivot points.

0134In <figref idref="f0009">FIG. 9</figref>, the hash index table (910) includes an entry for each possible value of hash index. In <figref idref="f0009">FIG. 9</figref>, the hash index has 8 bits, and the range of values for the hash index is 0x00 to 0xFF. An entry in the hash index table (910) can be empty or include a reference (<i>e.g.,</i> pointer) to a list of one or more pivot points. In some example implementations, a list of pivot point(s) initially includes entries for up to eight different pivot points, but can dynamically increase in size to store information for additional pivot points. Multiple pivot points represented in a given list can have different hash values that yield the same value of hash index. By using a multi-level scheme with lists of candidate pivot points that dynamically grow, the video encoder limits size of the data structure while providing fast access for search operations during global HBME.

0135<figref idref="f0009">FIG. 9</figref> shows two lists (920, 921) of pivot points. One list (920) has entries for three pivot points associated with the hash index 0x01, and the other list (921) has entries for two pivot points associated with the hash index 0xFD. For the sake of simplicity, other lists are not shown in <figref idref="f0009">FIG. 9</figref>. An entry in a list (920, 921) can be empty or include a reference (<i>e.g</i>., pointer) to a structure for a pivot point.

0136In <figref idref="f0009">FIG. 9</figref>, the structure (930) for a given pivot point includes entries for the location (<i>pivot_x</i> and <i>pivot_y</i>) and full hash value (<i>hash_value</i>) of the pivot point. For the sake of simplicity, entries for other pivot points are not shown in <figref idref="f0009">FIG. 9</figref>. Alternatively, the fields of a pivot point can be represented as follows: <pre listing-type="program-listing"> struct pivot { INT32 pivot_x; INT32 pivot_y; INT32 hash_value; }</pre>

0137A video encoder can maintain multiple hash index tables for global HBME, with one hash index table storing hash values for candidate pivot points in one or more previous pictures, and another hash index table storing hash values for pivot points in the current picture. In <figref idref="f0010">FIG. 10</figref>, a hash table (1010) includes references (<i>e.g</i>., pointers) to two hash index tables (1020, 1040). Each of the hash index tables (1020, 1040) includes an entry for each possible value of hash index. In <figref idref="f0010">FIG. 10</figref>, the hash index has 8 bits, and the range of values for the hash index is 0x00 to 0xFF. An entry in the hash index table (1020, 1040) can be empty or include a reference (<i>e.g.</i>, pointer) to a list of one or more pivot points. For the sake of simplicity, most lists are not shown in <figref idref="f0010">FIG. 10</figref>.

0138For the first hash index table (1020), each list includes entries for one or more candidate pivot points in the previous picture. <figref idref="f0010">FIG. 10</figref> shows one list (1030) of three candidate pivot points in the previous picture, which are associated with the hash index 0x03. For each of the candidate pivot points, an entry (not shown) in the list indicates a location in the previous picture and the hash value for the candidate pivot point.

0139For the second hash index table (1040), each list includes entries for one or more pivot points in the current picture. <figref idref="f0010">FIG. 10</figref> shows two lists (1050, 1051) of pivot points in the current picture, which are associated with the hash indices 0x03 and 0xFF, respectively. For each of the pivot points, an entry (not shown) in the list indicates a location in the current picture and the hash value for the pivot point.

0140When the video encoder performs global HBME for the current picture, the video encoder populates the hash index table (1040) for pivot points in the current picture. For example, the video encoder finds a pivot point in the current picture, calculates a hash value for the pivot point, calculates hash index 0x03 for the pivot point, and stores the location and hash value for the pivot point as an entry (1060) of the list (1050) of pivot points associated with hash index 0x03. Later, when the video encoder searches for a matching area, the video encoder retrieves the list (1030) of candidate pivot points associated with the hash index 0x03. The video encoder evaluates the three candidate pivot points in the list (1030), one after the other, until it finds a candidate pivot point (1033) that yields a matching area for the pivot point (1060) in the current picture.

0141After global HBME for the current picture is done, the video encoder can update the hash index tables (1020, 1040). For example, the video encoder merges pivot points from the two hash index tables (1020, 1040) into the hash index table (1020) that stores candidate pivot points for previous pictures, and initializes the hash index table (1040) for the next picture (as the current picture). Or, the video encoder merges points from the two hash index tables (1020, 1040) into the hash index table (1040) that stores pivot points for the current picture, which will be used as candidate pivot points in previous pictures, and initializes the hash index table (1020) for the next picture (as the current picture). When updating the data structure that includes the two hash index tables, the video encoder can retain at least one of the candidate pivot point(s) in the previous picture, remove at least one of the candidate pivot point(s) in the previous picture, and/or add at least one pivot point in the current picture. After the updating, the pivot points cover various sections of the current picture but redundant, outdated candidate pivot points (which have been superseded by newer pivot points in the current picture) have been removed.

4. Example Patterns for Pivot Points in Global HBME.

0142When finding pivot points in a picture during global HBME, the video encoder can search for various patterns of sample values. In general, to find a pivot point in a picture (<i>e.g.,</i> the current picture), the video encoder compares sample values for the current picture to one or more patterns. Each of the one or more patterns can be indicative of an edge, character, or other distinctive configuration of sample values. The sample values can be derivative sample values (<i>see</i> section V.B.1) or base sample values.

0143The video encoder can search for pivot points on a sample-by-sample basis. For example, for a current location in the picture, the video encoder checks sample values around the current location. When the video encoder finds a pivot point at the current location, the video encoder can jump ahead by an amount PIVOT_DISTANCE before searching for the next pivot point. The value of PIVOT_DISTANCE depends on implementation. For example, PIVOT_DISTANCE is a predetermined number of sample values (<i>e.g.,</i> 10 sample values, 20 sample values, 100 sample values) in scanning order or a predefined distance horizontally and/or vertically in the current picture. In this way, the video encoder avoids finding pivot points that are packed close together, which would not be useful for global HBME. Otherwise (the video encoder does not find a pivot point at the current location), the video encoder continues by searching for a pivot point at the next location.

0144The patterns used to find pivot points depend on implementation. <figref idref="f0011">FIGS. 11a, 11b, and 11c</figref> show example patterns (1110, 1120, 1130) for pivot points in global HBME. Alternatively, a video encoder uses other and/or additional patterns for pivot points.

0145For the first example pattern (1110), the video encoder compares sample values at five locations (shown as "a" in <figref idref="f0011">FIG. 11a</figref>) in a 4x4 arrangement and the sample value at a sixth location (shown as "<o ostyle="single">a</o>" for "not a" in <figref idref="f0011">FIG. 11a</figref>). If the sample values at the five locations are identical, but they are different from the sample value at the sixth location, the video encoder designates the current location (which can be the sixth location or the first location of the 4x4 arrangement) as a pivot point. The following pseudocode illustrates how a video encoder can find a pivot point having the first example pattern (1110). The condition checked is: <maths id="math0004"><formula-text>if (*pSource != *(pSource - 1) &amp;&amp; *(pSourcePrevLine - 1) == *(pSource - 1) &amp;&amp; *(pSourcePrevLine - 1) == *pSourcePrevLine &amp;&amp; *(pSourcePrevLine + 1) == *pSourcePrevLine &amp;&amp; *(pSourcePrevLine + 2) == *pSourcePrevLine).</formula-text></maths> The variable pSource represents the sixth location (with sample value "<o ostyle="single">a</o>" in <figref idref="f0011">FIG. 11a</figref>), and the variable pSourcePrevLine represents the location above the sixth location. If the sample value at the sixth location is different than the sample value to its left (at pSource-1), and the sample value at the left location (pSource-1) equals the sample values at the four locations pSourcePrevLine-1, pSourcePrevLine, pSourcePrevLine+1, pSourcePrevLine+2, then the video encoder designates the sixth location (pSource) to be a pivot point.

0146Variations of the first example pattern (1110) can include rotations of the example pattern (1110) by 90 degrees, 180 degrees, and/or 270 degrees, as well as mirror images of those patterns when flipped along a vertical axis of symmetry or horizontal axis of symmetry.

0147For the second example pattern (1120), the video encoder compares sample values in a first row and a third row of a 4x4 arrangement. If the four sample values within the first row are identical (shown as "a" in <figref idref="f0011">FIG. 11b</figref>) and the four sample values within the third row are identical (shown as "b" in <figref idref="f0011">FIG. 11b</figref>), but the sample values within the first row are different than the sample values within the third row (a <> b), the video encoder designates the current location as a pivot point. The current location can be the first location of the 4x4 arrangement. Variations of the second example pattern (1120) can include rotations of the example pattern (1120) by 90 degrees, 180 degrees, and/or 270 degrees.

0148For the third example pattern (1130), the video encoder compares sample values at locations of a 4x4 arrangement. If the sample values at the four locations in the first row and second location of the second row (shown as "a" in <figref idref="f0011">FIG. 11c</figref>) are identical, but they are different from all other sample values in the 4x4 arrangement (shown as "<o ostyle="single">a</o>" for "not a" in <figref idref="f0011">FIG. 11c</figref>), the video encoder designates the current location as a pivot point. The sample values that are "not a" can have different values from each other. The current location can be the first location of the 4x4 arrangement. Variations of the third example pattern (1130) can include rotations of the example pattern (1130) by 90 degrees, 180 degrees, and/or 270 degrees, as well as mirror images of those patterns when flipped along a vertical axis of symmetry or horizontal axis of symmetry.

