Multi-level significance map scanning

1. A method of reconstructing significant-coefficient flags for a transform unit from a bitstream of encoded data, wherein the transform unit is partitioned into non-overlapping blocks, each block containing a respective group of significant-coefficient flags, and the bitstream including encoded significant-coefficient-group flags, wherein each significant-coefficient-group flag corresponds to a respective block and its group of significant-coefficient flags, the method comprising:

for each of the respective groups of significant-coefficient flags, in a group scan order within the transform unit, wherein the group scan order is horizontal, vertical or diagonal,

reconstructing that group's corresponding significant-coefficient-group flag, and

reconstructing the significant-coefficient flags in that group in a scan order within the group, wherein the scan order is horizontal, vertical or diagonal, by, for each significant-coefficient flag in that group,

if that significant-coefficient flag is at position (0,0) in its group, the corresponding significant-coefficient-group flag is decoded to be non-zero, and all of the previous significant-coefficient flags in that group are zero, then inferring, without decoding that significant-coefficient flag from the bitstream, that significant-coefficient flag to be 1, and

otherwise

decoding that significant-coefficient flag from the bitstream if that significant-coefficient flag is in a group that has corresponding significant-coefficient-group flag that is non-zero, and

setting that significant-coefficient flag to zero, if that significant-coefficient flag is in a group that has corresponding significant-coefficient-group flag that is zero.

2. The method claimed in claim 1 , wherein the group scan order is a prescribed order in which the respective groups of significant-coefficient flags are to be reconstructed.

3. The method claimed in claim 2 , wherein the significant-coefficient flags of a next one of the respective groups are reconstructed only after all significant-coefficient flags of all groups earlier in the group scan order are reconstructed.

4. The method claimed in claim 1 , wherein the significant-coefficient flags of a next one of the respective groups are reconstructed only after all significant-coefficient flags of all groups earlier in the group scan order are reconstructed.

5. The method claimed in claim 1 , wherein reconstructing the significant-coefficient-group flags includes decoding at least some of the significant-coefficient-group flags from the encoded data of the bitstream.

6. The method claimed in claim 1 , wherein reconstructing that group's corresponding significant-coefficient-group flag comprises inferring, without decoding that significant-coefficient-group flag from the bitstream, that group's significant-coefficient-group flag to be 1 if that group contains the significant-coefficient flag for a last-significant coefficient.

7. The method claimed in claim 1 , wherein reconstructing that group's corresponding significant-coefficient-group flag comprises inferring, without decoding that significant-coefficient-group flag from the bitstream, that group's significant-coefficient-group flag to be 1 if that group contains the significant-coefficient flag for a coefficient at position [0,0] within the transform unit.

8. The method claimed in claim 1 , wherein the groups are contiguous square blocks.

9. The method claimed in claim 1 , wherein each group is a non-square rectangular block.

10. The method claimed in claim 1 , wherein reconstructing one of the significant-coefficient-group flags includes determining a context for that significant-coefficient-group flag based on the significant-coefficient-group flags reconstructed for two neighbor groups.

11. The method claimed in claim 10 , wherein the two neighbor groups include a right neighbor and a below neighbor, and wherein the context is 0 if both of the significant-coefficient-flags for the two neighbor groups are 0, and wherein otherwise the context is 1.

12. The method of claim 1 , wherein the group scan order and the scan order within the group are the same.

13. A decoder for decoding a bitstream of encoded data to reconstruct significant-coefficient flags for a transform unit, wherein the transform unit is partitioned into non-overlapping blocks, each block containing a respective group of significant-coefficient flags, and the bitstream including encoded significant-coefficient-group flags, wherein each significant-coefficient-group flag corresponds to a respective block and its group of significant-coefficient flags, the decoder comprising:

a processor;

a memory; and

a decoding application stored in memory and containing instructions for configuring the processor to:

for each of the respective groups of significant-coefficient flags, in a group scan order within the transform unit, wherein the group scan order is horizontal, diagonal or vertical,

reconstruct that group's corresponding significant-coefficient-group flag, and

reconstruct the significant-coefficient flags in that group in a scan order within the group, wherein the scan order is horizontal, diagonal or vertical, by, for each significant-coefficient flag in that group,

