Hybrid display system.

0001The invention relates to synthetically generated displays for aircraft flight instrumentation, and more particularly to a hybrid cathode ray tube display using digitally generated rasters and stroke vectors which superimposes stroke or raster symbols over the vector display in accordance with a desired priority.

0002In the use of electronic display systems for aircraft instruments it has become increasingly important to devise methods for presenting the information to the flight crew in a clear, uncluttered manner. A conventional hybrid display system includes a stroke vector generator and a raster generator which supply alternately and sequentially a single CRT with a picture that includes both raster and stroke information. Stroke written CRT displays trace the shape of figures to be presented by deflecting the electron beam in a manner which connects a successive sequence of strokes or vectors, which may be straight or curved. In a raster system the beam is caused to trace a repetitive pattern of parallel scan lines and the information is presented by intensity modulating the electron beam at the appropriate points along each line.

0003As the size of cathode ray tube (CRT) displays increases, more symbology is placed on an individual indicator. Correspondingly, as the quantity and the complexity of the symbology increases, the risk of misinterpretation of data due to clutter and overlapping may increase. A key objective of the electronic CRT display technique is the assignment of priority of symbology to defined areas of the screen. A symbol of importance, when it intersects one of less importance, should appear to be on a plane closer to the viewer. The less important symbol should disappear behind it. It is also desired that this effect be attained even if both symbols are allowed to move. This minimises any conflict of data presentation and is particularly effective, for example, as compared to the clutter and parallax of the flight director command cue presentation that is typical of conventional electromechanical attitude director indicators.

0004Prior art schemes for priority implementation generally employed a combination of stroke vectors for generating numerical data and index lines and raster generators for generating other symbology and background colours. One technique is a "cell" storage technique, which divides the display into a matrix of incremental display cell areas of the display screen. This approach is shown in Applicants' U.S. Patent Specification No. 4,070,662, wherein a symbol memory provides storage for a plurality of patterns and symbols which can be selectively fetched to form a display picture. This technique is adequate for static priority areas, but is very cumbersome when the priority areas are changing. To produce a display pattern in which lower priority areas are masked, the display processor must first determine the pattern of on and off picture elements for each defined cell in the display. It must then generate the proper symbol representing the cell's pattern, or choose from a predetermined library of symbols, and place that pattern at the proper cell location on the display. This imposes a considerable time-consuming load on the display processor, and is particularly_ difficult when moving symbology is necessary. Such software intensive techniques have a primary disadvantage of using large amounts of valuable computer time in a real-time system, where processing time is critical. Thus, although such symbology can be aecomplished, it can be done only to a limited extent in practice in an aircraft instrumentation display and may require a significant amount of processor time to calculate the appropriate cell and symbol definitions.

0005A second approach is the full-field memory or bit-mapped technique, where each resolution element or picture element of the display is defined by a group of memory bits corresponding to the respective individual picture elements on the display screen. A picture is loaded into memory from a computer or other source of digital instructions and the entire memory is read out sequentially in synchronism with digital circuitry generating a raster. An image is produced by specifically setting, for each picture element, the colour and priority desired by writing the appropriate data into the full-field memory. In readout, serial digital memory output words are converted to analogue form and then transmitted to the CRT display for each frame refresh cycle. From a hardware standpoint, this approach is unattractive because of the size of the required support circuitry and processing time. Systems using a full-field memory typically provide loading of the memory with a processor. The number of storage elements in the full-field memory is the product of the vertical and horizontal resolution elements. For a display mask resolution of 256 lines by 512 pixels (picture elements) per line the number of memory bits required is 131,072. If colour information is encoded, additional memory is required to specify a colour. The time required for the processor to calculate the image pattern and to store so many elements in the full-field memory is considerable and may impose unacceptable restrictions on the display update rate and other required processor tasks.

0006The present invention utilises the repetitive nature inherent in a conventional raster symbol generator scan to provide an apparatus for loading a stroke priority full-field memory without requiring access to extensive processor time and permits efficient generation of dynamic stroke priority areas. The circuits that are disclosed herein permit masking of the stroke vector symbology in accordance with the desired priority, and optional super position of stroke vector and raster scan displays.

