Adaptive scanning technique for wavelet video coding

BACKGROUND OF THE INVENTION

1. Industrial Fields of the Invention

0001This invention relates to wavelet decomposition, especially to wavelet based video coding.

2. Related art of the Invention

0002Besides DCT (Digital Cosine Transform) used in video coding, recently wavelet transform has been widely studied in video signal coding, since wavelet decomposition has spatio-frequency localization properties.

0003Wavelet transform is an alternative representation of image data. It decomposes an input raster image hierarchically in the vertical and horizontal direction, and forms many different multiresolution bands.

0004Figure 1 illustrates an example, wavelet decomposition process, where an input image is decomposed into 13-band of wavelet coefficient, after 4 times of decomposition.

0005The three B4 bands are obtained from the first horizontal/vertical wavelet decomposition; the three B3 bands are obtained from the second decomposition; and so on. The lower bands B1, B2, and B3 contain much of the information of the original image, and the higher band B4 contributes detailed information to the original image.

0006Figure 2 shows the real wavelet coefficients for Carphone Sequence, which is decomposed once by wavelet transform. It can be seen that there are some non-zero wavelet coefficients vertically distributed in the horizontal band , Hh, because of vertical edges in the original image, and some non-zero wavelet coefficients horizontally distributed in the vertical band, Hv, because of horizontal edges in the original image.

0007It has been known that if those non-zero wavelet coefficients in Figure 2 could be represented and coded by using less bits, then the efficiency of the wavelet compression would be high.

0008In the paper <img file="EP0797358A2_D0001.tif" />Wavelet Coefficients Scanning Methods for Video Coding", three scanning methods for wavelet coefficients were presented. One of them is called H_Band/V_Band/D_Band Scan, which is the best scan among the three scanning methods.

0009Figure 3 illustrates the scanning mechanism for H_Band/V_Band/D_Band Scan.

0010Firstly wavelet coefficients are diveided into three kinds of bands:horizontal band, vertical band, and Diagonal band. Within the horizontal bands, B1h, B2h, B3h, and B4h, different number of wavelet coefficients are taken from different horizontal bands, one from B1h, 2x2 from B2h, 4x4 from B3h, 8x8 from B4h, and then they are grouped into a macro band with total number of 85 items. Finally the macro band is quantized and Huffman VLC coded.

0011Figure 4 shows an example of three macro bands to be formed from horizontal bands, vertical bands, and diagonal bands, respectively. In the same paper <img file="EP0797358A2_D0002.tif" />Wavelet Coefficients Scanning Methods for Video Coding", one dimension array will be further formed just by using normal raster scan no matter what band it is from. But it can be seen from Figure 4 that in the horizontal band, there are a lot of cases where non-zero wavelet coefficients are vertically distributed, and inversely in the vertical band, there are a lot of cases where non-zero wavelet coefficients are horizontally distributed.

0012So if the situation could be considered, scanning wavelet coefficients vertically in horizontal bands and scanning wavelet coefficients horizontally in vertical bands, instead of raster scan, then the coding efficiency would be increased further.

0013In the same time, since a run-length Huffman VLC like entropy encoder will be applied to the array here, if non-zero wavelet coefficients in the array appear as the front of the array as much as possible, as well as to be small as much as possible, then less bits will be consumed.

0014The invention here is going to propose an adaptive scanning and DPCM process for the H_Band/V_Band/D_Band Scan to improve its coding efficiency further, and the concept is also applicable to the similar cases where wavelet or subband decomposition is used and non-zero coefficients appear vertically in horizontal bands and horizontally in vertical bands.

0015Wavelet coding is composed of Wavelet Decomposition, Scanning, Quantizaition, as well as entropy coding. Besides choosing a bettter wavelet transform, scanning and quantising wavelet coefficients efficient ly is also very important.

0016Edge information in an original image is often corresponding to non-zero wavelet coefficients after wavelet decomposition, and normally these coefficients are vertically distributed in horizontal bands and horizontally distributed in vertival bands. The invention is trying to make use of the phenomena to scan wavelet coefficients adaptively, so as to improve the coding efficiency of wavelet.

0017Further, it is shown that non-zero wavelet coefficients are distributed in different vertical line for different images, taken horizontal macro band as an example shown in Figure 4. In order to use less bits to code them by Huffman VLC like entropy coder, those non-zero coefficients are needed to scan and put in the front of a 1-Dimension array as much as possible.

0018There are a lot of cases where wavelet coefficients in the same vertical line for horizontal band have the same value, at least have the same sign. So one dimension of DPCM coding could be used here for reducing the redundancy between neighbour coefficients.

SUMMARY OF THE INVENTION

0019In this invention wavelet coefficients are scanned vertically for a horizontal band, horizontally scanned for a vertical band, and zig-zag for a diagonal band.

0020Non-zero wavelet coefficients are scanned and put in the front of an array as much as possible by searching the first vertical line with non-zero coefficients in its first value in horizontal bands, and the first horizontal line with non-zero coefficients in its first value in vertical bands.

0021DPCM coding is done vertical within a vertical line for a horizontal band, and horizontally done within a horizontal line for a vertical band.

0022Hereinafter the operation of the present invention is described

0023An input image is decomposed by wavelet transform, quantized by using different quantization step. H_Band/V_Band/D_Band scan is applied here.

0024In the meantime, for horizontal bands each of 1-D array is obtain ed after using one dimensional DPCM coding within a vertival line, searcing the first vertical line with non-zero coefficients in its first value, and scanning all the coefficients vertically, and for vertical bands each of 1-D array is obtained after using one dimensional DPCM within a horizontal line , searching the first horizontal line with non-zero coefficients in its first value, and scanning all the coefficients horizontally.