5. Example Hashing Functions in Global HBME.

0149When calculating hash values for pivot points during global HBME, the video encoder uses a hashing function. The hashing function depends on implementation. The hashing function can yield a hash value with 32 bits, 64 bits, or some other number of bits, depending on implementation. For example, the hashing function yields a hash value (hash_value) based on sample values around a pivot point as shown in the following pseudocode. <img file="EP4583510A2_D0004.tif" /><img file="EP4583510A2_D0005.tif" /> In this hashing function, the hash value (hash_value) is calculated from a string of sample values starting at pData, where the length of the string is len. The length can be 8, 16, or some other number of sample values. The sample values that contribute to the hashing function are not necessarily the same as the sample values evaluated according to a pattern for the pivot point, but they can be the same. For example, the sample values that contribute to the hashing function can be the 16 sample values of a 4x4 arrangement of sample values in one of the example patterns (1110, 1120, 1130) shown in <figref idref="f0011">FIGS. 11a, 11b, and 11c</figref>, respectively.

0150Alternatively, the video encoder uses a Cantor pairing function as the hashing function. In general, a pairing function is a process to uniquely encode two natural numbers into a single natural number. The Cantor pairing function is generally defined as: <maths id="math0005"><math display="block"><mi>hash_value</mi><mo>=</mo><mfenced><mfenced><mi mathvariant="normal">d</mi><mn>0</mn><mo>+</mo><mi mathvariant="normal">d</mi><mn>1</mn></mfenced><mo>*</mo><mfenced><mi mathvariant="normal">d</mi><mn>0</mn><mo>+</mo><mi mathvariant="normal">d</mi><mn>1</mn><mo>+</mo><mn>1</mn></mfenced></mfenced><mo>/</mo><mn>2</mn><mo>+</mo><mi mathvariant="normal">d</mi><mn>1</mn><mo>,</mo></math><img file="EP4583510A2_D0006.tif" /></maths> where d0 and d1 represent a pair of input values combined according to the Cantor pairing function. When the Cantor pairing function accepts 32-bit input values, a group of four 8-bit sample values (<i>e.g.,</i> in a single row or single column) can be combined into a single 32-bit value for input to the hashing function. Thus, for the example pattern (1110, 1120, 1130) shown in <figref idref="f0011">FIG. 11a, 11b, or 11c</figref>, d0 can contain the four 8-bit sample values of the first row, and d1 can contain the four 8-bit sample values of the second row. Or, for the example pattern (1120) shown in <figref idref="f0011">FIG. 11b</figref>, d0 can contain the four 8-bit sample values of the first row, and d1 can contain the four 8-bit sample values of the third row. Sample values in more lines (rows, columns) of sample values can be combined successively according to the Cantor pairing function. For example, for the example pattern (1130) shown in <figref idref="f0011">FIG. 11c</figref>, the video encoder can calculate a first hash value using d0 equal to the four 8-bit sample values of the first row and d1 equal to the four 8-bit sample values of the second row, calculate a second hash value using d0 equal to the four 8-bit sample values of the third row and d1 equal to the four 8-bit sample values of the fourth row, and then calculate a third hash value with the first hash value as d0 and second hash value as d1. In general, the sample values that contribute to the Cantor pairing function can, but need not, be the same as the sample values evaluated according to a pattern for the pivot point.

0151Alternatively, the video encoder uses a murmur hashing function. For the murmur hashing function, the sample values that contribute the "key" value can, but need not, be the same as the sample values evaluated according to a pattern for the pivot point. For example, for the example pattern (1110, 1120, 1130) shown in <figref idref="f0011">FIG. 11a, 11b, or 11c</figref>, the video encoder can combine sample values of the first row, second row, third row, and/or fourth row, depending on implementation, to provide the key value for the murmur hashing function. The seed value for the murmur hashing function can be a random value or defined value for the video encoder.

0152Alternatively, the video encoder uses a different hashing function in global HBME.

6. Examples of Detecting Changed Regions for Global HBME.

0153Before global HBME for a current picture, a video encoder can identify one or more changed regions in the current picture relative to the previous picture. The video encoder can then limit motion estimation operations to be within the changed region(s). For example, when global HBME includes hashing of sample values for pivot points <i>(see</i> section V.B.2), the video encoder can find pivot points in the changed region(s) of the current picture, evaluating only sample values within the changed region(s) of the current picture and ignoring sample values outside the changed region(s) of the current picture. Other (unchanged) regions of the current picture can be encoded using inter-picture prediction without motion estimation, by copying from the previous picture.

0154When it starts video encoding, or periodically during encoding, the video encoder encodes a key picture. Although the video encoder does not perform motion estimation when encoding the key picture, the video encoder can find pivot points in the key picture to use in subsequent encoding. As shown in <figref idref="f0012">FIG. 12</figref>, for a key picture (1210), the video encoder finds pivot points in the entire picture. Then, for a non-key picture (1220), the video encoder detects changed regions in the picture (1220) relative to the previous picture (key picture (1210) in <figref idref="f0012">FIG. 12</figref>). The changed regions are shown as shaded regions in <figref idref="f0012">FIG. 12</figref>. The video encoder finds pivot points in the changed regions and performs global HBME using the pivot points in the changed regions of the non-key picture (1220).

0155The video encoder can detect changed regions in the current picture in various ways. For example, the video encoder can detect changed regions using hint information provided by a rendering engine or other component of an operating system, which records the hint information when rendering images to a screen for display and capturing the images for encoding. As pictures of screen capture video are provided to the video encoder, the video encoder also receives the corresponding hint information for those pictures. The hint information can be a list of candidate changed rectangles, which might or might not include content changed from the previous picture to the current picture. During color space conversion (<i>e.g</i>., when converting sample values from an RGB sample space to a YUV sample space), the video encoder can check for differences in sample values within the candidate changed regions (identified in the hint information) to determine which rectangles actually changed.

0156Alternatively, the video encoder can detect changed regions using sample-by-sample comparisons, without using hint information provided by the operating system.

0157When the video encoder uses a multi-level data structure to track hash values for pivot points for global HBME (<i>see</i> section V.B.3), the video encoder can consider which regions of the current picture have changed relative to the previous picture when updating the data structure. For example, if the data structure includes a list of one or more candidate pivot points in the previous picture, the video encoder retains any of the candidate pivot point(s) in the previous picture that is outside the changed region(s), removes any of the candidate pivot point(s) in the previous picture that is inside the changed region(s), and adds at least one pivot point in the current picture that is inside the changed region(s). In this way, the video encoder merges pivot points in the previous picture and current picture, keeping the pivot points in the previous picture that are outside the changed region(s) but replacing pivot points in the previous picture that are inside the changed region(s). After the update, the data structure includes retained pivot points for unchanged regions and newly added pivot points for the changed regions.

0158Alternatively, the video encoder can skip detection of changed regions, performing global HBME for all parts of the current picture. This can be much slower, however, and typically does not detect much additional motion between pictures.

7. Examples of Global Motion Metadata.

0159As part of global HBME for a current picture, a video encoder can aggregate local motion information for multiple smaller areas into global motion metadata for a larger area that includes the multiple smaller areas. The video encoder can successively enlarge a matching area (as described in section V.B.2) or combine multiple adjacent areas having the same motion into a larger matching area.

0160Either way, the video encoder can use the global motion metadata to skip block-based motion estimation operations for multiple partitions of the current picture. For example, the video encoder assigns MVs for the multiple partitions based on the global motion metadata covering the area that includes the multiple partitions. The MVs are then encoded normally according to a standard or format. In this way, the video encoder can quickly make motion estimation decisions for the partitions while producing a bitstream that conforms to the standard or format.

0161Alternatively, the video encoder can set syntax elements based on the global motion metadata and signal the syntax elements as part of the bitstream (e.g., in an SEI message). When a video decoder decodes the current picture, during a pre-processing stage, the video decoder can parse the syntax elements from the bitstream, determine the global motion metadata from the syntax elements, and assign MVs for partitions of the current picture in the area covered by the global motion metadata. Then, during regular decoding (conforming to a standard or format according to which MVs are signaled in the bitstream), the video decoder can perform motion compensation for the partitions. Signaling global motion metadata in this way potentially provides an efficient way to signal motion data for partitions of the current picture, reducing the bitrate used by the motion data.

0162Alternatively, the video decoder can perform global motion compensation based on the global motion metadata. In this case, when a video decoder decodes the current picture, during a pre-processing stage, the video decoder can parse the syntax elements from the bitstream and determine the global motion metadata from the syntax elements. Then, the video decoder can perform global motion compensation for the entire area covered by the global motion metadata, potentially processing all of the partitions in the area in a single pass. The video decoder can skip motion compensation for individual partitions within the area covered by global motion metadata.

0163The area covered by global motion metadata can be a rectangle that aligns with MBs (for H.264 encoding/decoding) or CUs (for H.265 encoding/decoding). Or, the area covered by global motion metadata can be a rectangle that aligns with smaller units (<i>e.g.</i>, partitions for H.264 encoding/decoding or H.265 encoding/decoding), but is shifted relative to MB boundaries or CU boundaries. Thus, for example, the rectangle can be enlarged, merged, etc. such that it aligns with arbitrary 4x4 partitions in the current picture (for H.264 encoding/decoding) or potentially smaller partitions for other standards/formats.

8. Example Combined Implementations of Global HBME.

0164A video encoder can use the preceding features of global HBME in combination. <figref idref="f0013">FIGS. 13a</figref> and <figref idref="f0014">13b</figref> show an example technique (1300) for video encoding that includes global HBME with hashing of derivative sample values for pivot points for changed regions of a current picture. A video encoder as described with reference to <figref idref="f0002">FIGS. 3</figref>, <figref idref="f0003">4a</figref>, and <figref idref="f0004">4b</figref> or other video encoder can perform the technique (1300).