0007The present invention is defined in the appended claims and provides apparatus for superimposing symbols on an electronic display, responsive to a source of digital instructions. The apparatus comprises means for providing a stroke vector positional signal and means responsive to a priority signal and the digital instructions for generating signals to position stroke vectors. The display is energised by the stroke vector signals and responds preferentially to the prioritry signal, thereby masking a portion of the display within selected regions.

0008In a preferred embodiment, the apparatus includes a raster symbol generator for loading a bit-mapped full-field memory with the priority signal, which is coupled to control a stroke vector generator for energising the display. The raster generator may optionally be used to superimpose raster and stroke vector signals in a hybrid display with the masked stroke vectors. The display is sequentially and alternately energised by the stroke vector positional signals and the raster symbol character signals, thereby providing a display comprised of a raster symbol character display superimposed on a stroke vector display, with the raster display disposed preferentially to mask stroke vector characters of lesser priority.

0009In a further preferred embodiment, the means for preferentially masking on the stroke vector display includes a clock pulse source for providing timing signals. A counter coupled to the clock pulse source provides a corresponding pixel count signal and raster X and Y axis count signals, corresponding to sequential raster lines and sequential picture elements along the raster lines. The count signals, timing signals, and stroke vector positional signals from a stroke vector generator are coupled to a multiplexer-type switch which addresses a stroke priority full-field bit-mapped memory. A logic switch coupled to the clock pulse source provides control signals for reading and writing in the full-field memory. Priority data from a raster symbol generator is thereby entered into the full-field memory and read out in synchronism with the stroke vector positional signals to provide priority masking data to the stroke vector generator. The video output of the stroke vector generator is thereby masked by the prioritised symbology, and applied with the stroke vector positional data to control the position and colour of the beam of a cathode ray tube.

0010A hybrid display system in accordance with the invention will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which:- <ul id="ul0001" list-style="none"><li>Figure 1 is a pictorial representation of a stroke priority mask and raster image superimposed on an unmasked stroke vector image,</li><li>Figure 2 is a pictorial representation of an integrated flight instrument display face using stroke vector and raster scan symbology,</li><li>Figure 3 is a schematic block diagram of the hybrid display system according to the invention,</li><li>Figure 4 shows a timing sequence diagram for a stroke priority display, and</li><li>Figure 5 shows pictorial representations of composite raster images and composite stroke vector images used to provide a priority-masked stroke vector display and a raster display.</li></ul>

0011In general terms, the face of a display apparatus such as a CRT is sequentially and alternately excited by a raster symbol character generator and a stroke vector generator. Horizonal and vertical count signals increment at the raster line rate and the picture element rate, respectively, and when applied to the raster symbol character generator load corresponding priority command data into a stroke priority full-field memory. By means of the repetitive nature of the raster scan lines and the corresponding pixel counts, raster colour symbology may be defined by points where selected pixels intersect with a stroke vector and the resultant outlines filled in with a desired colour during the actual raster display time. This "filling in" procedure may also be used to produce and write stroke priority data into a full-field memory. The X and Y counts produced for the raster display, corresponding to beam deflections across Cartesian coordinates, may be used to address locations in the stroke priority full-field memory, and the serial video bit stream produced thereby can be stored as the priority data. For each raster picture element, the video bit is written into the location in the full-field memory addressed by the X and Y position counts. In this manner, the stroke priority full-field memory can be addressed with data corresponding to the priority areas on the screen without consuming large amounts of processor time. The use of edge-defined, "filled-in" raster type devices, such as disclosed in Applicants' copending European Patent Application No. (SPEY5277), may be used with the present invention, but any conventional raster-type generator may also be used to fill a stroke priority full-field memory as previously described.

0012During the stroke display time, which follows a frame of the raster scan in sequence, the priority data stored in memory may optionally mask stroke symbology of lower priority as it intersects selected regions of greater importance. The stroke vector generator provides X and Y positional data and colour video data during the stroke period of the refresh cycle. By masking the stroke vector signal at the point of interesection with a raster scan line, the stroke vector display may be blanked in selected areas, with the raster scan optionally superimposed thereon, thereby imposing minimal requirements for data storage and process time.

