Sub-block transform coding of prediction residuals
Techniques and tools for sub-block transform coding are described. For example, a video encoder adaptively switches between 8×8, 8×4, and 4×8 DCTs when encoding 8×8 prediction residual blocks; a corresponding video decoder switches between 8×8, 8×4, and 4×8 inverse DCTs during decoding. The video encoder may determine the transform sizes as well as switching levels (e.g., frame, macroblock, or block) in a closed loop evaluation of the different transform sizes and switching levels. The encoder and decoder may use different scan patterns for different transform sizes when scanning values from two-dimensional blocks into one-dimensional arrays, or vice versa. The encoder and decoder may use sub-block pattern codes to indicate the presence or absence of information for the sub-blocks of particular blocks.
1. A computer system comprising one or more processors and memory, wherein the computer system implements a video encoder configured to perform operations to encode one or more video frames, the operations comprising:
encoding one or more video frames to produce encoded data using a variable-block-size frequency transform with support for switching of transform size at varying levels within the one or more video frames,
wherein the encoding includes:
testing each of plural available transform sizes, including evaluating rate and/or distortion that results from applying the transform size to blocks of prediction residual data for at least part of the one or more video frames;
selecting a transform size to use during the encoding based on results of the evaluating; and
for one of the blocks of prediction residual data:
evaluating values of transform coefficients of prediction residual data for the block; and
determining, based at least in part on the values of the transform coefficients, sub-block pattern information that indicates an information pattern for presence or absence in a bitstream of the prediction residual data for plural sub-blocks of the block; and
outputting the encoded data in the bitstream, wherein the encoded data includes the sub-block pattern information.
2. The computer system of claim 1 , wherein the block is an 8×8 block and the plural sub-blocks are 4×4 sub-blocks.
3. The computer system of claim 1 , wherein the plural available transform sizes include 8×8 and 4×4.
4. The computer system of claim 1 , wherein the varying levels include frame level and block level.
5. The computer system of claim 1 , wherein, for each of the respective sub-blocks of the block, if the sub-block has at least one non-zero transform coefficient, the sub-block pattern information indicates presence in the bitstream of prediction residual data for that sub-block.
6. The computer system of claim 1 , wherein, for each of the respective sub-blocks of the block, if the sub-block has mostly zero-value transform coefficients, the sub-block pattern information indicates absence in the bitstream of prediction residual data for that sub-block.
7. A computer system comprising one or more processors and memory, wherein the computer system implements a video decoder configured to perform operations to decode one or more video frames, the operations comprising:
receiving encoded data in a bitstream, wherein the encoded data includes sub-block pattern information,
the sub-block pattern information indicating an information pattern for presence or absence in the bitstream of prediction residual data for plural sub-blocks of a block of the one or more video frames; and
decoding the encoded data to reconstruct the one or more video frames using a variable-block-size inverse frequency transform with support for switching of transform size at varying levels within the one or more video frames,
wherein the decoding for each of the respective sub-blocks of the block includes:
using the sub-block pattern information for the sub-block to determine whether the sub-block has prediction residual data in the bitstream; and
if the sub-block has prediction residual data in the bitstream, getting, from the encoded data in the bitstream, encoded transform coefficients for the sub-block and reconstructing the transform coefficients for the sub-block.
8. The computer system of claim 7 , wherein the block is an 8×8 block and the plural sub-blocks are 4×4 sub-blocks.
9. The computer system of claim 7 , wherein the varying levels include frame level and block level.
10. The computer system of claim 7 , wherein the block is decoded using a transform size selected from among plural available transform sizes.
11. The computer system of claim 10 , wherein the plural available transform sizes include 8×8 and 4×4.
12. The computer system of claim 7 , wherein, for each of the respective sub-blocks of the block, if the sub-block has at least one non-zero transform coefficient, the sub-block pattern information indicates presence in the bitstream of prediction residual data for that sub-block.
13. In a computer system that implements a video decoder, a method comprising:
receiving encoded data in a bitstream, wherein the encoded data includes sub-block pattern information, the sub-block pattern information indicating an information pattern for presence or absence in the bitstream of prediction residual data for plural sub-blocks of a block of one or more video frames; and
decoding the encoded data to reconstruct the one or more video frames using a variable-block-size inverse frequency transform with support for switching of transform size at varying levels within the one or more video frames,
wherein the decoding for each of the respective sub-blocks of the block includes:
using the sub-block pattern information for the sub-block to determine whether the sub-block has prediction residual data in the bitstream; and
if the sub-block has prediction residual data in the bitstream, getting, from the encoded data in the bitstream, encoded transform coefficients for the sub-block and reconstructing the transform coefficients for the sub-block.
14. The method of claim 13 , wherein the block is an 8×8 block and the plural sub-blocks are 4×4 sub-blocks.
15. The method of claim 13 , wherein the varying levels include frame level and block level.
16. The method of claim 13 , wherein the block is decoded using a transform size selected from among plural available transform sizes.
17. The method of claim 16 , wherein the plural available transform sizes include 8×8 and 4×4.
18. The method of claim 13 , wherein, for each of the respective sub-blocks of the block, if the sub-block has at least one non-zero transform coefficient, the sub-block pattern information indicates presence in the bitstream of prediction residual data for that sub-block.
19. The method of claim 13 , wherein the decoding for each of the respective sub-blocks of the block further includes:
inverse quantizing the transform coefficients for the sub-block;
with an inverse frequency transform, converting the transform coefficients for the sub-block to prediction residual values for the sub-block; and
combining the prediction residual values for the sub-block with predicted values for the sub-block.
20. The computer system of claim 7 , wherein the decoding for each of the respective sub-blocks of the block further includes:
inverse quantizing the transform coefficients for the sub-block;
with an inverse frequency transform, converting the transform coefficients for the sub-block to prediction residual values for the sub-block; and
combining the prediction residual values for the sub-block with predicted values for the sub-block.