0165The video encoder receives a picture (current picture) in a video sequence and converts (1310) base sample values of the current picture to derivative sample values. For example, the video encoder performs conversion operations as described in section VB. 1.

0166The video encoder checks (1320) whether the current picture is a key picture. If so, the video encoder finds (1330) one or more pivot points (if any) in the current picture (<i>see</i> sections V.B.2 and V.B.4). For a key picture, the video encoder assumes the entire picture is new (entire picture is a changed region) and attempts to find pivot points throughout the picture. For each of the pivot point(s) in the current picture, the video encoder calculates (1332) a hash value for the pivot point in the current picture and stores (1334) the pivot point in a data structure used for hashing (<i>see</i> section V.B.3). For example, as described in section V.B.3, the video encoder calculates a hash index from the hash value (<i>e.g.</i>, hash value &amp; 0xFF), determines a list of pivot points associated with the hash index, and stores the hash value and location of the pivot point in the list. The video encoder checks (1336) whether to continue with the next pivot point found in the current picture and, if so, calculates (1332) the hash value for that pivot point. In this way, the video encoder finds the pivot point(s) in the current picture and populates the data structure used for hashing.

0167The video encoder encodes the current picture normally (with intra-picture compression) and outputs the encoded data for the current picture in a bitstream. The current picture is designated as the previous picture, for purposes of motion estimation of a subsequent picture. The video encoder checks (1390) whether to continue with the next picture in the video sequence and, if so, receives the next picture (as the current picture).

0168If the current picture is not a key picture (at decision 1320), the video encoder attempts to encode the current picture using inter-picture compression. The video encoder identifies (1340) changed regions, if any, in the current picture (<i>see</i> section V.B.6). The video encoder checks (1342) whether any changed regions were identified. If no changed regions were identified, the video encoder encodes the current picture using simple inter-picture prediction without motion estimation (copying sections of the previous picture) and outputs the encoded data for the current picture in the bitstream. Then, the video encoder checks (1390) whether to continue with the next picture in the video sequence and, if so, receives the next picture (as the current picture).

0169Otherwise, if changed regions are identified (at decision 1342), the video encoder finds (1350) one or more pivot points (if any) in the changed region(s) in the current picture (<i>see</i> sections V.B.2 and V.B.4). For each of the pivot point(s) in the changed region(s) in the current picture, the video encoder calculates (1352) a hash value for the pivot point and searches (1354) for a matching area, if any, in the previous picture based at least in part on the hash value for the pivot point. As explained below, <figref idref="f0015">FIGS. 14a</figref> and <figref idref="f0016">14b</figref> show an example technique (1400) for searching for the matching area in the previous picture during global HBME. The video encoder checks (1356) whether to continue with the next pivot point found in the changed region(s) in the current picture. If so (that is, there is at least one pivot point left to evaluate), the video encoder calculates (1352) the hash value for the next pivot point in the changed region(s) in the current picture and searches (1354) for a matching area. In this way, the video encoder finds the pivot point(s) in the current picture, populates the data structure used for hashing with the pivot point(s) in the current picture, and performs global HBME operations using the pivot point(s) in the data structure (from the current picture and previous picture(s)).

0170Using the results of the global HBME operations, the video encoder encodes the current picture with inter-picture compression (if successful) or other compression (otherwise). The other compression can include local HBME, full block-matching motion estimation, or intra-picture compression for different blocks of the current picture. The video encoder outputs the encoded data for the current picture in the bitstream. In particular, when matching areas have been found in the global HBME, partitions in the matching areas are assigned MVs and encoded using motion compensation. When matching areas have not been found in global HBME, partitions can be encoded using local HBME, conventional block-based motion estimation, or intra-picture compression. The video encoder updates (1360) the data structure used for hashing, as described in sections V.B.3 and V.B.6. The current picture is designated as the previous picture, for purposes of motion estimation of a subsequent picture. The video encoder checks (1390) whether to continue with the next picture in the video sequence and, if so, receives the next picture (as the current picture).

0171With reference to <figref idref="f0015">FIGS. 14a</figref> and <figref idref="f0016">14b</figref>, as described in sections V.B.2 and V.B.3, the video encoder calculates (1410) a hash index from the hash value for the pivot point in the current picture (<i>e.g.</i>, hash value &amp; 0xFF) and retrieves (1420) a list, if any, of candidate pivot points (in the previous picture) associated with the hash index from the data structure used for hashing. The video encoder checks (1422) whether a list of candidate pivot point(s) in the previous picture was retrieved. If not, the video encoder finishes searching (1354) for the pivot point in the current picture, and checks (1356) whether to continue with the next pivot point found in the changed region(s) in the current picture, as shown in <figref idref="f0014">FIG. 13b</figref>.

0172On the other hand, if a list of candidate pivot point(s) in the previous picture was retrieved, the video encoder compares (1430) the hash value for the pivot point in the changed region(s) in the current picture against the hash value for the next candidate pivot point in the list. If the hash values match (at decision 1432), the video encoder compares (1440) sample values in an area around the pivot point in the current picture and corresponding sample values around the candidate pivot point in the previous picture, as described in section V.B.2. For example, the area for comparison of sample values is a rectangular area. If the sample values match in the area (at decision 1442), the video encoder checks (1450) whether the area overlaps another pivot point. If so, the video encoder updates (1452) the pivot points in the changed region(s) in the current picture to remove a pivot point (as being unhelpful), as described in section V.B.2. The video encoder continues by checking (1460) whether the size of the matching area is less than a maximum size. If so, the video encoder enlarges (1462) the area and compares (1440) sample values in the enlarged area around the pivot point in the current picture and corresponding sample values around the candidate pivot point in the previous picture.

0173Otherwise (the matching area has reached the maximum size), the video encoder designates (1470) the matching area as a global motion area, which will be encoded by assigning MVs to partitions based on the motion of the global motion area, and continues (at 1356) by evaluating the next pivot point, if any, in the changed region(s) in the current picture.

0174If the sample values do not match in the area (at decision 1442), the video encoder checks (1444) whether an area previously matched. If so, the video encoder checks (1446) whether the size of that matching area has satisfied a threshold size. If so, the video encoder designates (1470) the matching area as a global motion area, which will be encoded by assigning MVs to partitions based on the motion of the global motion area, and continues (at 1356) by evaluating the next pivot point, if any, in the changed region(s) in the current picture.

0175If there was no previous matching area (at decision 1444), or if the area size of a previous matching area did not satisfy the threshold size (at decision 1446), or if hash values do not match between the pivot points being compared (at decision 1432), the video encoder checks (1434) whether there is another candidate pivot point in the list. If so, the encoder compares (1430) the hash value for the pivot point in the changed region(s) in the current picture against the hash value for the next candidate pivot point in the list. Otherwise (no more candidate pivot points to evaluate in the list), the motion estimation using pivot points fails, and local HBME, conventional block-matching motion estimation, or intra-picture compression can be used instead.

C. Local Hash-Based Motion Estimation.

0176This section describes various features of local hash-based motion estimation ("HBME") adapted for screen remoting scenarios. Some of the features described in this section (<i>e.g</i>., examples of hashing functions) can also be used in global HBME. Other features described in this section (<i>e.g.</i>, multi-level data structures for tracking hash values) are similar in some respects to features used in global HBME.

0177In some example implementations, various features of local HBME allow a video encoder to efficiently detect matches for blocks of screen content that include text or other distinctive patterns, then encode the blocks with motion vectors (and possibly residual values). For example, for a block of a current picture, the video encoder checks whether the block includes a distinctive pattern of sample values (pivot point). (For brevity, a block that contains a pivot point is, in some examples of local HBME described herein, simply called a pivot point.) If so, the video encoder uses LBME to detect one or more possible matching blocks in a previous picture. The video encoder can use a hashing function and data structure that tracks hash values to speed up the matching process. In this way, the video encoder can quickly detect and encode a block with text or another distinctive pattern, which reduces overall latency and improves compression efficiency. For many cases, local HBME reduces the number of time-consuming comparisons between sample values of a current block and candidate blocks, which characterize full block-matching motion estimation.

0178New features of local HBME described herein include, but are not limited to, calculating derivative sample values for local HBME operations, applying a spatial constraint to limit a search area in HBME operations, checking conditions for content of a block before performing local HBME for the block, creating and updating a multi-level data structure for use in hashing operations (<i>e.g</i>., a dynamic grid-based multi-level data structure that tracks hash values in different grid areas of a reference picture), hashing of sample values for pivot points, applying thresholds when calculating hash values so as to detect approximate matches for sample values, detecting changed regions in which local HBME operations are performed, and calculating/updating hash values for blocks of a reference picture for local HBME. These different features can be used in combination or separately.

1. Using Derivative Sample Values in Operations for Local HBME.

0179As part of local HBME, a video encoder can calculate derivative sample values to use in motion estimation operations. In this case, when performing motion estimation operations for a current picture, the video encoder calculates multiple derivative sample values for the current picture based on base sample values for the current picture. When local HBME includes hashing of sample values for blocks, the video encoder can use the derivative sample values to evaluate whether LBME should be performed for the blocks and calculate hash values for the blocks. Thus, derivative sample values can be used to find a pivot point in the current picture and calculate a hash value for the pivot point (current block) in the current picture, which is compared to hash values for candidate blocks in a previous picture (also calculated from derivative sample values).

0180The way that derivative sample values are calculated depends on implementation. Example approaches to calculating derivative sample values are described in section V.B.1. Although section V.B.1 describes calculation of derivative sample values for use in global HBME operations, a video encoder can calculate derivative sample values in the same ways for use in local HBME operations.

