GB2247132A - Digital video data compression - Google Patents
Digital video data compression Download PDFInfo
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- GB2247132A GB2247132A GB9018170A GB9018170A GB2247132A GB 2247132 A GB2247132 A GB 2247132A GB 9018170 A GB9018170 A GB 9018170A GB 9018170 A GB9018170 A GB 9018170A GB 2247132 A GB2247132 A GB 2247132A
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
- H04N19/60—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using transform coding
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
- H04N19/85—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using pre-processing or post-processing specially adapted for video compression
- H04N19/86—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using pre-processing or post-processing specially adapted for video compression involving reduction of coding artifacts, e.g. of blockiness
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Abstract
In order to avoid edge effects in a block transform based video data reduction scheme, the video data is divided into overlapping blocks, with each block corresponding to a portion of the video picture such that each portion overlaps with adjacent blocks. The block form video data is transform coded, such as by a discrete cosine transform, on a block-by-block basis. Coefficient selection is then performed on the transform coded data, such as by ordering the coefficients by magnitude and selecting the first N coefficients, thereby generating compressed data. In order to reconstitute the video data, such as after transmission, the compressed data is regenerated according to the coefficient selection scheme, and is inverse transformed so as to recover the block form video data. Edge reconstruction of the blocks is then performed by weighted addition of corresponding video data samples in overlapping parts of adjacent blocks. Preferably, the weighted addition involves averaging each two corresponding data samples at the overlapping block edges, and averaging each four corresponding data samples at the overlapping block corners. <IMAGE>
Description
DIGITAL VIDEO DATA COMPRESSION
This invention relates to digital video data compression, and in particular to methods of conveying digital video data in a compressed manner, and to apparatus for compressing and restoring the digital video data.
Techniques for data rate reduction have previously been proposed in various implementations of digital data processing. The requirement is to configure digital data, for example for storage or transmission, in a manner which minimises the quantity of data to be stored or transmitted whilst preserving, or at least substantially preserving, the information content of the data. In digital television/video applications, this requirement is manifested by efforts to balance optimum picture quality against the lowest possible rate (of storage or transmission) of digital video data.
It has been proposed, for video data reduction, to subject the video data (for example, according to the CCIR Recommendation 601 4:2:2 standard) to one of a number of orthogonal transform schemes such that the resulting transformed signal is a more efficient representation in terms of data rate or quantity than the basic video data. Such transform-based data reduction schemes can offer significant reductions in data. The transform is chosen such that the information within the picture is transformed into as few significant components as possible.
The significant components are then retained to give the required data rate. However, a problem with such schemes arises in the context of real-time use, for example in television broadcast applications. The problem arises from the fact that it is not at present practical to perform the transform over the entire image to be displayed, since the required processing speeds, and to a certain extent the necessary memory capacity for storage of the video data for manipulation, cannot be realistically achieved. If it is attempted to split the image into a number of blocks, video data within each block being subjected to the transform process such that the blocks are processed in turn, subsequent reassembly of the image by recombination of inverse transformed block data is found to lead to distortion at block edges when the transform components are discarded.
Figures 1A to 1D of the accompanying drawings illustrate this problem in greater detail. Figure 1A shows, by way of example, a signal representing a one-dimensional ramp. When the signal is subjected to the transform process, it is assumed that the data outside the block is a repeat of the data within the block. Thus, for the purposes of the transform process, the data can be represented by the sawtooth waveform shown in Figure 1B. After transforming and component selection, the waveform can typically be represented as shown in Figure 1C. However, when the signal is finally reconstituted by inverse transformation and block recombination, it is found that the ramp appears as shown in Figure 1D, namely with significant errors at each block edge.
