Apparatus and methods for re-compression having a monotonic relationship between extent of compression and quality of compressed image

1. A computerized method for generating a quantization matrix usable for independent disjoint block-level recompression of an image, the image generated by independent disjoint block level compression of a precursor image using at least one first quantization matrix, the method comprising:

providing a second quantization matrix;

updating, using a processor, the second quantization matrix to provide a monotonic relationship between a compression extent and a compression quality when performing independent disjoint block-level recompression of the image using the second quantization matrix, including, for at least one (row, column) position, performing the following, giving rise to the quantization matrix:

calculating a ratio between values located at the position in the first quantization matrix and in the second quantization matrix;

calculating a rounding of the ratio;

calculating a rounding error, being a difference between the ratio and the rounding; and

if the rounding error fulfills an unacceptability criterion, decreasing, using the processor, the value at the position in the second quantization matrix.

2. A method according to claim 1 wherein said providing comprises generating the second quantization matrix by: scaling a third quantization matrix by a scaling factor, thereby to provide a scaled matrix, and subsequently computing a weighted average of said at least one scaled matrix with the first quantization matrix.

3. A method according to claim 1 wherein said providing comprises generating

the second quantization matrix according to a proposed compression level.

4. A method according to claim 1 wherein the second quantization matrix is a default JPEG matrix.

5. A computerized system for generating a quantization matrix usable for independent disjoint block-level recompression of an image, the image generated by independent disjoint block level compression of a precursor image using at least one first quantization matrix, the system comprising a processor configured to:

receive a second quantization matrix;

update the second quantization matrix to provide a monotonic relationship between a compression extent and a compression quality when performing independent disjoint block-level recompression of the image using the second quantization matrix, including, for at least one (row, column) position, performing the following, giving rise to the quantization matrix:

calculating a ratio between values located at the position in the first quantization matrix and in the second quantization matrix;

calculating a rounding of the ratio;

calculating a rounding error, being a difference between the ratio and the rounding; and

if the rounding error fulfills an unacceptability criterion, decreasing the value at the position in said second quantization matrix.

6. A method according to claim 3 wherein said proposed compression level is selected by a user using at least one of an Modified Quality Factor and a Quality Factor.

7. A method according to claim 1 wherein said unacceptability criterion comprises said rounding error being more negative than a threshold acceptable rounding value.

8. A method according to claim 7 wherein said threshold acceptable rounding value comprises −0.1.

9. A method according to claim 1 wherein said updating is performed for all (row, column) positions in the first quantization matrix and in the second quantization matrix.

10. The method of claim 1 further comprising utilizing the second quantization matrix for recompressing the image while maintaining the monotonic relationship between the compression extent and the compression quality.

11. The method according to claim 10 wherein said recompressing is a JPEG recompression.

12. A method according to claim 2 wherein the third quantization matrix is a default JPEG matrix.

13. At least one non-transitory computer readable medium comprising computer readable instructions that control at least one processor to implement the method of claim 1 .

14. A method according to claim 2 wherein the scaling factor is calculated using a Modified Quality Factor.

15. A system according to claim 5 wherein said providing comprises generating the second quantization matrix by: scaling a third quantization matrix by a scaling factor, thereby to provide a scaled matrix, and subsequently computing a weighted average of said at least one scaled matrix with the a first quantization matrix.

16. A system according to claim 5 wherein said providing comprises generating the second quantization matrix according to a proposed compression level.

17. A system according to claim 5 wherein the second quantization matrix is a default JPEG matrix.

18. A system according to claim 16 wherein said proposed compression level is selected by a user using at least one of an Modified Quality Factor and a Quality Factor.

19. A system according to claim 5 wherein said unacceptability criterion comprises said rounding error being more negative than a threshold acceptable rounding value.

20. A system according to claim 19 wherein said threshold acceptable rounding value comprises −0.1.

21. A system according to claim 5 wherein said update is performed for all (row, column) positions in the first quantization matrix and in the second quantization matrix.

22. A system according to claim 5 further comprising utilizing the second quantization matrix for recompressing the image while maintaining the monotonic relationship between the compression extent and the compression quality.

23. A system according to claim 22 wherein said recompressing is a JPEG recompression.

24. A system according to claim 15 wherein the third quantization matrix is a default JPEG matrix.