0181By using only derivative sample values (and not base sample values), a video encoder can perform certain local HBME operations more quickly while still detecting motion accurately. For example, a video encoder can find pivot points and hash values using only Y_deriv sample values (rather than YUV sample values), which provides a quick and accurate way to detect motion in screen capture video. Later motion estimation operations (<i>e.g</i>., sample-by-sample comparisons when hash values match) can be performed using Y_deriv sample values or YUV sample values. In general, using YUV sample values for motion estimation operations requires more comparisons but is more accurate. Using Y_deriv sample values is faster but potentially not as accurate.

0182Alternatively, a video encoder skips conversion of base sample values to derivative sample values, and instead performs local HBME operations with the base sample values or a subset of the base sample values. For example, the video encoder performs local HBME operations using Y, U, and V sample values when finding motion data for partitions of a current picture. Or, the video encoder performs local HBME operations using only Y sample values when finding motion data for partitions of a current picture. Or, the video encoder finds different motion data for different color components of a current picture, using sample values of the respective color components.

2. Example Spatial Constraints for Local HBME.

0183As part of local HBME for a current block of a current picture, a video encoder can limit searching in a reference picture according to a spatial constraint. The spatial constraint defines a search area in the reference picture, within which hash values for candidate blocks in the reference picture may be compared to a hash value for the current block. That is, hash values for candidate blocks outside the search area are not compared to the hash value for the current block.

0184To implement the spatial constraint for HBME, the video encoder splits a reference picture into regions and organizes hash values for candidate blocks by region. When checking for hash values that match the hash value for a current block of a current picture, the video encoder limits the search area in the reference picture to be one or more regions of the reference picture. The region(s) of the search area typically include the region that contains a collocated block for the current block. The region(s) of the search area can also include one or more adjacent regions, <i>e.g.,</i> one adjacent region in each direction, within the reference picture, relative to the region that contains the collocated block. (Adjacent regions that would be outside the reference picture are ignored.) By applying a spatial constraint during HBME, the video encoder can control the extent of the search process while also detecting most of the possible matching candidate blocks.

0185For example, the video encoder splits a reference picture into grid areas along horizontal and vertical grid lines. For example, a plane of derivative sample values <i>(see</i> section V.C.1) for the reference picture, or plane of luma sample values, is split into grid areas. The size of the grid areas depends on implementation and can be, for example, 256x256 sample values, 128x128 sample values, or some other size. Grid areas can have uniform size or non-uniform size within the reference picture. For a given candidate block among the candidate blocks in the reference picture, the video encoder can store the hash value for the given candidate block and location of the given candidate block in a multi-level data structure for hashing (<i>see</i> section V.C.8). The multi-level data structure organizes the hash values for the candidate blocks by grid area and, within a grid area, by hash index. For local HBME for a current block of a current picture, the video encoder can limit the search area to be a given grid area, among the grid areas of the reference picture, that includes the block collocated with the current block. Alternatively, the video encoder can limit the search area to be the given grid area (which includes the collocated block) and one or more adjacent grid areas within the reference picture.

0186<figref idref="f0017">FIGS. 15a and 15b</figref> illustrate examples of spatial constraints along lines of grid areas of a reference picture in local HBME. In <figref idref="f0017">FIG. 15a</figref>, a reference picture (1510) is divided into grid areas. For example, the reference picture (1510) is a 2048x1536 plane of derivative sample values or luma sample values, and the video encoder divides the reference picture (1510) into 256x256 grid areas. One grid area (1520) includes the collocated block for a current block of a current picture. For local HBME, the search area (shown as shaded) is the single grid area (1520) that includes the collocated block. Similarly, in <figref idref="f0017">FIG. 15b</figref>, a reference picture (1511) is divided into grid areas, and one grid area (1521) includes the collocated block for a current block of a current picture. In the example of <figref idref="f0017">FIG. 15b</figref>, for local HBME, the search area (shown as shaded) is the single grid area (1521) that includes the collocated block and multiple adjacent grid areas. The multiple adjacent grid areas include one adjacent grid area in each direction horizontally, vertically, and diagonally. An adjacent grid area that would be outside the reference picture (1511) is not considered part of the search area.

3. Example Block Conditions for Local HBME.

0187As described below (<i>see</i> section V.C.7), a video encoder can limit local HBME so that it is performed only for blocks within changed regions of the current picture. Even within a changed region of the current picture, the video encoder can further limit local HBME so that it is performed only for blocks that are likely to benefit from the local HBME. (In some examples described herein, such blocks are termed "qualifying blocks.") In this way, the video encoder can limit the resources used to compute and manage hash values for candidate blocks of a reference picture for local HBME. Similarly, the video encoder can limit the resources uses to compute hash values and search for blocks of the current picture.

0188For example, the video encoder can limit local HBME to blocks that contain text. To determine whether a block (<i>e</i>.<i>g</i>., current block of a current picture or candidate block of a reference picture) contains text, the video encoder can use any of various approaches, depending on implementation. For example, the video encoder can make a decision based on hint information from an operating system component, which indicates whether a block contains text. Or, as another example, the video encoder can make a decision based on information produced during color space conversion operations for sample values of a block. The information produced during color space conversion operations can be a histogram which shows the number of distinct sample values. The video encoder can determine that a block contains text if it has more than one distinct sample value but less than x distinct sample values, where <i>x</i> is 2, 3, or some other small non-zero integer. Or, the video encoder can determine whether a block includes black and white sample values (suggesting typical textual content) by evaluating whether chroma sample values are zero or mostly zero. Or, as another example, the video encoder can apply an image classification test to sample values of the block to determine whether the block contains text. To make a decision about whether a block contains text, the video encoder can make a decision for a larger region that includes the block, and then apply the decision to the various blocks within the larger region.

0189Or, as another example, the video encoder can limit local HBME to blocks that contain pivot points. To determine whether a block (<i>e</i>.<i>g</i>., current block of a current picture or candidate block of a reference picture) contains a pivot point, the video encoder can use any of various approaches, as described in section V.C.4. Or, as another example, the video encoder can limit local HBME to blocks that contain text and also contain pivot points.

0190These conditions can be checked when the video encoder determines hash values for candidate blocks in grid areas of a reference picture. For example, for a given reference picture, the video encoder splits the reference picture into grid areas (<i>see</i> section V.C.2). The video encoder checks candidate blocks in a given grid area. For a given candidate block, the video encoder checks whether the candidate block contains text content, as described above. If so, the video encoder checks whether the candidate block contains a pivot point, as described above. If the candidate block contains text content and contains a pivot point, the video encoder calculates a hash value for the candidate block and registers the candidate block in a data structure that tracks hash values for the reference picture (<i>see</i> section V.C.8). The video encoder continues this process for the next candidate block in the given grid area, and then repeats the process for other grid areas of the reference picture.

0191When encoding a current picture, the video encoder checks conditions for a current block of the current picture before performing local HBME for the current block. The video encoder checks whether the current block contains text content, as described above. If so, the video encoder checks whether the current block contains a pivot point, as described above. If the current block contains text content and contains a pivot point, the video encoder performs local HBME for the current block, calculating a hash value for the current block and searching for a matching block in the reference picture. Otherwise (the current block does not contain text content and/or does not contain a pivot point), the current block is encoded in some other way (<i>e.g.,</i> full block-matching motion estimation or intra-picture compression). The video encoder repeats this process for other blocks in the current picture.

0192Alternatively, instead of calculating hash values only for blocks that satisfy certain conditions (<i>e</i>.<i>g</i>., contain text and/or contain a pivot point), the video encoder can calculate hash values for all blocks. In typical encoding scenarios, however, this tends to increase computational complexity without improving effectiveness of local HBME very much.

4. Example Patterns for Pivot Points in Local HBME.

0193When finding pivot points in a picture during local HBME, the video encoder can search for various patterns of sample values. In general, to find a pivot point in a picture (<i>e.g.,</i> a reference picture, the current picture), the video encoder compares sample values for the picture to one or more patterns. Each of the one or more patterns can be indicative of an edge, character, or other distinctive configuration of sample values. The sample values can be derivative sample values (<i>see</i> section V.C.1) or base sample values.

0194Section V.B.4 describes several examples of patterns for pivot points in global HBME. The same patterns can be used for pivot points in local HBME.

0195Alternatively, for pivot points in local HBME, the video encoder uses another pattern, which is more likely to result in the video encoder finding pivot points. For example, <figref idref="f0017">FIG. 16</figref> shows an example pattern (1600) for pivot points in local HBME. For the example pattern (1600), the video encoder evaluates sample values in a first row and a fourth row of a 4x4 arrangement. The first row includes sample values a, b, c, and d, which can be the same or different from each other. The fourth row includes sample values e, f, g, and h, which can be the same or different from each other. The video encoder aggregates sample values a, b, c, and d of the first row into a first aggregate value, aggregates sample values e, f, g, and h of the fourth row into a second aggregate value, and compares the first and second aggregate values. If the two aggregate values are different, the video encoder designates the current location (<i>e.g.,</i> for a current block, for a candidate block) as a pivot point. For example, for 8-bit sample values, the first aggregate value is d0 = (a << 24) + (b << 16) + (c << 8) + d, which can be determined simply by reading a, b, c, and d as a 32-bit value from memory when a, b, c, and d are consecutively stored. Similarly, the second aggregate value is d1 = (e << 24) + (f << 16) + (g << 8) + h, which can be calculated simply by reading e, f, g, and h as a 32-bit value from memory when e, f, g, and h are consecutively stored. If d0 <> d1, the video encoder designates the current location as a pivot point. The current location can be the first location (top-left location) of a 4x4 arrangement for a 4x4 block. Thus, if any of the pairs of sample values a/e, b/f, c/g, or d/h in the example pattern (1600) is different, the current location is designated as a pivot point.

