Multi-level significance maps for encoding and decoding
Methods of encoding and decoding for video data are described in which multi-level significance maps are used in the encoding and decoding processes. The significant-coefficient flags that form the significance map are grouped into contiguous groups, and a significant-coefficient-group flag signifies for each group whether that group contains no non-zero significant-coefficient flags. If there are no non-zero significant-coefficient flags in the group, then the significant-coefficient-group flag is set to zero. The set of significant-coefficient-group flags is encoded in the bitstream. Any significant-coefficient flags that fall within a group that has a significant-coefficient-group flag that is non-zero are encoded in the bitstream, whereas significant-coefficient flags that fall within a group that has a significant-coefficient-group flag that is zero are not encoded in the bitstream.
1. A method of determining whether to encode a value of a significant-coefficient flag corresponding to a transform coefficient of a block among one or more non-overlapping blocks within a transform unit, the significant-coefficient flag being a member of a group of significant-coefficient flags corresponding to the block, and wherein a significant-coefficient-group flag corresponds to the group of significant-coefficient flags, the value of the significant-coefficient-group flag indicating whether or not there is a non-zero significant-coefficient flag in the group of significant-coefficient flags, the method comprising:
determining the value of each significant-coefficient flag in the group of significant-coefficient flags;
when the group contains the significant-coefficient flag corresponding to the DC coefficient in the transform unit or the group contains the significant-coefficient flag corresponding to the last non-zero coefficient in the transform unit, skipping determining and encoding of a value of the significant-coefficient-group flag corresponding to the group, into a bitstream;
otherwise:
determining the value of the significant-coefficient group flag corresponding to the group based on whether or not there is a non-zero significant-coefficient flag in the group, and
encoding, into the bitstream, the determined value of the significant-coefficient-group flag corresponding to the group;
for each significant-coefficient flag in the group:
when the encoded value of the significant-coefficient-group flag for the group is one:
when all other significant-coefficient flags of the group have been previously determined to have a value of zero,
skipping encoding of the value of the significant-coefficient flag at position (0,0) in the group, into the bitstream,
otherwise:
encoding the value of the significant-coefficient flag into the bitstream; and
when the encoded value of the significant-coefficient-group flag for the group is zero,
skipping encoding of the value of the significant-coefficient flag, into the bitstream; and
otherwise,
encoding the value of the significant-coefficient flag into the bitstream.
2. The method of claim 1 , wherein encoding the determined value of the significant-coefficient-group flag into the bitstream comprises:
determining a context for the significant-coefficient-group flag based on values of significant-coefficient-group flags corresponding to two blocks which are, respectively, adjacent to the right and below the block, wherein the context is determined to be a first context when the values of significant-coefficient-group flags of the two blocks are both zero and the context is determined to be a second context otherwise; and
encoding the determined value of the significant-coefficient-group flag into the bitstream based on the context.
3. The method of claim 1 , wherein encoding the value of the significant-coefficient flag is performed in a reverse scan order.
4. The method of claim 3 , wherein when the group contains the significant-coefficient flag corresponding to the last non-zero coefficient in the transform unit, the encoding the value of the significant-coefficient flag is performed in the reverse scan order starting from a position in the group corresponding to a position following the position of the last non-zero coefficient in the transform unit, through position (0,0) in the group.
5. The method of claim 3 , wherein the reverse scan order is one of a horizontal, vertical, or diagonal order.
6. The method of claim 1 , wherein the block is a contiguous square block or a non-square rectangular block.
7. The method of claim 1 , wherein the transform unit size is one of 8×8, 16×16, 32×32, or 64×64.
8. A device configured to determine whether to encode a value of a significant-coefficient flag corresponding to a transform coefficient of a block among one or more non-overlapping blocks within a transform unit, the significant-coefficient flag being a member of a group of significant-coefficient flags corresponding to the block, and wherein a significant-coefficient-group flag corresponds to the group of significant-coefficient flags, the value of the significant-coefficient-group flag indicating whether or not there is a non-zero significant-coefficient flag in the group of significant-coefficient flags, the method comprising:
a processor; and
a memory having instructions which, when executed by the processor, cause the device to:
determine the value of each significant-coefficient flag in the group of significant-coefficient flags;
when the group contains the significant-coefficient flag corresponding to the DC coefficient in the transform unit or the group contains the significant-coefficient flag corresponding to the last non-zero coefficient in the transform unit,
skip determining and encoding of a value of the significant-coefficient-group flag corresponding to the group, into a bitstream;
otherwise:
determine the value of the significant-coefficient group flag corresponding to the group based on whether or not there is a non-zero significant-coefficient flag in the group, and
encode, into the bitstream, the determined value of the significant-coefficient-group flag corresponding to the group;
for the each significant-coefficient flag in the group:
when the encoded value of the significant-coefficient-group flag for the group is one:
when all other significant-coefficient flags of the group have been previously determined to have a value of zero, skipping encoding of the value of the significant-coefficient flag at position (0,0) in the group, into the bitstream,
otherwise:
encoding the value of the significant-coefficient flag into the bitstream;
when the encoded value of the significant-coefficient-group flag for the group is zero,
skipping encoding of the value of the significant-coefficient flag, into the bitstream; and
otherwise,
encoding the value of the significant-coefficient flag into the bitstream.
9. The device of claim 8 , wherein encoding the determined value of the significant-coefficient-group flag into the bitstream comprises:
determine a context for the significant-coefficient-group flag based on values of significant-coefficient-group flags corresponding to two blocks which are, respectively, adjacent to the right and below the block, wherein the context is determined to be a first context when the values of significant-coefficient-group flags of the two blocks are both zero and the context is determined to be a second context otherwise; and
encode the determined value of the significant-coefficient-group flag into the bitstream based on the context.
10. The device of claim 8 , wherein encoding the value of the significant-coefficient flag is performed in a reverse scan order.
11. The device of claim 10 , wherein when the group contains the significant-coefficient flag corresponding to the last non-zero coefficient in the transform unit, the encoding the value of the significant-coefficient flag is performed in the reverse scan order starting from a position in the group corresponding to a position following the position of the last non-zero coefficient in the transform unit, through position (0,0) in the group.
12. The device of claim 10 , wherein the reverse scan order is one of a horizontal, vertical, or diagonal order.
13. The device of claim 8 , wherein the block is a contiguous square block or a non-square rectangular block.
14. The device of claim 8 , wherein the transform unit size is one of 8×8, 16×16, 32×32, or 64×64.