According to one aspect of the present invention there is provided a method of conveying digital video data in a compressed manner, the method comprising:
dividing the video data into a plurality of overlapping blocks, each block corresponding to a portion of a picture represented by the video data such that each portion overlaps with adjacent portions;
transform coding the video data in block form in accordance with an orthogonal transform scheme so as to generate transformed data;
selecting coefficients of the transformed data in accordance with a predetermined selection scheme such that the selected coefficients of the transformed data represent a reduction in data compared to the original video data;
transmitting the selected coefficients of the transformed data as compressed data;;
receiving the transmitted compressed data and regenerating the transformed data in block form in accordance with the predetermined selection scheme;
inverse transform decoding the transformed data in block form in accordance with the orthogonal transform scheme to recover the video data in block form; and
reconstituting the video data into a form representing the picture by selective combination of the blocks of video data and edge reconstruction of the blocks by weighted adding of corresponding video data samples in adjacent blocks from each overlapping part of the blocks.
According to another aspect of the present invention there is provided apparatus for compressing digital video data, the apparatus comprising:
means for dividing the video data into a plurality of overlapping blocks, each block corresponding to a portion of a picture represented by the video data such that each portion overlaps with adjacent portions;
means for transform coding the video data in block form in accordance with an orthogonal transform scheme so as to generate transformed data; and
means for selecting coefficients of the transformed data in accordance with a predetermined selection scheme such that the selected coefficients of the transformed data represent a reduction in data compared to the original video data and thus constitute compressed data.
The present invention also provides apparatus for restoring digital video data compressed by the above-defined data compressing apparatus, the restoring apparatus comprising:
means for receiving the compressed data and regenerating the transformed data in block form in accordance with the predetermined selection scheme;
means for inverse transform decoding the transformed data in block form in accordance with the orthogonal transform scheme to recover the video data in block form; and
means for reconstituting the video data into a form representing the picture by selective combination of the blocks of video data and edge reconstruction of the blocks by weighted addition of corresponding video data samples in adjacent blocks from each overlapping part of the blocks.
In a preferred embodiment of the invention, to be described in greater detail below, the transform process is performed on a blockby-block basis, but the blocks within the original image are arranged to overlap. Component or coefficient selection may typically be achieved by retaining a predetermined number of the components arranged in order of magnitude (the predetermined number being chosen to give the desired compression factor) of the transformed data in each block, and then each block is inverse transformed and the block edges regenerated. In the preferred overlap scheme, the video image is formatted with an overlap of one pixel at each block boundary.
Preferably, the weighted addition at the overlap areas involves averaging the two inverse transformed estimates for each edge-of-block pixel, and averaging the four inverse transformed estimates for each corner-of-block pixel. A larger overlap than one pixel can be used, but this leads to a corresponding increase in the data which reduces the effectiveness of the technique. Should a larger overlap nevertheless be required, the weighted addition of estimates at the overlap areas is modified to take this into account.
The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which:
Figures 1A to 1D are signal diagrams showing the manner in which block edge effects can arise in a block divided transform based data reduction method;
Figure 2 shows an overlapping block structure used in one embodiment of the present invention;
Figure 3 is a block circuit diagram of video data compressing apparatus according to an embodiment of the invention;
Figure 4 is a block circuit diagram of video data restoring apparatus according to an embodiment of the invention;
Figure 5 shows a discrete cosine transform matrix for an 8 by 8 block; and
Figure 6 shows an overlapping window function as applied by the apparatus of Figure 4 in order to restore the video data.
As described above, it is at present impractical in real-time applications to compress video data by transform coding an entire video image to be displayed. However, if the image is split into a number of blocks with transform coding being performed on a block-by-block basis, it is found upon reconstituting the image by combining the blocks that pronounced edge effects are visible at the junctions of the blocks.
Referring to Figure 2, there is shown an overlapping block structure as used in an embodiment of the invention, in which, in order to avoid the undesirable edge effects, each block (of 8 x 8 pixels) of the video image is formatted with an overlap, preferably of one pixel as shown. Thus it will be seen that there is an overlap of four corresponding pixels from different blocks in each corner of a block, and an overlap of two corresponding pixels from different blocks elsewhere along the block edges. It is possible to use larger overlaps than one pixel, but since this leads to a corresponding increase in data, the effectiveness of the data reduction technique is decreased.