0025For diagonal bands,a conventional zig-zag scan is applied directly.

0026Then Huffman VLC coding can be used to code each of array obtain ed from above-mentioned method.

BRIEF DESCRIPTION OF THE DRAWINGS

0027Figure 1 shows wavelet decomposion with 13 bands after 4 times of decomposition.

0028Figure 2 is an example of the distribution of wavelet coefficients for carphone sequence.

0029Figure 3 is an illustration of H_Band/V_Band/D_Band Scan.

0030Figure 4 shows a real case of horizontal macro band, vertical macro band, and diagonal macro band for Carphone Sequence.

0031Figure 5 is a general diagram of H_Band/V_Band/D_Band with DPCM &amp; Adaprive Scanning.

0032Figure 6 shows the detai of DPCM &amp; Adaptive Scanning.

0033Figure 7 is an explanation for Adaptive Scanning technique.

0034Figure 8 is an encoder of Wavelet Coding by using HVDS with DPCM &amp; Adaptive Scanning.

PREFERRED EMBODIMENTS

0035The preferred embodiment of the present invention would be best explained by following Figures 5, 6, 7, and 8.

0036Figure 5 is a general diagram for H_Band/V_Band/D_Band scan, where wavelet transform in unit 1 is applied to an input image, in unit 2 wavelet coefficients for whole frame are divided into horizontal band, vertical band and diagonal band. Then in unit 3 horizontal macro bands will be obtained by taking one coefficient from B1h, 2x2 coefficients from B2h, 4x4 coefficients from B3h, and 8x8 codfficients from B4h, as shown in Figure 3 for horizontal bands; in unit 4 vertical macro bands will be obtained by taking one coefficients from B1v, 2x2 from B2v, 4x4 from B3v, and 8x8 from B4v for vertical bands; in unit 5 diagonal macro bands will be obtained by taking one coefficients from B1d, 2x2 from B2d, 4x4 from B3d, and 8x8 from B4d for diagonal bands.

0037In units 6, 7 and 8 many 1-D arrays will be formed from the unit 3, unit 4, and unit 5, respectively, by use of the current DPCM &amp; Adaptive scan, which will be explained in Figure 6 in detail.

0038In unit 9 an entropy coding is applied to code each of 1-D array formed in the units 6, 7, and 8.

0039The detailed procedure is shown in Figure 6 and 7, and in Figure 6 horizontal bands are taken as an example.

0040As shown in Figure 6, in the unit 10 1-D DPCM is applied between neighbour coefficients within the same vertical line for a horizontal band, for example 4x4, or 8x8. Then in the unit 11 the first vertical line with non-zero coefficients in its first value will be searched, shown in Figure 7.

0041Taking 8x8 as an example, shown in Figure 7(a), searching is done along the first horizontal line until the first non-zero coefficient is found, i. e., the coefficient of "3" for Horizontal Macro Band, then in the unit 12 just by starting from that coefficient, scanning will be done vertically for that line, followed by next vertical line, formed the order of a, b, c, d, e, f, g, h, as shown in Figure 7(a).

0042Figure 7(a) is forward adaptive scanning method, and Figure 7(b) is continuous adaptive scanning method.

0043In Figure 8 it is shown that H_Band/V_Band/D_Band Scan (also call ed HVDS) with DPCM &amp; Adaptive scanning is used in an encoder.

0044As shown in Figure 8, an input frame is input into Block Sampling in the unit 13, and subjected to Motion Estimation in the unit 15, and motion Compensation in the unit 16. In the unit 14 Reference Frame Memory stores previous frame for motion estimation.

0045Wavelet Decomposition is done in the unit of 17 for original image, as well as for motion compensated image from the unit of 16.

0046In the unit of 18, H_Band/V_Band/D_Band Scan is applie to wavelet coefficients from the unit of 17, then an Adaprive Quantization in the unit of 19 is used here, which is adjusted by the unit of 22, Rate-Controller.

0047In the unit of 20 Rearrangement by DPCM Coding&amp;Adaptive Scanning is done for the coefficients quantized in the unit of 19.

0048VLC and Huffman Coding is done in the unit of 21, and the coded bitstream is output from the unit of 21.

0049An inverse process:Inverse Quantization, Wavelet Composition, are shown in the unit of 23 and 24, respectively. In the unit of 25 Local Decoded Frame Memory is used to store local decoded images for motion compensation.

0050The invention can improve the coding efficiency of wavelet, especially when used together with H_Band/V_Band/D_Band Scan. Compared with the raster scan employed in H_Band/V_Band/D_Band Scan previously, the DPCM &amp; Adaptive Scan presented in the invention has 1.7% to 9.7% bit saving for different images, which are table tennis, flower garden, susie, foreman, mother&amp;daughter, carphone and claire.

0051In any cases where wavelet or subband is applid to code images, it can be found that non-zero coefficients are vertically distributed in horizontal bands, and horizontally distributed in vertical bands. The concept of the invention could be applied to scan these coefficients to make bit use as less as possible no matter what the following coding method, i. e., entropy coding, is. That is to say, a rearrangement process for those coefficients could be done by use of the concept presented in the invention, before any following coding method, such as Huffman VLC, arithm etic coding, vector coding, is used.

0052The method of the invention becomes more efficient when there are a lot of vertical or horizontal edges in an original image, since it can save a lot of bits in that case, in the same time the coded picture quality would be improved much accordingly.

0053The concept of the invention is also very useful when we study what is the most efficient scanning method for wavelet coefficients, because it tells us that non-zero coefficients are distributed vertically in horizontal bands, and distributed horizontally in vertical bands, so that we could have a good scanning method to be combined with the idea of the invention.