Method and apparatus for limiting data rate and image quality loss in lossy compression of sequences of digital images
First Claim
1. A computer-implemented process for compressing a sequence of digital images, comprising:
- performing compression on at least part of a first image in the sequence of digital images according to a quantization table and a scale factor to provide an amount of compressed data;
if the amount of compressed data is greater than a target data rate, adjusting the scale factor such that a smaller amount of compressed data will be obtained by performing compression according to the adjusted scale factor and the quantization table than if the scale factor is not adjusted;
if the amount of compressed data is less than the target data rate, adjusting the scale factor such that a larger amount of data will be obtained by performing compression according to the adjusted scale factor and the quantization table than if the scale factor is not adjusted;
[and] comparing a magnitude of the scale factor to the magnitude of a limit factor; and
limiting the magnitude of the scale factor according to the comparison.
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Abstract
By limiting the extent to which the degree of quantization is lowered to increase the amount of compressed data, problems of data rate overshoots and image quality degradation in multi layer composites may be avoided. In particular, when a more complex image occurs after a simple image, the quantization used to compress the complex image will not cause as large of a change in the total amount of compressed data. Recovery from such a change also may occur more quickly. Where quantization tables are adjusted using a scaling factor, a limit on the scaling factor may be established such that the target data rate is not achieved for simple images. When rendering multi layer composites, this limit is such that recompression of previously compressed data does not result in additional loss of information. As a result, degradation of image quality in each layer of the composite is avoided. Where quantization tables are adjusted using a scaling factor, a limit on the scaling factor is established such that the same quantization table is used for each layer of the composite.
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Citations
36 Claims
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1. A computer-implemented process for compressing a sequence of digital images, comprising:
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performing compression on at least part of a first image in the sequence of digital images according to a quantization table and a scale factor to provide an amount of compressed data;
if the amount of compressed data is greater than a target data rate, adjusting the scale factor such that a smaller amount of compressed data will be obtained by performing compression according to the adjusted scale factor and the quantization table than if the scale factor is not adjusted;
if the amount of compressed data is less than the target data rate, adjusting the scale factor such that a larger amount of data will be obtained by performing compression according to the adjusted scale factor and the quantization table than if the scale factor is not adjusted;
[and] comparing a magnitude of the scale factor to the magnitude of a limit factor; and
limiting the magnitude of the scale factor according to the comparison. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12)
transforming blocks of the first image into matrices of values in a frequency domain; and
quantizing the matrices of values using the quantization table scaled by the scale factor[; and
wherein the steps of adjusting include changing the scale factor subject to the limit factor].
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3. The computer-implemented process of claim 2, wherein the step of performing compression compresses the first image, and the process further comprises the step of, after adjusting the scale factor and limiting the magnitude of the scale factor, compressing a next image in the sequence of digital images using the adjusted scale factor having the limited magnitude and the quantization table.
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4. The computer-implemented process of claim 3, wherein the step of performing compression compresses the first image only once.
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5. The computer-implemented process of claim 2, wherein the step of performing compression compresses a first block of the first image in the sequence of digital images, and the process further comprises the step of, after adjusting the scale factor and limiting the magnitude of the scale factor, compressing a next block in the first image using the adjusted scale factor having the limited magnitude and the quantization table.
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6. The computer-implemented process of claim 5, wherein the step of performing compression compresses the first block only once.
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7. The computer-implemented process of claim 1, wherein the step of performing compression compresses the first image in the sequence of digital images, and the step of performing compression compresses the first image only once.
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8. The computer-implemented process of claim 7, wherein both the step of adjusting the scale factor and the step of limiting the magnitude of the scale factor are performed only once on the first image.
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9. The computer-implemented process of claim 1, wherein the step of performing compression compresses the first image in the sequence of digital images, and both the step of adjusting the scale factor and the step of limiting the magnitude of the scale factor are performed only once for the first image.
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10. The computer-implemented process of claim 1, wherein the step of performing compression compresses a block of the first image in the sequence of digital images, and the step of performing compression compresses the block only once.
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11. The computer-implemented process of claim 10, wherein both the step of adjusting the scale factor and the step of limiting the magnitude of the scale factor are performed only once on the block.