0196The video encoder can apply any of these patterns for pivot points when searching for candidate blocks (qualifying blocks) in regions (<i>e.g.,</i> grid areas) of a reference picture or when searching for qualifying blocks in the current picture. If a pivot point is found in a reference picture, the video encoder adds an entry for the pivot point (that is, for the candidate block that contains the pivot point, at the location for the pivot point) to a data structure that tracks hash values for the reference picture. If a pivot point is found in the current picture, the video encoder performs local HBME for the pivot point (that is, for the current block that contains the pivot point).

5. Example Hashing Functions in Local HBME.

0197When calculating hash values for blocks (<i>e</i>.<i>g</i>., candidate blocks in a reference picture, qualifying blocks in a current picture) for local HBME, the video encoder uses a hashing function. The hashing function depends on implementation. The hashing function can yield a hash value with 32 bits, 64 bits, or some other number of bits, depending on implementation. Section V.B.5 describes examples of hashing functions for global HBME. The same hashing functions (<i>e</i>.<i>g</i>., Cantor pairing function, murmur hashing function) can be used for local HBME. Alternatively, the video encoder uses another hashing function for local HBME.

0198The sample values that contribute to the hashing function are not necessarily the same as the sample values evaluated according to a pattern for the pivot point, but they can be the same. For example, the sample values that contribute to the hashing function can selected from the 16 sample values of a 4x4 arrangement of sample values in one of the example patterns (1110, 1120, 1130, 1600) shown in <figref idref="f0011">FIGS. 11a, 11b, 11c</figref>, and <figref idref="f0017">16</figref>, respectively. Alternatively, hash values can be computed for larger blocks (<i>e.g.,</i> 8x8 blocks or 16x16 blocks).

0199In some example implementations, the hashing function for local HBME is a Cantor pairing function with 32-bit inputs d0 and d1. For the example pattern (1600) shown in <figref idref="f0017">FIG. 16</figref>, the input d0 is d0 = (a &lt;&lt; 24) + (b &lt;&lt; 16) + (c &lt;&lt; 8) + d, which can be determined simply by reading a, b, c, and d as a 32-bit value from memory when a, b, c, and d are consecutively stored. The input d1 is d1 = (e &lt;&lt; 24) + (f &lt;&lt; 16) + (g &lt;&lt; 8) + h, which can be calculated simply by reading e, f, g, and h as a 32-bit value from memory when e, f, g, and h are consecutively stored. The Cantor pairing function is calculated as: <maths id="math0006"><math display="block"><mi>hash_value</mi><mo>=</mo><mfenced><mfenced><mi mathvariant="normal">d</mi><mn>0</mn><mo>+</mo><mi mathvariant="normal">d</mi><mn>1</mn></mfenced><mo>*</mo><mfenced><mi mathvariant="normal">d</mi><mn>0</mn><mo>+</mo><mi mathvariant="normal">d</mi><mn>1</mn><mo>+</mo><mn>1</mn></mfenced></mfenced><mo>/</mo><mn>2</mn><mo>+</mo><mi mathvariant="normal">d</mi><mn>0</mn><mo>.</mo></math><img file="EP4583510A2_D0007.tif" /></maths>

6. Example Threshold Operations for Local HBME.

0200When computing the hash value for a block (<i>e</i>.<i>g</i>., current block of a current picture or candidate block of a reference picture), the video encoder can count all bits of sample values that are input to the hashing function. Alternatively, the video encoder can ignore one or more least significant bits of the respective sample values that are input to the hashing function. For example, the video encoder ignores the x least significant bits per sample value input to the hashing function, where x is, for example 1, 2, 3, or 4. The value of x defines a threshold for the hashing function and hash values. When the threshold is low, blocks may have the same hash value even when there are small differences in the respective sample values contributing to the hashing function for the blocks. When the threshold is higher, blocks may have the same hash value even when there are larger differences in the respective sample values contributing to the hashing function for the blocks.

0201This permits similar blocks of sample values to map to the same hash value, within a threshold of differences in sample values. For example, when the hashing function is a Cantor pairing function, inputs d0 and d1 can be computed from sample values with a threshold count of least significant bits ignored per sample value.

7. Examples of Detecting Changed Regions for Local HBME.

0202Before local HBME for a current picture, a video encoder can identify one or more changed regions in the current picture relative to the previous picture. The video encoder can then limit motion estimation operations to be within the changed region(s). For example, when local HBME includes hashing of sample values for qualifying blocks (<i>e</i>.<i>g</i>., containing textual content, containing pivot points), the video encoder can find pivot points in the changed region(s) of the current picture, evaluating only sample values within the changed region(s) of the current picture and ignoring sample values outside the changed region(s) of the current picture. Other (unchanged) regions of the current picture can be encoded using inter-picture prediction without motion estimation, by copying from the previous picture. Section V.B.6 describes various approaches to detecting changed regions in a current picture relative to the previous picture, in the context of global HBME. The same approaches can be used in the context of local HBME.

0203When the video encoder uses a multi-level data structure to track hash values for candidate blocks for local HBME (<i>see</i> section V.C.8), the video encoder can consider which regions of the current picture have changed relative to the previous picture when updating the data structure. If the data structure includes a list of one or more candidate blocks in the previous picture, the video encoder retains any of the candidate block(s) in the previous picture that is outside the changed region(s), removes any of the candidate blocks(s) in the previous picture that is inside the changed region(s), and adds at least one candidate block (from the current picture) that is inside the changed region(s). Thus, the video encoder updates hash values for changed regions by replacing obsolete entries. In this way, the video encoder merges candidate blocks in the previous picture and current picture, keeping the candidate blocks in the previous picture that are outside the changed region(s) but replacing candidate blocks in the previous picture that are inside the changed region(s). After the update, the data structure includes retained candidate blocks for unchanged regions and newly added candidate blocks for the changed regions.

0204Alternatively, the video encoder can skip detection of changed regions, potentially performing local HBME for all parts of the current picture. This can be much slower, however, and typically does not detect much additional motion between pictures.

8. Example Data Structures Tracking Hash Values for Candidate Blocks for Local HBME.

0205When using hash values for candidate blocks (<i>e.g.,</i> for pivot points) of a previous picture to speed up local HBME, a video encoder can use data structures to track the hash values for the candidate blocks organized in different regions. <figref idref="f0018">FIG. 17</figref> shows an example data structure (1700) used in local HBME with hashing of sample values for candidate blocks. The data structure (1700) uses a multi-level, dynamic array scheme to store hash values for candidate blocks, with hash values organized by grid area. This approach enables fast, accurate local HBME by searching hash values of candidate blocks within the one or more regions (<i>e.g.,</i> grid areas) of a search area.

0206The data structure (1700) includes different hash index tables for different grid areas in the previous picture (example of reference picture). For each grid area in the previous picture, the grid area table (1710) includes a reference (<i>e</i>.<i>g</i>., pointer) to a hash index table for that grid area. For example, at a first index (grid area index 0), the grid area table (1710) includes a reference to a hash index table (1720) for grid area 0. At another index (grid area index <i>n</i>), the grid area table (1710) includes a reference to a hash index table (1740) for grid area <i>n</i>. For the sake of simplicity, hash index tables for other grid areas are not shown.

0207Each hash index table manages hash values for candidate blocks (<i>e.g.,</i> for pivot points) in one grid area, and the hash values/candidate blocks are further organized by hash index. In <figref idref="f0018">FIG. 17</figref>, for hash values in one grid area, each hash index table (1720, 1740) includes an entry for each possible value of hash index. In <figref idref="f0018">FIG. 17</figref>, the hash index has 8 bits, and the range of values for the hash index is 0x00 to 0xFF. Examples of ways to compute hash index values from hash values are described section V.B.2.

0208An entry in the hash index table (1720, 1740) can be empty or include a reference (<i>e.g.,</i> pointer) to a list of one or more candidate blocks (<i>e.g.,</i> for pivot points) in a specific grid area. In some example implementations, a list of candidate block(s) initially includes entries for up to eight different candidate blocks, but can dynamically increase in size to store information for additional candidate blocks (<i>e.g.,</i> up to a size of 128 candidate blocks indexed 0x00 to 0x7F). Multiple candidate blocks represented in a given list can have different hash values that yield the same value of hash index. By using a multi-level scheme with lists of candidate blocks that dynamically grow, the video encoder limits size of the data structure while providing fast access for search operations during local HBME.

0209<figref idref="f0018">FIG. 17</figref> shows three lists (1730, 1750, 1751) of candidate blocks (<i>e.g.,</i> for pivot points). For the sake of simplicity, other lists are not shown in <figref idref="f0018">FIG. 17</figref>. An entry in a list (1730, 1750, 1751) can be empty or include a reference (<i>e</i>.<i>g</i>., pointer) to a structure for a candidate block (<i>e.g.,</i> for a pivot point). The structure for a given candidate block includes entries for the location (<i>pivot_x</i> and <i>pivot_y</i>) and full hash value (<i>hash_value</i>) of the candidate block. For the sake of simplicity, entries for candidate blocks are not shown in <figref idref="f0018">FIG. 17</figref>. Alternatively, the fields of a candidate block can be represented as follows: <pre listing-type="program-listing">struct pivot { INT32 pivot_x; INT32 pivot_y; INT32 hash_value; }</pre>

0210For example, for the hash index table (1720) associated with grid area 0, a list (1730) includes entries for three candidate blocks in grid area 0 of the previous picture. The three candidate blocks in the list (1730) have hash values associated with the hash index 0x03. For each of the candidate blocks, an entry (not shown) in the list indicates a location in the previous picture and the hash value for the candidate block.