The manner in which the overlapping block structure overcomes the edge effect problem is explained below.
Figure 3 shows a block circuit diagram of apparatus for compressing video data, according to one embodiment of the invention.
Digital video data is supplied via an input terminal 10 to an overlapping block generating circuit 12. The circuit 12 preferably includes a temporary store (not shown) capable of storing sufficient of the video data so as to generate overlapping video data blocks (for example, as shown in Figure 2) on a block-by-block basis by appropriate addressing of the temporary store. Thus if, for example, the video data is being supplied to the input terminal 10 on a line-by-line basis, the temporary store should be capable of storing at least eight full lines. This allows blocks for the first eight lines to be created, and then the next seven lines are written into the store, the last line of the previous eight being retained so as to provide the horizontal overlap. The vertical overlap is readily created by allocating each pixel column at the block overlaps to adjacent blocks.
It may in practice be convenient to provide two temporary stores, one having the video data written in while the other is reading out block for video data, the sequence being reversed in alternating fashion.
The block form video data is then supplied to a transform coding circuit 14. A number of orthogonal transforms are available for this purpose, the aim being to provide a transformed signal which is a more efficient representation than the original signal, that is as much of the energy or information should be represented by as few components or coefficients as possible. It can be shown that any signal may be represented by a linear combination of mutually orthogonal functions.
The attribute of any image transform is that the transform should compact the image energy into as few transform components as possible.
A high degree of energy compaction can be achieved if there is a correspondence between the basis vectors of the compaction scheme and the typical characteristics of the image. Thus the best transform for video data compression of an image depends on matching the transform to the image. However, an idealised scheme along these lines would not be practical or efficient since a different transform would need to be selected for each image.
Since typical pictures have correlation between pixels, transform coding can be used to produce a set of coefficients with reduced correlation between coefficients. Redundancy is therefore redistributed so that it can more easily be removed.
A number of transform schemes are available, including the slant transform, the Walsh Hadamard transform, the KLT transform, and the discrete cosine transform (DCT). It has been found that the discrete cosine transform is effective for video image transform purposes. It consists of a set of basis vectors which are sampled cosine functions.
The corresponding transformation matrix for an 8 by 8 matrix (as used in the preferred embodiment) is shown in Figure 5.
The transform coding circuit 14 thus supplies transformed data, such as data transformed according to the DCT matrix of Figure 5, to a coefficient selecting circuit 16. Coefficient selection is very important in transform based data reduction schemes since it determines the final picture quality and the compression factor achievable. An optimum scheme would involve testing every combination of coefficient selection and reduction and choosing that scheme which achieves the desired compression with the best picture quality. This could in theory be done by inverse transforming the output of the coefficient selection circuit 16 and finding the mean square error between this and the input video data to the apparatus. The selection scheme providing the minimum mean square error would be selected.However, since such a scheme is impractical, some modifications need to be made to arrive at a realisable system.
Coefficient selection can be either variable or fixed. In variable selection, the coefficients to be selected are varied according to some criterion, whereas in fixed selection, the same coefficients are always retained. A preferred criterion for variable selection is the magnitude of a coefficient, in other words the coefficients are ordered magnitude-wise and the first N are retained to achieve the required data rate. This technique does have the disadvantage in that additional addressing (for identifying the selected coefficients) must be sent with each coefficient retained from the transformed data. For 8 by 8 blocks, this additional addressing amounts to a further 6 bits. The addressing overhead reduces for smaller block sizes, but the performance of transforms with smaller blocks is much poorer. Thus it is preferable to retain the 8 by 8 block configuration.