0013Referring to Figure 1, which is a simplified example of masking stroke vector images to provide a stroke-masked display, a pictorial representation is shown on the face of a display apparatus generally denoted by reference number 10, which is comprised of a composite of stroke vector mask lA, stroke vector image 1B and raster image 1C to form image 1D. It includes a mask lA, wherein a zone 12 denotes an area designated for displaying digital characters, which in this example may be representative of airspeed in knots, and wherein a zone 14 represents an indicator area containing a plurality of preferentially displayed triangular reference symbols 16. Image 1B shows a pictorial representation of an unmasked stroke vector image which it is desired to present on the display face 10 with selected regions masked by instructions stored in the mask lA. An unmasked horizon line 18, digits 10, 20, etc., and index lines 10-10, 20-20, and 30-30 are formed by a stroke vector generator in a conventional manner to define image 1B. Sky and earth colour zones 25 and 26, respectively, may also be provided as in image 1C by using a conventional raster symbol generator. Image 1D shows the resulting hybrid display. Alphanumeric symbology is displayed in stroke vector format, while priority information is provided by the full-field memory mask. Desirably, a portion of selected regions may be preferentially displayed or masked to remove clutter and assure presentation of essential monitoring functions, such as an index 27. Thus, a full faced integrated flight instrument display format with prioritised symbology, as'shown in Figure 2, is provided to enhance the display of critical instrument functions.

0014Figure 2, which represents a fully integrated hybrid display, shows a rectangular area 32 which represents the aircraft nose or fuselage and a pair of laterally extending bars 33, 33' representing its wings. The rectangular symbol 32 includes a numeric stroke vector readout representing the flight path angle of the aircraft. Sky shading 34 and ground shading 35 are provided by raster generation and are separated by a stroke vector representing horizon on line 36. The area of the nose symbol 32 is preferentially masked to provide priority over the sky-ground shading 34, 35 and a stroke vector flight director symbol or cue 37. The position of the flight director cue 37 relative to the aircraft nose symbol 32 indicates the direction of corrective action required to satisfy the control commands. When the commands are satisfied, the crossed lines of the cue 37 will be masked by the nose symbol 32. It is thus appreciated that the masking of the flight director symbol 37 by the higher priority nose symbol 32 results in a clear and uncluttered presentation of control commands or adjustments. A fuller disclosure of the applications of such a flight instrument display in a raster scan system is provided by Applicants' U.S. Patent Specification No. 4,247,843.

0015The display face 10 may be, for example, the face of a conventional CRT, but the invention is applicable to other types of displays as well, such as gas plasma displays, liquid crystal displays, or other electrically-actuated displays, for example.

0016Referring now to Figure 3, there is shown in block diagram form a schematic drawing of a stroke priority full-field memory system including a raster scan. A bit-mapped full-field memory 30 is organised to have the same number of resolution elements as the raster scan. In a system having 256 raster lines with 512 picture elements (pixels) along each line, a random access memory will require 131,072 bits. This assumes that there is a two-state control of each bit; that is, that the bit may be turned on or off in response to a command. A memory 30 includes in a conventional manner means for selectively addressing particular bits within the memory, writing video information into such bits, and reading out the stored data at a commanded address and time sequence. A display timing module 40 includes a clock pulse source for providing a plurality of timing signals. Thus, the module 40 outputs an X count signal on a bus 41, representing the sequence of raster lines being generated along the X (horizontal) axis, and a raster Y count, also on bus 41, representing the sequence of pixels corresponding to a raster scan line along the Y (vertical) axis. The timing module 40 also produces a command signal on a line 42 to a stroke vector generator 50 to initiate the generation of the vector display refresh cycle. A pixel clock signal on a line 43 provides a timing signal corresponding to the sequential pixel rate to a raster symbol generator 60 and an AND gate 45. A mode control signal derived from the clock pulse source is provided on a line 46 to the AND gate 45 and to a multiplexer 70 to initiate appropriate storage and readout functions during the respective stroke vector and raster scan portions of the refresh cycle. Further, the timing module 40 also provides horizontal and vertical synchronisation pulses on a line 100 in a conventional manner for energising X and Y sweeps in an interface 90 for the raster scan on a CRT 80.

