Adaptive scanning technique for wavelet video coding

Abstract: A video coding method using wavelet with an adaptive scanning method has such main steps of: subjecting the input video signal to a horizontal and a vertical wavelet decomposition to obtain the base, horizontal, vertical and diagonal band of wavelet coefficients; recursively subjecting the resulting base band to further wavelet decomposition to obtain lower layers of base, horizontal, vertical and diagonal bands of wavelet coefficients; grouping the co-sited coefficients from the different layers of horizontal, vertical and diagonal bands into a plurality of horizontal, vertical and diagonal macro-bands, respectively; applying an adaptive quantization to quantize the above coefficients in the said horizontal, vertical and diagonal macro-bands, respectively; adaptively scanning and differentially coding the quantized coefficients in the said macro-bands into a plurality of one dimensional arrays, using a technique depending on orientation of the macro-bands and the distribution of the coefficients, and applying an entropy coding method to code each of the said arrays.

Description: This invention relates to wavelet decomposition,especially to wavelet based video coding. Besides 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. Wavelet 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. Figure 1 illustrates an example,wavelet decomposition process, where an input image is decomposed into 13-band of wavelet coefficient, after 4 times of decomposition. The 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. Figure 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. It 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. In the paper "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. Figure 3 illustrates the scanning mechanism for H_Band/V_Band/D_Band Scan. Firstly wavelet coefficients are divided 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. Figure 4 shows an example of three macro bands to be formed from horizontal bands, vertical bands,and diagonal bands, respectively. In the same paper "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. So 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. EP-A-0 642 272 describes an image data compression technique employing a wavelet separation of image data. Coefficients of the obtained subbands are sequentially scanned by taking coefficients from each subband. Coefficients of a subband are taken using a horizontal raster, a vertical raster or a zig-zag pattern, varying between subbands. US-A-5 276 525 describes a subband decomposition of an image. Subband coefficients are quantized, divided into blocks and scanned block by block. Each block is scanned according to the orientation of the subband, using a vertical raster scan for blocks of a horizontal subband and a horizontal raster scan for blocks of a vertical subband. In 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. The 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. Wavelet coding is composed of Wavelet Decomposition, Scanning, Quantizaition, as well as entropy coding. Besides choosing a better wavelet transform, scanning and quantising wavelet coefficients efficient ly is also very important. Edge 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 vertical 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. Further,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. There 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. It is the object of the present invention to provide a video coding method, a video coding apparatus, a decoding method and a decoding apparatus having improved coding/decoding efficiency. This is achieved by the features of claim 1 for a video coding method, the features of claim 11 for a video coding apparatus, the features of claim 14 for a decoding method and the features of claim 16 for a decoding apparatus. Further embodiments are the subject-matter of dependent claims. Figure 1 shows wavelet decomposion with 13 bands after 4 times of decomposition. Figure 2 is an example of the distribution of wavelet coefficients for carphone sequence. Figure 3 is an illustration of H_Band/V_Band/D_Band Scan. Figure 4 shows a real case of horizontal macro band, vertical macro band,and diagonal macro band for Carphone Sequence. Figure 5 is a general diagram of H_Band/V_Band/D_Band with DPCM & Adaprive Scanning. Figure 6 shows the details of DPCM & Adaptive Scanning. Figure 7 is an explanation for Adaptive Scanning technique. Figure 8 is an encoder of Wavelet Coding by using HVDS with DPCM & Adaptive Scanning. The preferred embodiment of the present invention would be best explained by following Figures 5,6,7,and 8. Figure 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 coefficients from B4h, as shown in Figure 3 for horizontal bands; in unit 4 vertical macro bands will be obtained by taking one coefficient 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 coefficient from B1d, 2x2 from B2d,4x4 from B3d, and 8x8 from B4d for diagonal bands. In 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 & Adaptive scan, which will be explained in Figure 6 in detail. In unit 9 an entropy coding is applied to code each of 1-D array formed in the units 6,7,and 8. The detailed procedure is shown in Figure 6 and 7,and in Figure 6 horizontal bands are taken as an example. As 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. Taking 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). Figure 7(a) is forward adaptive scanning method, and Figure 7(b) is continuous adaptive scanning method. In Figure 8 it is shown that H_Band/V_Band/D_Band Scan(also call ed HVDS)with DPCM & Adaptive scanning is used in an encoder. As 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, end motion Compensation in the unit 16.In the unit 14 Reference Frame Memory stores previous frame for motion estimation. Wavelet Decomposition is done in the unit of 17 for original image,as well as for motion compensated image from the unit of 16. In 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. In the unit of 20 Rearrangement by DPCM Coding&Adaptive Scanning is done for the coefficients quantized in the unit of 19. VLC and Huffman Coding is done in the unit of 21,and the coded bitstream is output from the unit of 21. An 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. The 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 & 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&daughter, carphone and claire. In 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. The 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. The 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.

