Methods and devices for context set selection
Methods of encoding and decoding for video data are described for encoding or decoding multi-level significance maps. Distinct context sets may be used for encoding the significant-coefficient flags in different regions of the transform unit. In a fixed case, the regions are defined by coefficient group borders. In one example, the upper-left coefficient group is a first region and the other coefficient groups are a second region. In a dynamic case, the regions are defined by coefficient group borders, but the encoder and decoder dynamically determine in which region each coefficient group belongs. Coefficient groups may be assigned to one region or another based on, for example, whether their respective significant-coefficient-group flags were inferred or not.
1. A method of decoding a bitstream of encoded video by reconstructing significant-coefficient flags for a transform unit, the transform unit being partitioned into a plurality of non-overlapping blocks, the method comprising:
for each block of the plurality of non-overlapping blocks:
determining whether a significant-coefficient group flag of the block is set;
when it is determined that the significant-coefficient group flag of the block is set,
for each significant-coefficient flag in the block:
determining a first context index from a context set;
when a position of the block of the transform unit corresponds to an x-coordinate of one, two or three and a y-coordinate of one, two or three, setting the first context index as a selected context index, and
when the position of the block of the transform unit corresponds to at least one of an x-coordinate of greater than three or a y-coordinate of greater than three,
modifying the first context index by adding an offset to the first context index; and
setting the modified first context index as the selected context index; and
entropy decoding the significant-coefficient flag using the selected context index.
2. The method of claim 1 , wherein the block is a 4×4 block, and the position of the block is determined according to whether the x-coordinate and the y-coordinate of the block are each within 4 positions of the upper-left corner of the transform unit.
3. The method of claim 2 , wherein the x-coordinate and the y-coordinate correspond to a first significant-coefficient flag of the block to be decoded in a reverse scan order.
4. The method of claim 1 , wherein when the position of the block is at the upper-left corner of the transform unit, selecting a specific context index for the significant-coefficient flag in the DC position of the block.
5. The method of claim 1 , wherein the value of the offset is equal to the number of context indices in the context set.
6. A decoder for decoding a bitstream of encoded data by reconstructing significant-coefficient flags for a transform unit, the transform unit being partitioned into a plurality of non-overlapping blocks, the decoder comprising:
a processor;
a memory; and
a decoding application stored in memory and containing instructions that, when executed by the processor, cause the processor to:
for each block of the plurality of non-overlapping blocks:
determining whether a significant-coefficient group flag of the block is set;
when it is determined that the significant-coefficient group flag of the block is set,
for each significant-coefficient flag in the block:
determining a first context index from a context set;
when a position of the block of the transform unit corresponds to an x-coordinate of one, two or three and a y-coordinate of one, two or three, setting the first context index as a selected context index, and
when the position of the block of the transform unit corresponds to an x-coordinate of greater than three or a y-coordinate of greater than three,
modifying the first context index by adding an offset to the first context index; and
setting the modified first context index as the selected context index; and
entropy decoding the significant-coefficient flag using the selected context index.
7. The decoder of claim 6 , wherein the block is a 4×4 block, and the position of the block is determined according to whether the x-coordinate and the y-coordinate of the block are each within 4 positions of the upper-left corner of the transform unit.
8. The decoder of claim 7 , wherein the x-coordinate and the y-coordinate correspond to a first significant-coefficient flag of the block to be decoded in a reverse scan order.
9. The decoder of claim 6 , wherein when the position of the block is at the upper-left corner of the transform unit, selecting a specific context index for the significant-coefficient flag in the DC position of the block.
10. The decoder of claim 6 , wherein the value of the offset is equal to the number of context indices in the context set.
11. A non-transitory processor-readable medium storing processor-executable instructions which, when executed by a processor, cause the processor to decode a bitstream of encoded video by reconstructing significant-coefficient flags for a transform unit, the transform unit being partitioned into a plurality of non-overlapping blocks, wherein the decoding comprises:
for each block of the plurality of non-overlapping blocks:
determining whether a significant-coefficient group flag of the block is set;
when it is determined that the significant-coefficient group flag of the block is set,
for each significant-coefficient flag in the block:
determining a first context index from a context set;
when a position of the block of the transform unit corresponds to an x-coordinate of one, two or three and a y-coordinate of one, two or three, setting the first context index as a selected context index, and
when the position of the block of the transform unit corresponds to an x-coordinate of greater than three or a y-coordinate of greater than three,
modifying the first context index by adding an offset to the first context index; and
setting the modified first context index as the selected context index; and
entropy decoding the significant-coefficient flag using the selected context index.
12. The non-transitory processor-readable medium of claim 11 , wherein the block is a 4×4 block, and the position of the block is determined according to whether the x-coordinate and the y-coordinate of the block are each within 4 positions of the upper-left corner of the transform unit.
13. The non-transitory processor-readable medium of claim 12 , wherein the x-coordinate and the y-coordinate correspond to a first significant-coefficient flag of the block to be decoded in a reverse scan order.
14. The non-transitory processor-readable medium of claim 11 , wherein when the position of the block is at the upper-left corner of the transform unit, selecting a specific context index for the significant-coefficient flag in the DC position of the block.
15. The non-transitory processor-readable medium of claim 11 , wherein the value of the offset is equal to the number of context indices in the context set.