0017The timing module 40 is provided with a clock oscillator (not shown) for generating regular clock pulses. In the preferred embodiment, this clock operates at 13.1 MHz. However, other clock rates suitable for the required display updating and associated circuitry are also suitable. The frequency of the clock oscillator is determined by the required resolution of the X and Y counts, a higher frequency being required for higher resolution systems. The clock pulses are sent to the pixel clock output and to a controller (not shown), which may be a programmable read only memory and latch to provide mode control and stroke vector initiate timing functions. The raster X count, raster Y Count, and pixel clock are generated in synchronism with the pixel sweep to provide synchronised digital timing signals. In a manner to be shown, during a stroke display refresh interval, the X and Y counts produced by the timing module 40 will be used to address locations in the stroke priority full-field memory, and the serial video bit stream produced on readout by the raster symbol generator in response to instructions from a computer appropriately programmed on readout will be used as the input video masking data to the stroke vector generator. For each raster picture element, a video bit is written into a location in the full field memory corresponding to the X and Y position counts. In this manner, the stroke priority full-field memory can be loaded with data corresponding to priority areas on the screen without consuming substantial amounts of processor time.

0018During the display interval, the raster symbol generator 60 is used to load full-field memory 30 with priority video data. A conventional computer interface (not shown) applies digital instruction signals to an address bus 61 and a data bus 62 in accordance with the masking display presentation to be generated on the display face 10 of CRT 80. Video data output in digital form from the raster generator 60 is provided on a bus 63 to the memory 30 and to the CRT 80 via the interface 90. While for simplicity a common bus 63 is shown herein, in practice, as will be illustrated below, where a multiplicity of raster symbol generators may be used, the raster video provided to the interface 90 on bus 63' may be a composite of the individual raster symbol generators, and the information provided to the memory 30 on the bus 63 will be a subset thereof defining the priority content.

0019Continuing during the raster display refresh interval, the mode control signal on the line 46 when applied to the multiplexer 70 will first select the inputs on the bus 41, providing raster X and Y count signals. The output thereof is coupled on a bus 71 to the address section of the memory block 30. The AND gate 45 is also gated by the mode control signal on the line 46 and the pixel clock signal on the line 43 to permit writing data into memory 30 at each pixel clock interval. Thus, for each address in the memory 30 corresponding to a predetermined raster X line count and raster Y pixel count, video priority data on line 63 is written into the full-field memory 30.

0020The pixel clock is initialised at the beginning of each raster scan and counts in synchronism with the pixels being generated to provide the raster Y count signal. The raster line counter, triggered by the pixel count, counts in synchronism with the raster lines being generated to provide the timing signal for the raster X count. Thus, the raster symbol generator 60 is energised by the raster X and Y counts taken together to generate a video output corresponding to the pixel address currently being provided to the generator by the raster X and Y counts from the timing module 40. As the raster scan proceeds, the raster X and raster Y counts increment accordingly and provide addressing to all the locations in the memory 30. At each location addressed by the raster X and Y counts, the data represented by the raster video signal on the line 63 is written into full-field memory 30. At the completion of the raster display refresh interval, memory 30 will have been updated with a complete pattern corresponding to the image loaded in the raster symbol generator 60.

0021The display timing module 40 may be organised to provide either sequential raster scan or interlaced fields. In the case of an interlaced raster, one half of the memory 30 will be filled during each of the two raster fields. Typically, the field would be organised to command video modulation at a rate of 80 fields per second where a field is comprised of 128 raster lines with a resolution of 512 pixels per line. A rate of 80 fields per second is preferred in order to obtain a flicker-free presentation on the CRT face. Preferably, interlaced raster fields of 128 lines alternate every 12.5 milliseconds with a stroke vector field to form a complete display format on the face of the CRT at a frame rate of 40 Hz. The odd-even fields are generated conventionally.

