Adaptive truncation of transform coefficient data in a transform-based digital media codec

1. A method of decoding digital media data, the method comprising:

reading information of transform coefficients for a region of an image from a compressed bitstream, where a most significant information of the transform coefficients is contained in a core layer for the region in the bitstream and less significant information of the transform coefficients is contained in an optional layer of the bistream;

reading at least one syntax element from the bitstream signaling truncation information of any truncation applied to the least significant information of the transform coefficients;

processing the information of the transform coefficients in accordance with the signaled truncation information; and

reconstructing an image from the processed information of the transform coefficients.

2. The method of claim 1 , wherein the at least one syntax element comprises an image level syntax element signaling whether truncation is applied to less significant information of transform coefficients in the optional layers of any regions of the image.

3. The method of claim 1 , wherein the at least one syntax element comprises a tile level syntax element signaling whether truncation is applied to less significant information of transform coefficients in the optional layers of the tile.

4. The method of claim 1 , wherein the at least one syntax element comprises a tile level syntax element signaling a degree of truncation applied to less significant information of transform coefficients in the optional layers of the tile.

5. The method of claim 1 , wherein the at least one syntax element comprises a syntax element signaled in each of multiple independently decodable bitstream layers associate with separate partitions of the image, which specifies whether truncation is applied to less significant information of transform coefficients in the optional layer for the respective partition.

6. The method of claim 1 , wherein the at least one syntax element comprises a syntax element signaled in each of multiple independently decodable bitstream layers associate with separate partitions of the image, which specifies a degree of truncation applied to less significant information of transform coefficients in the optional layer for the respective partition, whereby the degree of truncation is variable among the separate partitions.

7. The method of claim 1 , wherein the at least one syntax element comprises a syntax element signaled in each of the separately decodable partitions of the compressed bitstream, which specifies whether truncation is applied to the less significant portion of transform coefficients in the optional layer for the respective separately decodable partition.

8. The method of claim 1 , wherein the at least one syntax element comprises a syntax element signaled in each of the separately decodable partitions of the compressed bitstream, which specifies a degree of truncation applied to the less significant portion of transform coefficients in the optional layer for the respective separately decodable partition, whereby the degree of truncation is variable among the separate partitions.

9. An image bitstream decoder, comprising:

a memory for holding a compressed image bitstream encoded using a bitstream syntax defining a core layer containing a most significant portion of transform coefficient data for a region of an image and an optional layer containing a less significant portion of transform coefficient data for the region, the bitstream syntax further defining at least one syntax element signaling truncation information relating to any truncation being applied to the less significant portion of transform coefficient data in the optional layer;

a processor operating to read the transform coefficient data and at least one syntax element from the compressed bitstream, and process the transform coefficient data based upon the truncation information to reconstruct the image; and

an image display for presenting the image for viewing by a user.

10. The image bitstream decoder of claim 9 , wherein the bitstream syntax defines an image layer containing information pertaining to the image as a whole, and separately decodable partitions corresponding to separate regions of the image each contain the core layer and optional layer, and wherein the at least one syntax element comprises an image level syntax element in the image layer to signal whether truncation is applied to less significant information of transform coefficients in the optional layers of the separately decodable partitions for regions of the image.

11. The image bitstream decoder of claim 10 , wherein the at least one syntax element further comprises a syntax element in each separately decodable partition signaling a degree of truncation applied to the less significant portion of transform coefficient data in the optional layers of the respective separately decodable partition.

12. The image bitstream decoder of claim 9 , wherein the at least one syntax element comprises a syntax element signaled in each of multiple independently decodable partitions of the bitstream associated with separate regions of the image, said syntax element specifying whether truncation is applied to the less significant portion of transform coefficient data in the optional layer of the respective independently decodable partition.

13. The image bitstream decoder of claim 9 , wherein the at least one syntax element comprises a syntax element signaled in each of multiple independently decodable partitions of the bitstream associated with separate regions of the image, said syntax element specifying a degree of truncation applied to the less significant portion of transform coefficient data in the optional layer for the respective partition, whereby the degree of truncation is variable between the independently decodable partitions.

14. A method of coding an image as a compressed bitstream, the method comprising:

for a separately decodable partition of the compressed bitstream associated with a region of the image, separately coding a most significant portion of transform coefficients into a core layer of the compressed bitstream and a less significant portion of the transform coefficients into an optional layer of the compressed bitstream;

determining to apply a truncation to the less significant portion of the transform coefficients in the optional layer to meet a desired bitrate constraint;

applying the truncation to the less significant portion of the transform coefficients in the optional layer; and

signaling the applied truncation in the compressed bistream using at least one syntax element.

15. The method of claim 14 wherein said determining action comprises:

counting a number of bits in a least bit position of the less significant portion of the transform coefficients in the optional layer; and

for a certain maximum file size constraint, calculating a degree of truncation of the less significant portion of the transform coefficient in the optional layer necessary to reduce the compressed bitstream in size to less than the maximum file size constraint.

16. The method of claim 14 , wherein the at least one syntax element comprises an image level syntax element signaling whether truncation is applied to the less significant portion of the transform coefficients in the optional layers of the separately decodable partitions of the compressed bitstream.

17. The method of claim 14 , wherein the at least one syntax element comprises a syntax element in each separately decodable partition signaling whether truncation is applied to the less significant portion of transform coefficients in the optional layer of the respective separately decodable partition.

18. The method of claim 14 , wherein the at least one syntax element comprises a syntax element in each separately decodable partition signaling a degree of truncation applied to the less significant portion of transform coefficients in the optional layer of the respective separately decodable partition.