0211For the second hash index table (1740), which is associated with grid area <i>n,</i> each list includes entries for one or more candidate blocks in grid area <i>n</i> of the previous picture. <figref idref="f0018">FIG. 17</figref> shows two lists (1750, 1751) of candidate blocks in grid area <i>n</i> of the previous picture. The two lists are associated with the hash indices 0x03 and 0xFF, respectively, for hash values of candidate blocks in grid area <i>n</i> of the previous picture. For each of the candidate blocks, an entry (not shown) in the list indicates a location in the previous picture and the hash value for the candidate block.

0212Before performing location HBME for blocks of a current picture, a video encoder performs various operations to register a reference picture for local HBME, populating the multi-level data structure (1700) with entries for candidate blocks in the reference picture. For example, the video encoder splits the reference picture into multiple grid areas, as described in section V.C.2. For a given grid area, the video encoder determines whether a block at a location in the grid area qualifies as one of the candidate blocks in the reference picture. For example, the video encoder checks the content of the block (<i>e</i>.<i>g</i>., checking that the block contains textual content as described in section V.C.3, checking that the candidate block contains a pivot point as described in section V.C.4).

0213If the block qualifies as one of the candidate blocks in the reference picture (<i>e.g.,</i> the block contains textual content and contains a pivot point), the video encoder calculates a hash value for the candidate block (<i>e.g.,</i> as described in section V.C.5). The hash value for the candidate block can be computed using sample values subjected to a threshold (<i>e</i>.<i>g</i>., ignoring the x least significant bits of the sample values, respectively), as described in section V.C.6. The video encoder registers the hash value and location for the candidate block in the data structure (1700). For example, the video encoder calculates a hash index of the hash value for the candidate block, then stores, as an entry of a list associated with the hash index of the hash value for the candidate block, the hash value for the candidate block and location of the candidate block. As noted above, the data structure (1700) organizes the hash values for the candidate blocks by grid area and, within a grid area, by hash index.

0214The video encoder repeats the process for the next block at another location in the given grid area. Candidate blocks may overlap to some extent. For example, the video encoder sets the location of the next block at an offset of <i>x</i> sample values horizontally and <i>y</i> sample values vertically from the block just evaluated. The values of <i>x</i> and <i>y</i> depends on implementation, and can be the same or different. For a low value of <i>x</i> and <i>y</i> (such as 1), more candidate blocks are potentially represented in the data structure (1700). For a higher value of <i>x</i> and <i>y</i> (such as 2 or 4), fewer candidate blocks are potentially represented in the data structure (1700). Making more candidate blocks available for local HBME can require more processing resources (to calculate hash values, to search for a matching block) and memory resources (to store entries for candidate blocks), but also increases the chance of local HBME yielding a matching block.

0215When blocks at all locations in the given grid area have been evaluated, the video encoder continues with the next grid area of the previous picture. In this way, the video encoder eventually registers qualifying candidate blocks (containing textual content and pivot points) in all of the grid areas of the previous picture into the data structure (1700). Compared to global HBME, local HBME may use a denser configuration of pivot points from the previous picture.

0216Alternatively, instead of registering candidate blocks in one grid area at a time, the video encoder can find a pivot point in the previous picture and register the candidate block that contains the pivot point in the appropriate hash index table for the grid area that includes the candidate block. The video encoder can determine the grid area from the location of the candidate block/pivot point, dimensions of grid areas, and dimensions of the previous picture. For example, suppose the width of the previous picture is w sample values, the height of the previous picture is <i>h</i> sample values, and the size of grid areas is 256x256 sample values. The variable max_grid_x is <i>w</i>/256, and the variable max_grid_y is <i>h</i>/256. For a pivot point/candidate block at location <i>x, y</i> in the previous picture, the grid area index can be computed as ((<i>y</i>>> 8) * max_grid_x) + (<i>x</i>>>8). The video encoder repeats that process for other pivot points in the previous picture.

0217When the video encoder performs local HBME for blocks of the current picture, the video encoder can populate another hash index table (not shown) for blocks (<i>e.g.,</i> for pivot points) in the current picture. After local HBME for the current picture is done, the video encoder can update the hash index tables for the previous picture, as described below.

9. Examples of Local HBME.

0218After candidate blocks in a reference picture have been registered for local HBME, a video encoder can perform local HBME for blocks of a current picture. <figref idref="f0019">FIGS. 18a and 18b</figref> show a generalized technique (1800) for local HBME. A video encoder such as the video encoder shown in <figref idref="f0002">FIGS. 3</figref>, <figref idref="f0003">4a</figref>, and <figref idref="f0004">4b</figref>, or other video encoder, can perform the technique (1800).

0219With reference to <figref idref="f0019">FIG. 18a</figref>, the video encoder receives (1810) a current picture in a video sequence. An input buffer can be configured to receive one or more pictures for encoding. The video encoder encodes (1820) the current picture to produce encoded data. An output buffer can be configured to store the encoded data for output. The video encoder outputs (1830) the encoded data as part of a bitstream. When encoding (1820) the current picture, the video encoder performs various operations for local HBME.

0220<figref idref="f0019">FIG. 18b</figref> shows one example (1821) of the encoding (1820) shown in <figref idref="f0019">FIG. 18a</figref> for the current picture. The video encoder calculates (1822) a hash value for a current block in the current picture. For example, the video encoder calculates the hash value as described in section V.C.5 or in some other way. The hash value for the current block can be computed using sample values subjected to a threshold (<i>e</i>.<i>g</i>., ignoring the <i>x</i> least significant bits of the sample values, respectively), as described in section V.C.6. The video encoder can store the hash value and location for the current block as an entry of a list in a data structure that tracks hash values for blocks of the current picture.

0221The video encoder searches (1824), subject to a spatial constraint, for a matching block in a reference picture based at least in part on the hash value for the current block. For example, the reference picture is the previous picture in display order. The spatial constraint defines a search area in the reference picture within which hash values for candidate blocks in the reference picture may be compared to the hash value for the current block. For example, the search area is a given grid area, in the reference picture, that includes a block collocated with the current block. Or, as another example, the search is the given grid area (which includes the collocated block) and multiple grid areas adjacent to the given grid area in the reference picture.

0222In some example implementations, the video encoder determines the search area for the current block as follows. Suppose the width of the current picture is w sample values, the height of the current picture is <i>h</i> sample values, and the size of grid areas is 256x256 sample values. The variable max_grid_x is <i>w</i>/256, and the variable max_grid_y is <i>h</i>/256. For a current block at location <i>x, y</i> in the current picture, the grid area index for the grid area that includes the collocated block can be computed as ((<i>y</i>>> 8) * max_grid_x) + (<i>x</i>>>8). The search area includes the grid area in the reference picture that contains the collocated block for the current block. Depending on implementation, the search area can also include one or more adjacent grid areas in the reference picture.

0223When the video encoder searches (1824) for a matching block in the search area in the reference picture, the video encoder can use the hash value for the current block to identify one or more possible matching blocks among the candidate blocks in the reference picture. Each of the one or more possible matching blocks has a hash value that matches the hash value for the current block. For example, the video encoder calculates a hash index from the hash value for the current block, and retrieves any list of candidate blocks associated with that hash index for the search area (<i>e.g.,</i> the list of candidate blocks for the hash index for each grid area in the search area). The video encoder compares the hash value for the current block to the respective hash values for the candidate blocks in the retrieved lists, so as to identify any candidate block (possible matching block) whose hash value matches the hash value for the current block.

0224For each of the possible matching block(s), the video encoder can perform limited block-matching motion estimation for the current block, starting from the possible matching block, and potentially continuing motion estimation around that candidate block according to a limited motion estimation search pattern. As part of the limited block-matching motion estimation for the current block, the video encoder can evaluate distortion cost and/or bit cost (for motion information, for residual values) for each of one or more blocks in the reference picture. The distortion cost can be measured as a sum of absolute difference of sample values between the current block and evaluated block in the reference picture, as sum of absolute transform differences for residual values, or using some other distortion metric. The video encoder can select the matching block based at least in part on results of the limited block-matching motion estimation. For example, the video encoder selects, for the matching block, the evaluated block in the reference picture that has the lowest overall cost (in terms of distortion cost and/or bit cost). Alternatively, for each of the possible matching block(s), the video encoder can evaluate a single candidate block, which is the candidate block associated with the matching hash value (no limited motion estimation search pattern). This approach is faster since fewer blocks in the reference picture are evaluated, but compression efficiency may suffer. In some example implementations, the video encoder passes the locations of possible matching blocks to a motion estimation module, which uses the locations of the possible matching blocks as hint information to guide limited block-matching motion estimation decisions, and thereby speeds up the motion estimation process.

0225The video encoder encodes (1826) the current block. For example, if local HBME finds a matching block, the current block is encoded using motion-compensated prediction and residual values (if motion-compensated prediction is not perfect) computed for the selected matching block. Otherwise (local HBME fails), the video encoder encodes the current block in some other way (<i>e.g.,</i> full block-matching motion estimation or intra-picture compression). The video encoder checks (1828) whether to continue with the next block in the current picture. If so, the video encoder calculates (1822) a hash value for the next block (as the current block). In this way the video encoder can perform local HBME for blocks in the current picture.