With fixed selection, the same coefficients are always selected, the data rate reduction being achieved either by selecting a subset of the transform domain coefficients or by reducing the precision with which coefficients are represented. In the former case, normally the
N lower frequency coefficients are retained, but this is not a particularly useful technique since it represents nothing more than low-pass filtering. In the case of reduced precision, the number of bits assigned to the transform coefficients is based on their probability of occurrence. In this case, more bits will generally be assigned to the lower frequency terms. This can be effective for some picture material, but it degrades significantly when the energy for a given block is concentrated away from the low frequency region.As a result, it is preferred to use a variable selection scheme, despite the additional addressing overhead, and the presently-preferred scheme is that described above, selecting the first N of magnitude-wise ordered coefficients.
The selected transformed coefficients are then supplied from the coefficient selecting circuit 16 to an output terminal 18 as compressed data, and this can be transmitted, recorded on a suitable recording medium or the like.
Figure 4 shows apparatus for restoring the compressed data. The compressed data, after transmission or upon reproduction from a recording medium, is supplied via an input terminal 20 to a block regeneration circuit 22 in which the transformed data in block form is regenerated according to the chosen selection scheme, such as the above-described variable selection scheme. In that case, the additional addressing is utilised to regenerate the block form transformed data correctly. The regenerated data is supplied to an inverse transform decoding circuit 24 in which the video data in block form is recovered by applying an inverse transform to that applied in the transform coding circuit 14, for example if appropriate the inverse of the DCT matrix shown in Figure 5.The block form video data, still including overlapping edges, is then supplied to an edge reconstruction circuit 26 in which the overlapping edges are effectively merged to recover the original video data then fed to an output terminal 28 for display or the like.
The edge reconstruction circuit 26 preferably includes a temporary store (not shown) capable of storing sufficient of the data blocks to allow read out of corresponding data samples in adjacent blocks for weighted addition. In other words, after inverse transforming, there are two estimates for edge-of-block pixels, and four for corner-of-block pixels. The required output pixel value for the output video data is calculated by weighted addition of each of these two or four estimates and preferably, the weighted addition involves averaging of the estimates. Thus the four corresponding corner-of-block pixels for each block corner are added and divided by four, whereas each two corresponding edge-of-block pixels along each block edge are added and halved. The resulting window function applied by the edge reconstruction circuit 26 over a block of eight samples is as shown in Figure 6, weightings of 0.25 being applied to the added corner pixels, weightings of 0.5 being applied to the added edge pixels, and weightings of 1.0 being applied to the remaining pixels.
It has been found that edge reconstruction as described above, and in particular as shown in Figure 6, is very effective in reduction of edge effects upon reconstruction of a video image represented by data conveyed by means of block transform-based data reduction schemes.
Claims (25)
1. A method of conveying digital video data in a compressed manner, the method comprising:
dividing the video data into a plurality of overlapping blocks, each block corresponding to a portion of a picture represented by the video data such that each portion overlaps with adjacent portions;
transform coding the video data in block form in accordance with an orthogonal transform scheme so as to generate transformed data;
selecting coefficients of the transformed data in accordance with a predetermined selection scheme such that the selected coefficients of the transformed data represent a reduction in data compared to the original video data;
transmitting the selected coefficients of the transformed data as compressed data;
receiving the transmitted compressed data and regenerating the transformed data in block form in accordance with the predetermined selection scheme;;
inverse transform decoding the transformed data in block form in accordance with the orthogonal transform scheme to recover the video data in block form; and
reconstituting the video data into a form representing the picture by selective combination of the blocks of video data and edge reconstruction of the blocks by weighted adding of corresponding video data samples in adjacent blocks from each overlapping part of the blocks.
2. A method according to claim 1, wherein each block portion of the picture is square.
3. A method according to claim 2, wherein each block comprises eight rows by eight columns of pixels.
4. A method according to claim 1, claim 2 or claim 3, wherein the overlap between adjacent blocks is one pixel wide.
5. A method according to claim 4, wherein the weighted addition of corresponding video data samples at the overlapping block edges comprises halving the sum of each two corresponding data samples.
