Claims
- 1. A method for transcoding compressed data for further compression including steps of
entropy decoding said compressed data to form quantized transform coefficient values, reducing a range of said quantized transform coefficient values to form quantized transform coefficient values of reduce range, altering at least one value in a quantization table to form an altered quantization table, entropy encoding said quantized transform coefficient values of reduced range to form further compressed data, and transmitting or storing said further compressed data with said altered quantization table.
- 2. The method as recited in claim 1, wherein said step of reducing a range reduces said range by a factor of two.
- 3. The method as recited in claim 1, wherein said altered quantization values are in the range of 1 to 255 for eight bit precision in original data samples.
- 4. The method as recited in claim 2, wherein said altered quantization values are in the range of 1 to 255 for eight bit precision in original data samples.
- 5. The method as recited in claim 1, including the further step of
altering a Huffman table based on the alteration of the quantized coefficients without collecting new statistics for an image.
- 6. The method as recited in claim 5, wherein said frequency of occurrence is estimated from original Huffman table data.
- 7. The method as recited in claim 5, wherein said frequency of occurrence is determined from a histogram of said quantized transform coefficient values.
- 8. The method of claim 1, wherein said entropy encoding step includes arithmetic coding.
- 9. The method of claim 1, wherein said quantized transform coefficient values are in packed format.
- 10. The method as recited in claim 1, wherein said step of altering said at least one value in said quantization table is complementary to said step of reducing a range for said at least one value.
- 11. The method as recited in claim 2, including the further step of
altering a Huffman table based on the alteration of the quantized coefficients without collecting new statistics for an image.
- 12. The method as recited in claim 11, wherein said frequency of occurrence is estimated from original Huffman table data.
- 13. The method as recited in claim 11, wherein said frequency of occurrence is determined from a histogram of said quantized transform coefficient values.
- 14. The method of claim 2, wherein said entropy encoding step includes arithmetic coding.
- 15. The method of claim 2, wherein said transform coefficient values are in packed format.
- 16. The method as recited in claim 1, including the further step of
preventing alteration of quantized transform coefficients at a zig-zag scam position based on the frequency of occurrence of non-zero quantized transform coefficients at said zig-zag scan position, and preventing alteration of the quantization value in said quantization table at said zig-zag scan position.
- 17. The method as recited in claim 2, including the further step of
preventing alteration of quantized transform coefficients at a zig-zag scan position based on the frequency of occurrence of non-zero quantized transform coefficients at said zig-zag scan position, and preventing alteration of the quantization value in said quantization table at said zig-zag scan position.
- 18. Apparatus for transcoding compressed data for further compression including steps of
means for entropy decoding said compressed data to form quantized transform coefficient values, means for reducing a range of said quantized transform coefficient values to form quantized transform coefficient values of reduced range, means for altering at least one value in a quantization table to form an altered quantization table, means for entropy encoding said quantized transform coefficient values of reduced range to form further compressed data, and means for transmitting or storing said further compressed data with said altered quantization table.
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is related to U.S. patent application Ser. No. 09/760,383, entitled “Enhanced Compression of Documents”, filed Jan. 16, 2001, Ser. No. 09/736,444, entitled “JPEG Packed Block structure” filed Dec. 15, 2000, Ser. No. 09/896,110, entitled “JPEG Packed Block Structure for enhanced Image Processing”, filed Jul. 2, 2001, and Ser. No. 10/______ (Docket CHA920010025US1) filed Jun. 28, 2002, entitled “Adaptive Generation of Q-Table2 for Enhanced Image Quality”, all of which are assigned to the assignee of the present invention and hereby fully incorporated by reference.