Multiple sign bit hiding within a transform unit
Methods of encoding and decoding for video data are described for encoding or decoding coefficients for a transform unit. In particular, the sign bits for the non-zero coefficients are encoded using sign bit hiding. Two or more sets of coefficients are defined for the transform unit and a sign bit may be hidden for each set, subject to satisfaction of a threshold test. The sets may correspond to coefficient groups that are otherwise used in multi-level significance map encoding and decoding.
1. A method of decoding a bitstream of encoded video by reconstructing coefficients for a transform unit, wherein the transform unit is partitioned into non-overlapping coefficient groups each containing a respective set of coefficients, and each of the non-zero coefficients having a sign bit indicating whether the coefficient is positive or negative, the method comprising:
determining, for each coefficient group in the transform unit, whether to decode all sign bits for non-zero coefficients in that coefficient group, or whether that coefficient group is to use sign bit hiding, and wherein the determining is based on whether an adjacent coefficient group contains at least one non-zero coefficient;
for each coefficient group in the transform unit that is to use sign bit hiding,
decoding a sign bit for each of the non-zero coefficients in that coefficient group except for one of the non-zero coefficients;
summing an absolute value of the coefficients in that coefficient group; and
based on whether the absolute value is even or odd, assigning a sign to said one of the non-zero coefficients within that coefficient group.
2. The method claimed in claim 1 , wherein the determining, for each coefficient group, is further based on determining that the number of coefficients within that coefficient group between a first non-zero coefficient in that coefficient group and last non-zero coefficient in that coefficient group, in a scan order within that coefficient group, exceeds a threshold value.
3. The method claimed in claim 1 , wherein the adjacent coefficient groups comprises either of the coefficient group to the right and the coefficient group below a current coefficient group.
4. The method claimed in claim 1 , wherein each coefficient group is square.
5. The method claimed in claim 1 , wherein each coefficient group is 4×4.
6. A decoder for decoding a bitstream of encoded video by reconstructing coefficients for a transform unit, wherein the transform unit is partitioned into non-overlapping coefficient groups each containing a respective set of coefficients, and each of the non-zero coefficients having a sign bit indicating whether the coefficient is positive or negative, the decoder comprising:
a processor;
a memory; and
a decoding application stored in memory and containing instructions that, when executed, cause the processor to:
determine, for each coefficient group in the transform unit, whether to decode all sign bits for non-zero coefficients in that coefficient group, or whether that coefficient group is to use sign bit hiding, and wherein the determining is based on whether an adjacent coefficient group contains at least one non-zero coefficient;
for each coefficient group in the transform unit that is to use sign bit hiding,
decode, sign bit for each of the non-zero coefficients in that coefficient group except for one of the non-zero coefficients;
sum an absolute value of the coefficients in that coefficient group; and
based on whether the absolute value is even or odd, assign a sign to said one of the non-zero coefficients within that coefficient group.
7. The decoder claimed in claim 6 , wherein the instructions, when executed, further cause the processor to determine, for each coefficient group, whether that coefficient group is to use sign bit hiding is further based upon whether the number of coefficients within that coefficient group between a first non-zero coefficient in that coefficient group and last non-zero coefficient in that coefficient group, in a scan order within that coefficient group, exceeds a threshold value.
8. The decoder claimed in claim 6 , wherein the adjacent coefficient group comprises either of the coefficient group to the right and the coefficient group below a current coefficient group.
9. The decoder claimed in claim 6 , wherein each coefficient group is square.
10. The decoder claimed in claim 6 , wherein each coefficient group is 4×4.
11. A non-transitory processor-readable medium storing processor-executable instructions which, when executed, configures one or more processors to decode a bitstream of encoded video by reconstructing coefficients for a transform unit, wherein the transform unit is partitioned into non-overlapping coefficient groups each containing a respective set of coefficients, and each of the non-zero coefficients having a sign bit indicating whether the coefficient is positive or negative, and wherein the instructions further cause the processor to:
determine, for each coefficient group in the transform unit, whether to decode all sign bits for non-zero coefficients in that coefficient group, or whether that coefficient group is to use sign bit hiding, and wherein the determining is based on whether an adjacent coefficient group contains at least one non-zero coefficient;
for each coefficient group in the transform unit that is to use sign bit hiding,
decode, a sign bit for each of the non-zero coefficients in that coefficient group except for one of the non-zero coefficients;
sum an absolute value of the coefficients in that coefficient group; and
based on whether the absolute value is even or odd, assign a sign to said one of the non-zero coefficients within that coefficient group.