6. A method according to claim 4 or claim 5, wherein the weighted addition of corresponding video data samples at the overlapping block corners comprises dividing the sum of each four corresponding data samples by four.
7. A method according to any one of the preceding claims, wherein the predetermined selection scheme is a fixed coefficient selection scheme in which the same coefficients are selected each time.
8. A method according to any one of claims 1 to 7, wherein the predetermined selection scheme is a variable coefficient selection scheme in which the coefficients to be selected are determined on the basis of a parameter of the transformed data.
9. A method according to claim 8, wherein N coefficients are to be selected, and wherein the coefficients are ordered by magnitude, the first N coefficients being selected.
10. Apparatus for compressing digital video data, the apparatus comprising:
means for dividing the video data into a plurality of overlapping blocks, each block corresponding to a portion of a picture represented by the video data such that each portion overlaps with adjacent portions;
means for transform coding the video data in block form in accordance with an orthogonal transform scheme so as to generate transformed data; and
means for selecting coefficients of the transformed data in accordance with a predetermined selection scheme such that the selected coefficients of the transformed data represent a reduction in data compared to the original video data and thus constitute compressed data.
11. Apparatus for restoring digital video data compressed by the apparatus according to claim 10, the restoring apparatus comprising:
means for receiving the compressed data and regenerating the transformed data in block form in accordance with the predetermined selection scheme;
means for inverse transform decoding the transformed data in block form in accordance with the orthogonal transform scheme to recover the video data in block form; and
means for reconstituting the video data into a form representing the picture by selective combination of the blocks of video data and edge reconstruction of the blocks by weighted addition of corresponding video data samples in adjacent blocks from each overlapping part of the blocks.
12. Apparatus according to claim 10, wherein the means for dividing the video data into a plurality of overlapping blocks comprises a temporary memory capable of storing sufficient video data so as to generate video data blocks on a block-by-block basis by appropriate addressing of the temporary memory.
13. Apparatus according to claim 11, wherein the means for reconstituting the video data comprises a temporary memory capable of storing sufficient of the blocks of video data so as to allow read out of corresponding video data samples in adjacent blocks for weighted addition thereof.
14. Apparatus according to any one of claims 10 to 13, wherein each block portion of the picture is square.
15. Apparatus according to claim 14, wherein each block comprises eight rows by eight columns of pixels.
16. Apparatus according to any one of claims 10 to 15, wherein the overlap between adjacent blocks is one pixel wide.
17. Apparatus according to claim 16 when dependent on claim 11, wherein the video data reconstituting means is operable to halve the sum of each two corresponding data samples at the overlapping block edges.
18. Apparatus according to claim 17, or claim 16 when dependent on claim 11, wherein the video data reconstituting means is operable to divide the sum of each four corresponding data samples at the overlapping block corners by four.
19. Apparatus according to any one of claims 10 to 18, wherein the orthogonal transform scheme is a discrete cosine transform scheme.
20. Apparatus according to any one of claims 10 to 19, wherein the predetermined selection scheme is a fixed coefficient selection scheme in which the same coefficients are selected each time.
21. Apparatus according to any one of claims 10 to 19, wherein the predetermined selection scheme is a variable coefficient selection scheme in which the coefficients to be selected are determined on the basis of a parameter of the transformed data.
22. Apparatus according to claim 21, wherein N coefficients are to be selected, and wherein the coefficients are ordered by magnitude, the first N coefficients being selected.