0226In the example (1821) of <figref idref="f0019">FIG. 18b</figref>, all blocks of the current picture may be encoded using local HBME. In practice, a video encoder can check various conditions that limit which blocks of the current picture may be encoded using local HBME. For example, the video encoder determines a changed region of the current picture, and the calculating (1822) the hash value for the current block and the searching (1824) are contingent on the current block being within the changed region. As another example, the video encoder determines whether the current block contains textual content, and the calculating (1822) the hash value for the current block and the searching (1824) are contingent on the current block containing textual content. Or, as another example, the video encoder determines whether the current block contains a pivot point, and the calculating (1822) the hash value for the current block and the searching (1824) are contingent on the current block containing a pivot point. If the current block is not encoded using local HBME (<i>e.g.,</i> because the current block fails to satisfy one of the foregoing conditions, or because local HBME fails to find a good enough matching block), the current block can be encoded in some other way (<i>e.g.,</i> full block-matching motion estimation or intra-picture prediction for a block in a changed region; or simple inter-picture prediction with zero motion for a block in a non-changed region).

0227With reference to <figref idref="f0019">FIG. 18a</figref>, the video encoder checks (1840) whether to continue with the next picture and, if so, receives (1810) the next picture in the video sequence. Thus, the video encoder can repeat the receiving (1810), encoding (1820), and outputting (1830) operations for one or more other pictures in the video sequence. In this way, the video encoder receives one or more pictures in a video sequence, encodes the one or more pictures, and outputs the encoded data for the one or more pictures as part of a bitstream.

10. Examples of Updating the Reference Picture for Local HBME.

0228After completing local HBME for the current picture, the video encoder can update the data structure that tracks hash values for the reference picture for local HBME, thereby registering the current picture into the data structure. As described in section V.C.7, the video encoder can consider which regions of the current picture have changed relative to the previous picture, and selectively update the data structure that tracks hash values for blocks in the changed region(s).

0229For example, the video encoder determines changed regions, if any, of the current picture relative to the previous picture in display order. The video encoder removes obsolete entries in the data structure for candidate blocks in any changed regions (for locations in the previous picture, which are in the changed regions). The video encoder adds entries in the data structure for candidate blocks in any changed regions of the current picture, and it retains entries in the data structure for candidate blocks in any non-changed regions. The candidate blocks added to the data structure may overlap, being defined at offsets for a dense configuration of candidate blocks, as described in section V.C.8.

0230With reference to the example data structure (1700) shown in <figref idref="f0018">FIG. 17</figref>, the video encoder can update the hash index tables (1720, 1740). For example, the video encoder removes any entries for candidate blocks from the previous picture in any changed region (between the previous picture and current picture), adds entries for candidate blocks in any changed region in the current picture, and retains other entries for candidate blocks outside the changed region(s). After the updating, the candidate blocks cover various sections of the current picture, but redundant, outdated candidate blocks (which have been superseded by newer blocks in the current picture) have been removed.

11. Example Combined Implementations of Local HBME.

0231A video encoder can use the preceding features of local HBME in combination. <figref idref="f0020">FIGS. 19a</figref> and <figref idref="f0021">19b</figref> show an example technique (1900) for video encoding that includes local HBME for changed regions of a current picture. A video encoder as described with reference to <figref idref="f0002">FIGS. 3</figref>, <figref idref="f0003">4a</figref>, and <figref idref="f0004">4b</figref> or other video encoder can perform the technique (1900).

0232The video encoder receives a picture (current picture) in a video sequence and converts (1910) base sample values of the current picture to derivative sample values. For example, the video encoder performs conversion operations as described in section V.C.1.

0233The video encoder checks (1920) whether the current picture is a key picture. If so, for use of the current picture as a reference picture in later local HBME, the video encoder finds (1930) one or more qualifying blocks (if any) in the current picture. In this example, a qualifying block is a candidate block that contains text and contains a pivot point. <i>See</i> sections V.C.3 and V.C.4. For a key picture, the video encoder assumes the entire picture is new (entire picture is a changed region) and attempts to find qualifying blocks throughout the picture.

0234For each of the qualifying block(s) (candidate blocks) in the current picture, the video encoder calculates (1932) a hash value for the candidate block in the current picture (<i>see</i> sections V.C.5 and V.C.6) and stores (1934) the candidate block in a grid-based data structure used for hashing (<i>see</i> section V.C.8). For example, the video encoder determines a grid area index for a grid area that includes the candidate block (if the grid area index is not previously known), calculates a hash index from the hash value for the candidate block, determines a list of pivot points associated with the hash index for the grid area, and stores the hash value and location of the candidate block in the list. The video encoder checks (1936) whether to continue with the next candidate block found for the current picture and, if so, calculates (1932) the hash value for that candidate block. In this way, the video encoder finds the candidate blocks(s) in the current picture and populates the data structure used for hashing.

0235The video encoder encodes the current picture normally (with intra-picture compression) and outputs the encoded data for the current picture in a bitstream. The current picture is designated as the previous picture (reference picture), for purposes of motion estimation of a subsequent picture. The video encoder checks (1990) whether to continue with the next picture in the video sequence and, if so, receives the next picture (as the current picture).

0236If the current picture is not a key picture (at decision 1920), the video encoder attempts to encode the current picture using inter-picture compression. The video encoder identifies (1940) changed regions, if any, in the current picture (<i>see</i> section V.C.7). The video encoder checks (1942) whether any changed regions were identified. If no changed regions were identified, the video encoder encodes the current picture using simple inter-picture prediction without motion estimation (copying sections of the previous picture) and outputs the encoded data for the current picture in the bitstream. Then, the video encoder checks (1990) whether to continue with the next picture in the video sequence and, if so, receives the next picture (as the current picture).

0237Otherwise, if changed regions are identified (at decision 1942), the video encoder finds (1950) one or more qualifying blocks (if any) in the changed region(s) in the current picture For example, a qualifying block is a block in the current picture that contains textual content and contains a pivot point. <i>See</i> sections V.C.3 and V.C.4. Also, when the local HBME is performed after global HBME (not shown in <figref idref="f0020">FIGS. 19a</figref> and <figref idref="f0021">19b</figref>), the qualifying blocks are selected from within portions of the current picture that were not encoded using the global HBME.

0238For each of the qualifying block(s) (current block) in the changed region(s) in the current picture, the video encoder calculates (1952) a hash value for the current block. <i>See</i> sections V.C.5 and V.C.6. The video encoder searches (1954) for a matching block, if any, in the previous picture based at least in part on the hash value for the current block in the changed region(s) in the current picture. <figref idref="f0022">FIG. 20</figref> shows an example technique (2000) for searching for the matching block in the previous picture during local HBME. The video encoder checks (1956) whether to continue with the next qualifying block found in the changed region(s) in the current picture. If so (that is, there is at least one qualifying block left to evaluate), the video encoder calculates (1952) the hash value for the next qualifying block (new current block) in the changed region(s) in the current picture and searches (1954) for a matching block. In this way, the video encoder finds the qualifying blocks(s) in the current picture and performs local HBME operations for the candidate blocks(s) in the reference picture.

0239The video encoder encodes the qualifying blocks of the current picture using the results of the local HBME operations (if local HBME was successful) or other compression (otherwise). The other compression can include full block-matching motion estimation or intra-picture compression for different blocks of the current picture. The video encoder outputs the encoded data for the current picture in the bitstream. In particular, when matching blocks have been found in the local HBME operations, partitions in the qualifying blocks are assigned MVs and encoded using motion compensation, and residual values are calculated and encoded for the qualifying blocks. The video encoder updates (1960) the grid-based data structure used for hashing, as described in sections V.C.7, V.C.8, and V.C.10. The current picture is designated as the previous picture, for purposes of motion estimation of a subsequent picture. The video encoder checks (1990) whether to continue with the next picture in the video sequence and, if so, receives the next picture (as the current picture).

0240<figref idref="f0022">FIG. 20</figref> shows an example technique (2000) for searching for a matching block in a previous picture based at least in part on a hash value for a current block in a changed region of a current picture during local HBME. As described in section V.C.8, the video encoder calculates (2010) a hash index from the hash value for the current block and retrieves (2020) one or more lists, if any, of candidate blocks (in the previous picture) associated with the hash index in the search area from the data structure used for hashing. For example, the video encoder determines a grid area index for a grid area that includes the current block, determines grid area indices for the search area, calculates a hash index from the hash value for the current block, and determines a list of candidate blocks associated with the hash index for each grid area in the search area.

0241The video encoder checks (2022) whether any list of candidate block(s) in the previous picture was retrieved. If not, the video encoder finishes searching (1954) and checks (1956) whether to continue with the next qualifying block found in the changed region(s) in the current picture, as shown in <figref idref="f0021">FIG. 19b</figref>.

0242On the other hand, if at least one list of candidate block(s) in the previous picture was retrieved, the video encoder compares (2030) the hash value for the current block in the changed region(s) in the current picture against the hash value for the next candidate block in the list(s). If the hash values match (at decision 2032), the video encoder performs (2040) limited block-matching motion estimation for the current block and candidate block, as described in section V.C.9. For example, the video encoder performs limited block-matching motion estimation for the current block, starting from the candidate block, and potentially continuing motion estimation around that candidate block according to a short motion estimation search pattern (<i>e</i>.<i>g</i>., evaluating blocks at offsets of no more than one sample value horizontally and/or one sample value vertically in the previous picture). As part of the limited block-matching motion estimation, the video encoder can evaluate distortion cost and/or bit cost for the respective blocks that are evaluated in the previous picture. The video encoder decides (2042) whether to terminate the search (1954) and, if so, finishes the search (1954). For example, the video encoder can terminate the search (and select one of the evaluated blocks in the previous picture to be the matching block) if one of the evaluated blocks has an overall cost that is sufficiently low (in terms of distortion cost and bit cost).