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12. The computer-implemented process of claim 1, wherein the step of performing compression compresses a block of the image in the sequence of digital images, and both the step of adjusting the scale factor and the step of limiting the magnitude of the scale factor are performed only once for the block.
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13. An apparatus for compressing a sequence of digital images, comprising:
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an encoder having a first input for receiving uncompressed image data defining at least part of a first image of the sequence of digital images, a second input for receiving a signal indicating a quantization table, a third input for receiving a signal indicating a scale factor, both the quantization table and the scale factor for compressing the uncompressed image data, and an output for providing an amount of compressed data according to the quantization table and the scale factor; and
a controller comprising;
a circuit having a first input for receiving a signal indicating the amount of compressed data, a second input for receiving a signal indicating a target amount of compressed data and an output for providing an updated scale factor updated according to the difference between the target amount of data and the amount of compressed data; and
a comparator having a first input for receiving a limit factor, a second input for receiving the updated scale factor, and an output for providing a signal indicating an adjusted scale factor to the encoder, the adjusted scale factor selected according to a comparison of a magnitude of the updated scale factor to a magnitude of the limit factor such that the adjusted scale factor has a magnitude limited by the limit factor. - View Dependent Claims (14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24)
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25. A system for compressing a sequence of digital images, comprising:
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means for performing compression on at least part of a first image in the sequence of digital images according to a quantization table and a scale factor to provide an amount of compressed data;
means for adjusting, if the amount of compressed data is greater than a target data rate, the scale factor such that a smaller amount of compressed data will be obtained by performing compression according to the adjusted scale factor and the quantization table than if the scale factor is not adjusted;
means for adjusting, if the amount of compressed data is less than the target data rate, the scale factor such that a larger amount of data will be obtained by performing compression according to the adjusted scale factor and the quantization table than if the scale factor is not adjusted;
means for comparing a magnitude of the scale factor to a magnitude of a limit factor; and
means for limiting a magnitude of the scale factor according to the comparison. - View Dependent Claims (26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36)
means for transforming blocks of the first image into matrices of values in a frequency domain; and
means for quantizing the matrices of values using the quantization table scaled by the scale factor.
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27. The system of claim 26, wherein the means for performing compression comprises means for compressing the first image, and
wherein the system further comprises means for compressing, after adjusting the scale factor and limiting the magnitude of the scale factor, a next image in the sequence of digital images using the adjusted scale factor having the limited magnitude and the quantization table. -
28. The system of claim 27, wherein the means for performing compression comprises means for compressing the first image only once.
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29. The system of claim 26, wherein the means for performing compression comprises means for compressing a first block of the first image in the sequence of digital images, and the system further comprises means for compressing, after adjusting the scale factor and limiting the magnitude of the scale factor, a next block in the first image using the adjusted scale factor having the limited magnitude and the quantization table.
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30. The system of claim 29, wherein the means for performing compression compresses the first block only once.
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31. The system of claim 25, wherein the means for performing compression comprises means for compressing the first image in the sequence of digital images, and the means for performing compression comprises means for compressing the first image only once.
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32. The system of claim 31, wherein the means for adjusting the scale factor comprises means for adjusting the scale factor only once on the first image, and wherein the means for limiting the scale factor comprises means for limiting the scale factor only once on the first image.
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33. The system of claim 25, wherein:
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the means for performing compression comprises means for compressing the first image in the sequence of digital images;
the means for adjusting the scale factor comprises means for adjusting the scale factor only once for the first image; and
the means for limiting the scale factor comprises means for limiting the scale factor only once for the first image.
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34. The system of claim 25, wherein the means for performing compression comprises means for compressing a block of the first image in the sequence of digital images;
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wherein the means for performing compression comprises means for compressing the block only once.
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35. The system of claim 34, wherein the means for adjusting the scale factor comprises means for adjusting the scale factor only once on the block, and wherein the means for limiting the scale factor comprises means for limiting the scale factor only once on the block.
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36. The system of claim 25, wherein:
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the means for performing compression comprises means for compressing a block of the image in a sequence of digital images;
the means for adjusting the scale factor comprises means for adjusting the scale factor only once for the block; and
the means for limiting the scale factor comprises means for limiting the scale factor only once for the block.
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Specification