Claims: A video coding method using a subband decomposition technique, comprising the steps of: subjecting (1,2) an input video signal to a subband decomposition to obtain a base band of two-dimensionally arranged coefficients and horizontal, vertical and diagonal subbands of two-dimensionally arranged coefficients, recursively subjecting (1,2) the resulting base band to further subband decomposition to obtain a lower layer of a base band and horizontal, vertical and diagonal subbands of two-dimensionally arranged coefficients, quantizing the coefficients of the subbands, scanning (6-8) the coefficients in the said subbands wherein coefficients of a horizontal subband are scanned in a vertical scanning direction and coefficients of a vertical subband are scanned in a horizontal scanning direction, and applying entropy coding (9) to the scanned coefficients, characterized in that the method further comprises the step of grouping (3-5) two-dimensionally arranged coefficients of layers of horizontal, vertical and diagonal subbands into horizontal, vertical and diagonal macro-bands, respectively, before scanning the coefficients, and the scanning of the two-dimensionally arranged coefficients of the horizontal or vertical subband of a macro-band comprises the steps of: (a) scanning (11) the coefficients in a direction orthogonal to the scanning direction of the subband until a first non-zero coefficient is scanned, (b) resuming (12) the scanning of the coefficients in the scanning direction of the subband, and (c) wrapping around to the coefficients that have not yet been scanned. A video coding method according to claim 1 wherein the grouping of the subband coefficients into macro-bands comprises the following steps of: keeping the lowest band as the base band (B1b), form a horizontal macro-band by taking one coefficient of the lowest horizontal band (B1h), 2 x 2 coefficients of the next lowest horizontal band (B2h), 4 x 4 coefficients of the next lowest horizontal band (B3h) and 8 x 8 coefficients of the next lowest horizontal band (B4h), each at corresponding spatial positions, form a vertical macro-band by taking one coefficient of the lowest vertical band (B1v), 2 x 2 coefficients of the next lowest vertical band (B2v), 4 x 4 coefficients of the next lowest vertical band (B3v) and 8 x 8 coefficients of the next lowest vertical band (B4v), each at corresponding spatial positions, and form a diagonal macro-band by taking one coefficient of the lowest diagonal band (B1d), 2 x 2 coefficients of the next lowest diagonal band (B2d), 4 x 4 coefficients of the next lowest diagonal band (B3d) and 8 x 8 coefficients of the next lowest diagonal band (B4d), each at corresponding spatial positions. A video coding method according to claim 1 or 2 wherein the scanning of the coefficients of each of a horizontal, vertical and diagonal macro-band starts with the coefficients of the lowest subband within the macro-band and after scanning said subband coefficients scans the coefficients of the next higher subband of said macro-band, finally scanning the coefficients of the highest subband of said macro-band. A video coding method according to any of claims 1 to 3 wherein a zig-zag scan is applied to the two-dimensionally arranged coefficients of a subband within a diagonal macro-band. A video coding method according to any of claims 1 to 4 wherein the step of resuming (12) the scanning comprises the step of: scanning two-dimensionally arranged coefficients of a horizontal subband column by column, from the first coefficient of each column to the last coefficient of that column, or scanning two-dimensionally arranged coefficients of a vertical subband row by row, from the first coefficient of each row to the last coefficient of that row. A video coding method according to any of claims 1 to 4 wherein the step of resuming (12) the scanning comprises the step of: scanning two-dimensionally arranged coefficients of a horizontal subband, alternating for successive columns of the two-dimensionally arranged coefficients, from the first coefficient of a column to the last coefficient of that column and from the last coefficient of a column to the first coefficient of that column, or scanning two-dimensionally arranged coefficients of a vertical subband, alternating for successive rows of the two-dimensionally arranged coefficients, from the first coefficient of a row to the last coefficient of that row and from the last coefficient of a row to the first coefficient of that row. A video coding method according to any of claims 1 to 6 further comprising a step of applying (10) a one-dimensional differential pulse modulation coding (DPCM) to vertical columns and horizontal rows of the two-dimensionally arranged coefficients of the horizontal and vertical macro-bands, respectively, thereby applying said differential pulse modulation coding to each column and each row, respectively. A video coding method according to any of claims 1 to 7 wherein the scanning direction of a subband is vertical for a horizontal subband and horizontal for a vertical subband, respectively. A video coding method according to any of claims 1 to 8, wherein said entropy coding (9) applies a run length Huffman like coding to the said scanned coefficients to produce the coded bits, and comprising further steps of including the coded bits into a bitstream, and sending the bitstream to a decoder. A video coding method according to any of claims 1 to 9 wherein the subband