0022Figure 4 shows a timing diagram of the stroke vector and raster refresh cycles of a hybrid display. A display refresh interval 92 is comprised of a 6.25 ms stroke display interval 93 followed by a 6.25 ms raster display interval 94. During the stroke display interval 93, the raster symbol generator 60 is loaded from the address bus 61 and data bus 62. All stroke masking data is written into the full-field memory during the raster display interval 92.

0023Referring again to Figure 3, with continued reference to Figure 4, during the display refresh interval 92, a stroke vector generator 50 is also loaded with instructions on the address bus 61 and data bus 62 from an external computer, not shown. While the buses are shown here for simplicity in common with the raster symbol generator 60, they may if desired be comprised of separate and distinct buses. The computer interface provides for generation of a stroke vector display presentation on the face 10 of the CRT 80 in accordance with the instructions provided therefrom. These instructions are stored sequentially and completely define the stroke vector presentation. A vector generator (not shown) within the stroke vector generator 50 provides the necessary X and Y deflection position signals 51 to the CRT interface 90, as well as stroke video information which may provide colour and intensity modulation on a line 52. The stroke vector generator 50 may be comprised of a multiplexer for loading a stroke instruction memory, a vector generator for providing the X and Y deflection and stroke video signals, and the necessary control logic in a conventional manner. If desired, the stroke vector generator 50 may be integrated with a raster symbol generator of the type shown in the block 60 to permit filling in colour raster information with minimal demands on processor and memory, as set forth in Applicants' copending European Patent Application No. (SPEY5277) previously referred to.

0024During a successive display refresh interval 95, the stroke signal information is updated during display interval 96. The stroke priority full-field memory 30 is used to provide a mask for stroke symbology of lesser priority. During the stroke display portion 96 of the refresh interval, the raster symbol generator 60 is updated with current information on the address bus 61 and the data bus 62. During the subsequent raster display interval 97 the updated raster will be displayed while updated masking data is written into the stroke priority full-field memory 30 using the raster scan addressing scheme outlined above. During a stroke display interval, the mode control signal on the line 46 is switched to a state such that the multiplexer 70 accepts the input from the stroke vector generator 50 on the bus 51, which provides digital representations of the commanded CRT beam positions on the display screen. The stroke X and Y position values on the bus 51 are transferred to the bus 71 where they are allowed to address corresponding bits of memory in the full-field memory 30. At the same time, the change of state of the mode control signal on the line 46 disables the AND gate 45. This inhibits the transfer of the pixel clock signal on the line 43, thereby disabling the write signal on the line 47 and transferring the memory 30 to the read state. As the CRT beam is deflected across the display screen 10, the address X, Y to the memory 30 changes accordingly. In the read state, for each address a bit will be fetched from the memory 30 which identifies by its state (i.e., 0 or 1) the existence or non-existence of a priority bit. This bit is provided at the mask data output line 31 and drives the stroke vector generator 50 with a video enable signal. The mask data on the line 31 may be used to blank the stroke video output on the line 52 in regions where a stroke has been defined as a low priority area. In this case, the stroke area will be overwritten by any superimposed raster symbology or higher priority stroke images. When it is desired to overwrite any underlying symbols, in accordance with instructions programmed into the stroke vector generator 50, the stroke signal on the line 52 will be permitted to address the CRT interface 90. Stroke video on the line 52 and raster video on the line 63' may thereupon be combined by multiplexing in the CRT interface 90 in a conventional manner to provide a superposed video command signal 91 to the CRT 80.

0025Following completion of the stroke vector display and loading of the raster symbol generator 60 with update information, the raster is again displayed. Thus, stroke and raster information are alternately and sequentially updated to provide a hybrid display on the face 10 of the CRT 80.