23. A method of conveying digital video data in a compressed manner substantially as hereinbefore described.
24. Apparatus for compressing digital video data, substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.
25. Apparatus for restoring compressed digital video data, substantially as hereinbefore described with reference to Figure 4 of the accompanying drawings.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9018170A GB2247132A (en) | 1990-08-17 | 1990-08-17 | Digital video data compression |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB9018170A GB2247132A (en) | 1990-08-17 | 1990-08-17 | Digital video data compression |
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| Publication Number | Publication Date |
|---|---|
| GB9018170D0 GB9018170D0 (en) | 1990-10-03 |
| GB2247132A true GB2247132A (en) | 1992-02-19 |
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| Application Number | Title | Priority Date | Filing Date |
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| GB9018170A Withdrawn GB2247132A (en) | 1990-08-17 | 1990-08-17 | Digital video data compression |
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| GB (1) | GB2247132A (en) |
Cited By (5)
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| GB2330473A (en) * | 1997-09-30 | 1999-04-21 | Ricoh Kk | Compression and decompression system with reversible wavelets and lossy reconstruction |
| EP0942588A2 (en) * | 1998-03-11 | 1999-09-15 | Canon Kabushiki Kaisha | Image processing apparatus and method, and computer readable storage medium |
| WO2007023254A3 (en) * | 2005-08-26 | 2007-09-20 | Electrosonic Ltd | Image data processing |
| EP2309715A3 (en) * | 2009-10-06 | 2012-01-25 | Canon Kabushiki Kaisha | Image processing apparatus and method of controlling the apparatus |
| US9992252B2 (en) | 2015-09-29 | 2018-06-05 | Rgb Systems, Inc. | Method and apparatus for adaptively compressing streaming video |
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| EP0107426A1 (en) * | 1982-10-14 | 1984-05-02 | British Telecommunications | Image transmission |
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Cited By (17)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US7382925B2 (en) | 1994-09-21 | 2008-06-03 | Ricoh Company, Ltd. | Compression and decompression system with reversible wavelets and lossy reconstruction |
| US6141446A (en) * | 1994-09-21 | 2000-10-31 | Ricoh Company, Ltd. | Compression and decompression system with reversible wavelets and lossy reconstruction |
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| EP0942588A3 (en) * | 1998-03-11 | 2000-05-24 | Canon Kabushiki Kaisha | Image processing apparatus and method, and computer readable storage medium |
| US6671418B2 (en) | 1998-03-11 | 2003-12-30 | Canon Kabushiki Kaisha | Image processing apparatus and method which determines a block size in accordance with a filter structure |
| US9924199B2 (en) | 2005-08-26 | 2018-03-20 | Rgb Systems, Inc. | Method and apparatus for compressing image data using compression profiles |
| EP2928189A1 (en) * | 2005-08-26 | 2015-10-07 | RGB Systems, Inc. | Image data processing |
| US9204170B2 (en) | 2005-08-26 | 2015-12-01 | Rgb Systems, Inc. | Method for image data processing utilizing multiple transform engines |
| WO2007023254A3 (en) * | 2005-08-26 | 2007-09-20 | Electrosonic Ltd | Image data processing |
| US9930364B2 (en) | 2005-08-26 | 2018-03-27 | Rgb Systems, Inc. | Method and apparatus for encoding image data using wavelet signatures |
| US10051288B2 (en) | 2005-08-26 | 2018-08-14 | Rgb Systems, Inc. | Method and apparatus for compressing image data using a tree structure |
| US10244263B2 (en) | 2005-08-26 | 2019-03-26 | Rgb Systems, Inc. | Method and apparatus for packaging image data for transmission over a network |
| EP2309715A3 (en) * | 2009-10-06 | 2012-01-25 | Canon Kabushiki Kaisha | Image processing apparatus and method of controlling the apparatus |
| US8665484B2 (en) | 2009-10-06 | 2014-03-04 | Canon Kabushiki Kaisha | Processing tile images including overlap regions |
| US9992252B2 (en) | 2015-09-29 | 2018-06-05 | Rgb Systems, Inc. | Method and apparatus for adaptively compressing streaming video |
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|---|---|
| GB9018170D0 (en) | 1990-10-03 |
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