decomposition technique is a wavelet decomposition technique. A video coding apparatus using a subband decomposition technique, comprising: a subband decomposition unit (17) for decomposing an input video signal into a base band of two-dimensionally arranged coefficients and horizontal, vertical and diagonal subbands of two-dimensionally arranged coefficients, and for recursively decomposing the resulting base band into a lower layer of a base band and horizontal, vertical and diagonal subbands of two-dimensionally arranged coefficients, a quantizing unit (19) for quantizing the coefficients of the subbands, a scanning unit (20) for scanning the coefficients of said subbands wherein coefficients of a horizontal subband are scanned in a vertical scanning direction and coefficients of a vertical subband are scanned in a horizontal scanning direction, and an entropy encoder (21) for encoding the scanned coefficients, characterized in that the apparatus further comprises a grouping unit (18) for grouping the two-dimensionally arranged coefficients of layers of horizontal, vertical and diagonal subbands into horizontal, vertical and diagonal macro-bands, respectively, and said scanning unit (20) being adapted for scanning the two-dimensionally arranged coefficients of the horizontal or vertical subbands of a macro-band in a direction orthogonal to the scanning direction of the subband until a first non-zero coefficient is scanned, for resuming the scanning of the coefficients in the scanning direction of the subband, and for wrapping around to the coefficients that have not yet been scanned. A video coding apparatus according to claim 11 further comprising a DPCM coding unit (20) for applying a one-dimensional differential pulse modulation coding (DPCM) to vertical columns and horizontal rows of the two-dimensionally arranged coefficients of the horizontal and vertical macro-bands, respectively, thereby applying said differential pulse modulation coding to each column and each row, respectively. A video coding apparatus according to claim 11 or 12 wherein the subband decomposition unit (17) is adapted for performing a wavelet decomposition. A method for decoding a digital video signal containing entropy coded coefficients of a subband-coded image, comprising the steps of: entropy decoding the entropy encoded coefficients to obtain decoded coefficients of a base band and of layers of horizontal, vertical and diagonal subbands, inverse scanning the decoded coefficients, thereby two-dimensionally arranging the coefficients of the base band and the subbands of different layers, wherein coefficients of a horizontal subband are arranged in a vertical arranging direction and coefficients of a vertical subband are arranged in a horizontal arranging direction, inverse quantizing the two-dimensionally arranged coefficients of the subbands, reconstructing from said two-dimensionally arranged coefficients a base band and horizontal, vertical and diagonal subbands, and composing said base band and said horizontal, vertical and diagonal subbands to form a reconstructed image, characterized in that said inverse scanning step comprises for coefficients of a horizontal or a vertical subband the steps of: (a) arranging the coefficients of the subband in a direction orthogonal to the arranging direction of said subband up to the first non-zero coefficient, (b) resuming the arranging of coefficients in the arranging direction of said subband, and (c) wrapping around to complete the two-dimensional arrangement of said coefficients of said subband by arranging the remaining coefficients. A video decoding method according to claim 14 wherein the composition technique is a wavelet synthesis. An apparatus for decoding a digital video signal containing entropy coded coefficients of a subband-coded image, comprising: an entropy decoder for decoding the entropy coded coefficients to obtain decoded coefficients of a base band and of layers of horizontal, vertical and diagonal subbands, an inverse scanning unit for two-dimensionally arranging the decoded coefficients of the base band and the subbands of different layers, wherein coefficients of a horizontal subband are arranged in a vertical arranging direction and coefficients of a vertical subband are arranged in a horizontal arranging direction, an inverse quantizing unit for inverse quantizing the two-dimensionally arranged coefficients of the subbands, a reconstructing unit for reconstructing from said two-dimensionally arranged coefficients a base band and horizontal, vertical and diagonal subbands, and an image composition unit for composing said base band and said subbands to form a reconstructed image, characterized in that said inverse scanning unit is adapted for arranging coefficients of a horizontal or a vertical subband in a direction orthogonal to the arranging direction of the subband up to the first non-zero coefficient, for resuming the arranging of coefficients in the arranging direction of said subband, and for wrapping around to complete the two-dimensional arrangement of said coefficients of said subband by arranging the remaining coefficients. A decoder for decoding digital video signals according to claim 16 wherein the image composition unit is adapted for performing a wavelet synthesis.

Inventors: SHEN SHENG MEI, TAN THIOW KENG, SHEN, SHENG MEI, TAN, THIOW KENG
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1997
Main classification: H04N7
IPC codes: G06T9, H04N7, H04N1, H03M7