Multi-level significance map scanning

A decoder which reconstructs significant-coefficient flags for a transform unit, wherein the transform unit comprises non-overlapping blocks of transform coefficients, each of the non-overlapping blocks corresponds to a respective group of significant-coefficient flags, each significant-coefficient flag corresponds to a respective transform coefficient, and each respective group corresponds to a respective significant-coefficient group flag, groups of significant-coefficient flags are reconstructed in a group scan order, and significant-coefficient flags are reconstructed in a scan order within their group, the decoder comprising:

• a processor;
• a memory;
• and
• a decoding application stored in memory and containing instructions for configuring the processor to:
• decode a position for a last-significant coefficient of the transform unit from a bitstream of encoded data;
• in a first group corresponding to the position of the last-significant coefficient, decode from the bitstream each significant-coefficient flag in the scan order from a position following the position of the last-significant coefficient in the scan order through position (0,0) of the first group;
• for each group in the group scan order from a second group, which follows the first group in the group scan order, to a group preceding a DC group in the group scan order, wherein the DC group corresponds to a block containing a transform coefficient at a (0,0) position of the transform unit,
• decode a significant-coefficient-group flag from the bitstream for that group, and
• reconstruct significant-coefficient flags in that group in a scan order within that group, by, for each significant-coefficient flag in that group,
• if that significant-coefficient flag is at position (0,0) in its group, its corresponding significant-coefficient-group flag is decoded to be non-zero, and all of the significant-coefficient flags which precede that significant-coefficient flag in the scan order in that group are zero, then inferring, without decoding 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
• inferring, without decoding from the bitstream, 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;
• and
• in the DC group, decode each corresponding significant-coefficient flag in the scan order.

A non-transitory processor-readable medium storing processor-executable instructions for a decoder which reconstructs significant-coefficient flags for a transform unit, wherein the transform unit comprises non-overlapping blocks of transform coefficients, each of the non-overlapping blocks corresponds to a respective group of significant-coefficient flags, each significant-coefficient flag corresponds to a respective transform coefficient, and each respective group corresponds to a respective significant-coefficient group flag, groups of significant-coefficient flags are reconstructed in a group scan order, and significant-coefficient flags are reconstructed in a scan order within their group, the processor-executable instructions, when executed, configuring one or more processors to:

• decode a position for a last-significant coefficient of the transform unit from a bitstream of encoded data;
• in a first group corresponding to the position of the last-significant coefficient, decode from the bitstream each significant-coefficient flag in the scan order from a position following the position of the last-significant coefficient in the scan order through the (0,0) position;
• for each group in the group scan order from a second group, which follows the first group in the group scan order, to a group preceding a DC group in the group scan order, wherein the DC group corresponds to a block containing a transform coefficient at the (0,0) position of the transform unit,
• decode a significant-coefficient-group flag from the bitstream for that group, and
• reconstruct significant-coefficient flags in that group in a scan order within that group, by, for each significant-coefficient flag in that group,
• if that significant-coefficient flag is at position (0,0) in its group, its corresponding significant-coefficient-group flag is decoded to be non-zero, and all of the significant-coefficient flags which precede that significant-coefficient flag in the scan order in that group are zero, then inferring, without decoding 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
• inferring, without decoding from the bitstream, 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;
• and
• in the DC group, decode each corresponding significant-coefficient flag in the scan order.