0243Otherwise (search does not end at decision 2042), the video encoder can check (2034) whether there is another candidate block in the list(s). If so, the video encoder continues by comparing (2030) the hash value for the current block in the changed region(s) in the current picture against the hash value for the next candidate block in the list(s). Otherwise (no more candidate blocks to evaluate in the list(s)), the search

0244(1954) ends. At this point, the video encoder can select one of the evaluated blocks in the previous picture to be the matching block, if one of the evaluated blocks has an overall cost that is sufficiently low (in terms of distortion cost and bit cost), or the video encoder can encode the current block in some other way (<i>e.g.,</i> full block-matching motion estimation, or intra-picture compression).

0245<figref idref="f0020">FIGS. 19a</figref>, <figref idref="f0021">19b</figref>, and <figref idref="f0022">20</figref> show one possible ordering of operations for local HBME. The timing of operations can be varied, depending on implementation. For example, candidate blocks of a reference picture can be processed on a block-by-block basis, with each stage of processing (<i>e</i>.<i>g</i>., evaluating conditions, calculating hash value, registering in grid-based data structure) being performed for each block before processing starts for the next block. Similarly, blocks of a current picture can be processed on a block-by-block basis, with each stage of processing being performed for each block before processing starts for the next block. Alternatively, operations can be performed on a stage-by-stage basis for a picture, with each stage of processing being performed for all blocks of the picture before the next stage of processing starts.

0246In view of the many possible embodiments to which the principles of the disclosed invention may be applied, it should be recognized that the illustrated embodiments are only preferred examples of the invention and should not be taken as limiting the scope of the invention. Rather, the scope of the invention is defined by the following claims. We therefore claim as our invention all that comes within the scope and spirit of these claims. <b>The following is a list of further preferred embodiments of the invention:</b><ul id="ul0002" list-style="none"><li>Embodiment 1. A computer system comprising: <ul id="ul0003" list-style="none"><li>an input buffer configured to receive one or more pictures in a video sequence;</li><li>a video encoder configured to encode the one or more pictures to produce encoded data by performing operations that include: <ul id="ul0004" list-style="none"><li>calculating a hash value for a current block in a current picture of the one or more pictures; and</li><li>searching, subject to a spatial constraint, for a matching block in a reference picture based at least in part on the hash value for the current block, wherein the spatial constraint defines a search area in the reference picture within which hash values for candidate blocks in the reference picture may be compared to the hash value for the current block; and</li></ul></li><li>an output buffer configured to store the encoded data for output as part of a bitstream.</li></ul></li><li>Embodiment 2. The computer system of embodiment 1, wherein the reference picture is a previous picture, in display order, among the one or more pictures.</li><li>Embodiment 3. The computer system of embodiment 1, wherein the operations further include registering the reference picture in a multi-level data structure for hashing, including, for a given block among multiple blocks in the reference picture: <ul id="ul0005" list-style="none"><li>determining whether the given block qualifies as one of the candidate blocks in the reference picture;</li><li>if the given block qualifies as one of the candidate blocks in the reference picture: <ul id="ul0006" list-style="none"><li>calculating a hash value for the given block;</li><li>calculating a hash index of the hash value for the given block; and</li><li>storing, as an entry of a list associated with the hash index of the hash value for the given block, the hash value for the given block and location of the given block; and</li></ul></li><li>determining, at an offset of x sample values horizontally and y sample values vertically from the given block, a next block, among the multiple blocks in the reference picture, to process as the given block.</li></ul></li><li>Embodiment 4. The computer system of embodiment 1, wherein the operations further include: <ul id="ul0007" list-style="none" compact="compact"><li>splitting the reference picture into multiple grid areas; and</li><li>for a given candidate block among the candidate blocks in the reference picture, storing the hash value for the given candidate block and location of the given candidate block in a multi-level data structure for hashing, wherein the multi-level data structure organizes the hash values for the candidate blocks by grid area and, within a grid area, by hash index.</li></ul></li><li>Embodiment 5. The computer system of embodiment 4, wherein a given grid area among the multiple grid areas includes a block collocated with the current block, and wherein the search area is: <ul id="ul0008" list-style="none"><li>the given grid area; or</li><li>the given grid area and multiple grid areas adjacent to the given grid area in the reference picture.</li></ul></li><li>Embodiment 6. The computer system of embodiment 1, wherein the hash value for the current block and the hash values for the candidate blocks in the reference picture are each computed using sample values subjected to a threshold, and wherein the threshold ignores the x least significant bits of the sample values, respectively.</li><li>Embodiment 7. The computer system of embodiment 1, wherein the operations further include one or more of: <ul id="ul0009" list-style="none"><li>determining a changed region of the current picture, wherein the calculating the hash value for the current block and the searching are contingent on the current block being within the changed region;</li><li>determining whether the current block contains textual content, wherein the calculating the hash value for the current block and the searching are contingent on the current block containing textual content; and</li><li>determining whether the current block contains a pivot point, wherein the calculating the hash value for the current block and the searching are contingent on the current block containing a pivot point.</li></ul></li><li>Embodiment 8. The computer system of embodiment 1, wherein the searching comprises: <ul id="ul0010" list-style="none"><li>using the hash value for the current block to identify one or more possible matching blocks among the candidate blocks in the reference picture, each of the one or more possible matching blocks having a hash value that matches the hash value for the current block;</li><li>for each of the one or more possible matching blocks, performing block-based motion estimation for the current block, starting from the possible matching block; and</li><li>selecting the matching block based at least in part on results of the block-based motion estimation.</li></ul></li><li>Embodiment 9. The computer system of embodiment 1, wherein the operations further comprise registering the current picture in a multi-level data structure for hashing, including: <ul id="ul0011" list-style="none"><li>determining changed regions, if any, of the current picture relative to the previous picture in display order;</li><li>adding entries in the multi-level data structure for candidate blocks in any changed regions of the current picture; and</li><li>retaining entries in the multi-level data structure for candidate blocks in any non-changed regions of the current picture.</li></ul></li><li>Embodiment 10. In a computer system, a method comprising: <ul id="ul0012" list-style="none"><li>receiving one or more pictures in a video sequence;</li><li>encoding the one or more pictures to produce encoded data by performing operations that include, for a current picture of the one or more pictures: <ul id="ul0013" list-style="none"><li>determining one or more changed regions of the current picture relative to a reference picture;</li><li>within the one or more changed regions, performing global hash-based motion estimation ("HBME") to identify any blocks that have uniform motion relative to the reference picture;</li><li>within any remaining portions of the one or more changed regions after the global HBME, performing local HBME for any blocks that have distinct patterns; and</li><li>within any remaining portions of the one or more changed regions after the local HBME, performing full block-matching motion estimation; and</li></ul></li><li>outputting the encoded data as part of a bitstream.</li></ul></li><li>Embodiment 11. The method of embodiment 10, wherein the local HBME includes, for a current block within the current picture: <ul id="ul0014" list-style="none"><li>calculating a hash value for the current block; and</li><li>searching, subject to a spatial constraint, for a matching block in the reference picture based at least in part on the hash value for the current block, wherein the spatial constraint defines a search area in the reference picture within which hash values for candidate blocks in the reference picture may be compared to the hash value for the current block.</li></ul></li><li>Embodiment 12. The method of embodiment 11, wherein the local HBME further comprises: <ul id="ul0015" list-style="none"><li>splitting the reference picture into multiple grid areas; and</li><li>for a given candidate block among the candidate blocks in the reference picture, storing the hash value for the given candidate block and location of the given candidate block in a multi-level data structure for hashing, wherein the multi-level data structure organizes the hash values for the candidate blocks by grid area and, within a grid area, by hash index.</li></ul></li><li>Embodiment 13. The method of embodiment 11, wherein the hash value for the current block and the hash values for the candidate blocks in the reference picture are each computed using sample values subjected to a threshold.</li><li>Embodiment 14. The method of embodiment 11, wherein the local HBME further comprises one or more of: <ul id="ul0016" list-style="none"><li>determining a changed region of the current picture, wherein the calculating the hash value for the current block and the searching are contingent on the current block being within the changed region;</li><li>determining whether the current block contains textual content, wherein the calculating the hash value for the current block and the searching are contingent on the current block containing textual content; and</li><li>determining whether the current block contains a pivot point, wherein the calculating the hash value for the current block and the searching are contingent on the current block containing a pivot point.</li></ul></li><li>Embodiment 15. One or more computer-readable media storing computer-executable instructions for causing a computer system, when programmed thereby, to perform operations comprising: <ul id="ul0017" list-style="none"><li>receiving one or more pictures in a video sequence;</li><li>encoding the one or more pictures to produce encoded data by performing operations that include, for a current block of a current picture of the one or more pictures: <ul id="ul0018" list-style="none"><li>determining whether the current block contains textual content and contains a pivot point;</li><li>if the current block contains textual content and contains a pivot point: <ul id="ul0019" list-style="none"><li>calculating a hash value for the current block; and</li><li>searching, subject to a spatial constraint, for a matching block in a reference picture based at least in part on the hash value for the current block, wherein the spatial constraint defines a search area in the reference picture within which hash values for candidate blocks in the reference picture may be compared to the hash value for the current block; and</li></ul></li><li>otherwise, performing full block-matching motion estimation for the current block; and</li></ul></li><li>outputting the encoded data as part of a bitstream.</li></ul></li></ul>