Coding of transform coefficients for video coding
This disclosure describes devices and methods for coding transform coefficients associated with a block of residual video data in a video coding process. Aspects of this disclosure include the selection of a scan order for both significance map coding and level coding, as well as the selection of contexts for entropy coding consistent with the selected scan order. This disclosure proposes a harmonization of the scan order to code both the significance map of the transform coefficients as well as to code the levels of the transform coefficient. It is proposed that the scan order for the significance map should be in the inverse direction (i.e., from the higher frequencies to the lower frequencies). This disclosure also proposes that transform coefficients be scanned in sub-sets as opposed to fixed sub-blocks. In particular, transform coefficients are scanned in a sub-set consisting of a number of consecutive coefficients according to the scan order.
1. An apparatus for decoding transform coefficients associated with residual video data, the apparatus comprising a memory and one or more processors configured to:
decode, from a bitstream, a sub-set of transform coefficients associated with the residual video data that comprises fewer than all transform coefficients of a block using a plurality of passes, wherein each pass of the plurality of passes comprises a scan of the sub-set of transform coefficients in a scan order, wherein the scan order comprises a scan pattern and a scan direction in which the sub-set of transform coefficients are processed during decoding, and wherein the scan order is the same for each pass of the plurality of passes,
wherein to decode the sub-set of transform coefficients, the one or more processors are configured to:
decode, from the bitstream, significance information for the sub-set of transform coefficients using a first mode of a decoding process in a first pass;
decode, from the bitstream, a first bin of levels of the transform coefficients for the sub-set of transform coefficients using the first mode of the decoding process in a second pass, wherein the second pass is separate from the first pass; and
decode, from the bitstream, a sign of the levels of the transform coefficients for the sub-set of transform coefficients using a second mode of the decoding process in a third pass, wherein the third pass is separate from the first pass and the second pass.
2. The apparatus of claim 1 , wherein the first mode is a regular mode of a Context Adaptive Binary Arithmetic Coding (CABAC) process and the second mode is a bypass mode of the CABAC process.
3. The apparatus of claim 2 , wherein the regular mode of the CABAC process uses an adaptive context model to decode bins and the bypass mode of the CABAC process uses fixed probabilities to decode bins.
4. The apparatus of claim 1 , wherein the one or more processors are further configured to repeat the decoding for each of a plurality of sub-sets of transform coefficients in the block, wherein each sub-set comprises less than all of the entire set of transform coefficients in the block.
5. The apparatus of claim 1 , wherein each of the sub-sets comprises sixteen consecutive coefficients in the scan order.
6. The apparatus of claim 1 , wherein each of the sub-sets comprises a sub-block of transform coefficients in the scan order.
7. The apparatus of claim 1 , wherein the significance information of the first pass and the first bin of the levels of the second pass are grouped together for decoding for the sub-set.
8. The apparatus of claim 1 , wherein the significance information for the sub-set of transform coefficients comprises one or more significant coefficient flags for the sub-set of transform coefficients, the first bin of levels of the transform coefficients for the sub-set of transform coefficients comprises one or more coefficient absolute value level greater than 1 flags for the sub-set of transform coefficients, and the sign of the levels of the transform coefficients for the sub-set of transform coefficients comprises one or more coefficient sign flags for the sub-set of transform coefficients.
9. The apparatus of claim 1 , wherein to decode the sub-set of transform coefficients, the one or more processors are further configured to:
decode, from the bitstream, a second bin of levels of the transform coefficients for the sub-set of transform coefficients using the first mode of the decoding process; and
decode, from the bitstream, remaining bins of the levels of the transform coefficients for the sub-set of transform coefficients using the second mode of the decoding process.
10. The apparatus of claim 1 , wherein to decode the sub-set of transform coefficients, the one or more processors are further configured to:
determine transform coefficients for a block of video data from, at least, the significance information, the first bin of the levels of the transform coefficients, and the sign of the levels of the transform coefficients;
apply an inverse transform to the determined transform coefficients to generate a block of residual video data; and
combine the block of residual video data with a predictive block of video data to form a reconstructed block of video data.
11. An apparatus for encoding transform coefficients associated with residual video data, the apparatus comprising a memory and one or more processors configured to:
encode, in a bitstream, a sub-set of transform coefficients associated with the residual video data that comprises fewer than all transform coefficients of a block using a plurality of passes, wherein each pass of the plurality of passes comprises a scan of the sub-set of transform coefficients in a scan order, wherein the scan order comprises a scan pattern and a scan direction in which the sub-set of transform coefficients are processed during encoding, and wherein the scan order is the same for each pass of the plurality of passes,
wherein to encode the sub-set of transform coefficients, the one or more processors are configured to:
encode, in the bitstream, significance information for the sub-set of transform coefficients using a first mode of an encoding process in a first pass;
encode, in the bitstream, a first bin of levels of the transform coefficients for the sub-set of transform coefficients using the first mode of the encoding process in a second pass, wherein the second pass is separate from the first pass; and
encode, in the bitstream, a sign of the levels of the transform coefficients for the sub-set of transform coefficients using a second mode of the encoding process in a third pass, wherein the third pass is separate from the first pass and the second pass.
12. The apparatus of claim 11 , wherein the first mode is a regular mode of a Context Adaptive Binary Arithmetic Coding (CABAC) process and the second mode is a bypass mode of the CABAC process.
13. The apparatus of claim 12 , wherein the regular mode of the CABAC process uses an adaptive context model to encode bins and the bypass mode of the CABAC process uses fixed probabilities to encode bins.
14. The apparatus of claim 11 , wherein the one or more processors are further configured to repeat the encoding for each of a plurality of sub-sets of transform coefficients in the block, wherein each sub-set comprises less than all of the entire set of transform coefficients in the block.
15. The apparatus of claim 11 , wherein each of the sub-sets comprises sixteen consecutive coefficients in the scan order.
16. The apparatus of claim 11 , wherein each of the sub-sets comprises a sub-block of transform coefficients in the scan order.
17. The apparatus of claim 11 , wherein the significance information of the first pass and the first bin of the levels of the second pass are grouped together for encoding for the sub-set.
18. The apparatus of claim 11 , wherein the significance information for the sub-set of transform coefficients comprises one or more significant coefficient flags for the sub-set of transform coefficients, the first bin of levels of the transform coefficients for the sub-set of transform coefficients comprises one or more coefficient absolute value level greater than 1 flags for the sub-set of transform coefficients, and the sign of the levels of the transform coefficients for the sub-set of transform coefficients comprises one or more coefficient sign flags for the sub-set of transform coefficients.
19. The apparatus of claim 11 , wherein to encode the sub-set of transform coefficients, the one or more processors are further configured to:
encode, in the bitstream, a second bin of levels of the transform coefficients for the sub-set of transform coefficients using the first mode of the encoding process; and
encode, in the bitstream, remaining bins of the levels of the transform coefficients for the sub-set of transform coefficients using the second mode of the encoding process.