if that significant-coefficient flag is at position (0,0) in its group, a corresponding significant-coefficient-group flag is decoded to be non-zero, and all of the previous significant-coefficient flags in that group are zero, then inferring, without decoding that significant-coefficient flag from the bitstream, that significant-coefficient flag to be 1, and

otherwise

 decoding that significant-coefficient flag from the bitstream if that significant-coefficient flag is in a group that has corresponding significant-coefficient-group flag that is non-zero, and

 setting that significant-coefficient flag to zero, if that significant-coefficient flag is in a group that has corresponding significant-coefficient-group flag that is zero.

14. The decoder claimed in claim 13 , wherein the group scan order is a prescribed order in which the respective groups of significant-coefficient flags are to be reconstructed.

15. The decoder claimed in claim 14 , wherein the significant-coefficient flags of a next one of the respective groups are reconstructed only after all significant-coefficient flags of all groups earlier in the group scan order are reconstructed.

16. The decoder claimed in claim 13 , wherein the significant-coefficient flags of a next one of the respective groups are reconstructed only after all significant-coefficient flags of all groups earlier in the group scan order are reconstructed.

17. The decoder claimed in claim 13 , wherein the processor is configured to reconstruct the significant-coefficient-group flags by decoding at least some of the significant-coefficient-group flags from the encoded data of the bitstream.

18. The decoder claimed in claim 13 , wherein the processor is further configured to set a significant-coefficient-group flag to 1 for a group containing the significant-coefficient flag for a last-significant coefficient.

19. The decoder claimed in claim 13 , wherein the processor is further configured to set a significant-coefficient-group flag to 1 for a group containing the significant-coefficient flag for a coefficient at position [0,0] within the transform unit.

20. The decoder claimed in claim 13 , wherein the groups are contiguous square blocks.

21. The decoder claimed in claim 13 , wherein each group is a non-square rectangular block.

22. The decoder claimed in claim 13 , wherein the processor is configured to reconstruct one of the significant-coefficient-group flags by determining a context for that significant-coefficient-group flag based on the significant-coefficient-group flags reconstructed for two neighbor groups.

23. The decoder claimed in claim 22 , wherein the two neighbor groups include a right neighbor and a below neighbor, and wherein the context is 0 if both of the significant-coefficient-flags for the two neighbor groups are 0, and wherein otherwise the context is 1.

24. The decoder of claim 13 , wherein the group scan order and the scan order within the group are the same.

25. A non-transitory processor-readable medium storing processor-executable instructions which, when executed, configures one or more processors to perform the method claimed in claim 1 .

26. A method for encoding significant-coefficient flags for a transform unit, wherein the transform unit is partitioned into non-overlapping blocks, each block containing a respective group of significant-coefficient flags and having a corresponding significant-coefficient group flag, the method comprising:

for each respective group of significant-coefficient flags in a group scan order within the transform unit, wherein the group scan order is horizontal, diagonal or vertical,

determining that group's corresponding significant-coefficient-group flag, wherein that group's corresponding significant-coefficient-group flag is set to zero if that group contains no non-zero significant-coefficient flags, but setting the significant-coefficient group flag to one for a group containing a significant-coefficient flag at position [0,0] in the transform unit irrespective of whether that group has any non-zero significant-coefficient flags, and

encoding the significant-coefficient flags in that group in a scan order within the group, wherein the scan order within the group is horizontal, diagonal or vertical, if that group's corresponding significant-coefficient-group flag is non-zero, except the significant-coefficient-flag at position (0,0) in that group when both of the following conditions are met:

all the previous significant-coefficient flags in that group are zero, and the significant-coefficient flag is not at position [0,0] in the transform unit.

27. The method of claim 26 , wherein the group scan order and the scan order within the group are the same.

28. An encoder for encoding significant-coefficient flags for a transform unit, the encoder comprising:

a processor;

a memory storing a significance map; and

an encoding application stored in memory and containing instructions for configuring the processor to perform the method claimed in claim 26 .

29. A non-transitory processor-readable medium storing processor-executable instructions which, when executed, configures one or more processors to perform the method claimed in claim 26 .