0026The CRT interface 90 is conventional and provides for raster scanning, synchronised by a control signal 100 from the timing module 40, multiplexing or other combination of the raster video and stroke video signals on the lines 63 and 52, respectively, and conversion of the video signals and stroke X and Y positional signals from digital to analogue form for driving the CRT 80 in the conventional manner. The apparatus includes a conventional CRT 80 having a display face 10 energised by an electron beam whose position is controlled by X deflection and Y deflection signals applied to corresponding electrodes. A video signal applied to suitable control electrodes determines the colour and intensity of the displayed output. The interface 90 provides X and Y sweeps for the raster of the cathode ray tube, generated by conventional sweep generators. The sweep generators may be comprised of the usual sawtooth waveform X and Y sweep generators for providing a conventional linear raster. Such sweep circuits are well know in television and display systems employing a raster scan.

0027The digital memories used in the preferred embodiment can be commercially available RAM integrated circuit chips such as used in small or microdigital data processors for storage. The various control functions including storing, fetching, and applying digital values as described above can be implemented conveniently by processor or other control logic included in or associated with the CRT display. Such control facilities are well known in digital displays for effecting various operations in synchronism with the display raster, e.g. character generation, cursor location, and stroke vector generation.

0028The digital-to-analogue convertors and multiplexers may be of any suitable kind which combine binary voltages or currents to produce resultant outputs according to the inputs shown. Sweep amplifiers may be conventional analogue amplifiers, such as may be formed by hybrid and integrated circuit techniques.

0029In operation, the apparatus of Figure 3 may be applied for providing moving displays with priority status that are utilised, for example, in aircraft. On initiation of the stroke display interval by the timing module 40, the stroke vector generator 50 is commanded to execute a sequence of stroke instructions which have been stored in the stroke instruction memory integral therein by means of the computer address bus 61 and the computer data bus 62. The instructions result in the production of digital outputs on the bus 51 representing the digital X position and Y position and digital values of the video output on the line 52 for each commanded position of the electron beam of the cathode ray tube 80. The digital X position and digital Y position values on the bus 51 are converted to corresponding analogue X and Y deflection voltages and the digital stroke video signal 52 is converted to an analogue video driving voltage by the CRT interface 90 to drive the CRT 80.

0030During the following raster display interval, the timing module 40 generates a raster X count sequence to identify sequential raster scan line numbers and a raster Y count sequence to identify the sequential pixel numbers corresponding to the selected raster line on the bus 41. These two counts are then entered into the raster symbol generator 60. The raster X and Y counts on the bus 41 are also directed to the multiplexer 70 to provide the respective addresses on the bus 71 to the stroke priority full-field memory 30. during the stroke display interval, the computer information on the address bus 61 and the data bus 62 is further provided to load the raster symbol generator 60, which outputs video priority data for entry into the random access memory 30 during the subsequent raster display interval.

0031At each increment of change in position of X or Y or change in colour or intensity of video during the stroke refresh, the vector generator output on the bus 51 and the line 52 is updated. Simultaneously, the computer input data is updated and loaded into the raster symbol generator 60. Thus, during the stroke display interval, at the same time that the stroke information is displayed the raster symbol generator is loaded with the complete picture priority information for presentation of the raster display. On completion of the stroke vector portion of the refresh cycle, the timing module 40 initiates the raster scan portion of the display. During the raster display interval, the data previously loaded into the generator 60 is written into the stroke priority memory 30 utilising the raster scan addressing scheme previously described. During the raster display interval, the AND gate 45 permits the entry of data from the generator 60 into the memory 30 at an address corresponding to the raster X and Y counts from the timing module 40. This loads the full-field memory 30 with the priority data in which each bit in memory corresponds to a resolution element on the face 10 of the CRT 80. The stroke priority full-field memory 30 now contains a bit-map of the defined priority areas for the display. On completion of the raster display interval, a new stroke display refresh interval is initiated. The memory 30 is now addressed by the stroke vector generator 50 through the multiplexer 70 to read out the previously loaded video priority data corresponding to each X and _Y position. Thus, the display of the stroke vectors will be masked by the data output from the memory 30. During this time period, the raster symbol generator 60 will be loaded with updated information which is then written into the memory 30 upon the subsequent raster display interval. The cycle thereupon repeats, updating on each subsequent display refresh interval. During the stroke display interval, stroke symbology which has less display priority is thereby masked by the contents of the stroke priority full-field memory 30. This is done by reading a priority control bit out of the stroke full-field memory 30 for each change in the stroke X and Y beam position command. For each position, the control bit either enables, if not in a priority area, or disables, if in a priority area, the stroke video of the masked symbology. The masked symbology is presented along with other stroke symbology to produce a resulting stroke display on the CRT and may be superimposed with a raster display.

0032It will be appreciated by one skilled in the art that by duplicating the raster symbol generator functions described above and processing the raster video outputs in parallel or sequentially to provide additional output channels, additional colour combinations or more complex symbology may be realised. The above process may be illustrated for the more general case of a plurality of raster generators by referring to Figure 5. A raster 100 represents a raster image of the cell-mapped image type, as discussed in U.S. Patent Specification No 4,070,662 referred to above. The face of the tube is divided into a plurality of cells 101 where a library of stored data may be called upon to present images in digital raster form such as a block 102, circular area 103, and index 104. A second raster image is developed in the raster 110 which may employ the edge-defined colour display technique of the previously mentioned copending European Patent Application No. (SPEY5277) although conventional raster symbol generators employing software or hardware programming are also suitable. Vectors 111 and 112 define display boundaries and 116 and 117 the index elements. A raster 118 provides a further edge-defined image which may represent a moving horizon 119 in an attitude display.

0033Figures 3 and 5 illustrate the relationship of various display generator devices and buffers in the implementation to the resulting stroke and raster components of the image displayed-on the face of the CRT.

0034The order from left to right in the figure does not represent strict time sequence of events, but basic flow of data. Dashed line 105 denotes a raster symbol generator which is made up of various types of internal devices. The types of raster generator devices shown are the cell-mapped and edge-defined image generators. Although all the raster generators are conventionally designed for displaying raster symbology on the CRT, a particular device may be earmarked for usage only as a vehicle for generating data used for dynamic stroke priority, or it may serve a double purpose of producing displayed raster symbology as well as data for dynamic stroke priority. In the figure, the cell-mapped raster generator 100 is utilised for both displayed raster symbology (as shown in image 120) and for supplying data to the stroke priority full-field memory 121. Raster generator 118 is used exclusively for the production of displayed raster symbology. The remaining device 110 is used in part for displayed raster symbology, and is used fully for the generation of data 121 for the stroke priority full-field memory 30.

0035In operation, during the raster display interval all the raster symbol generator devices 100, 110, and 118 are read out simultaneously. The read-out is done in synchronism with the X and Y (line number and pixel number) counts associated with the raster scan. The outputs of the appropriate raster devices are combined in a parallel manner by a conventional logical operator indicated by the junctions 122-124. The logical operator 123 results in video (colour) data which is used to produce the displayed raster symbology shown in an image 125. The logical operator 124 results in the data which is written into RAM locations in the stroke priority full-field memory 30. At the conclusion of the raster display interval, the CRT face has been updated with the image 125 and the stroke priority full-field memory 121- has been filled with one bit words representing a stroke priority image.

0036During the following stroke display interval, the stroke vector generator produces the incremental beam position X and Y commands and vectors which comprise the stroke display. Certain stroke vectors are programmed to respond to the contents of the stroke priority full-field memory 30. As they are displayed on the CRT face 10, these incremental vector beam movements are compared to the data at the corresponding locations (or positions) in the stroke priority full-field memory 30 and are either intensified or blanked on the CRT face, depending on the data word (0 or 1) stored in the stroke priority full-field memory 30. The image 120 shows the actual displayed stroke symbology as it appears on the face of the CRT 10. It is a combination of the unmasked stroke image 130 masked by the contents of the stroke priority full-field memory as denoted by the block 121.

0037Since the displayed raster symbology and displayed stroke symbology appear on the face of the CRT sequentially and alternately at a high rate of refresh, the images are merged by the human eye to produce a single image which is an overlay of the raster symbology on the stroke symbology.