Multi-bit information hiding using overlapping subsets
Methods and devices for reconstructing coefficient levels from a bitstream of encoded video data for a coefficient group in a transform unit. Sign bits are hidden in the parity of partially overlapping subsets of a set of coefficients. This enables the hiding of multiple sign bits per coefficient group. Other information bits may be hidden instead of sign bits in some cases.
1. A method of decoding a bitstream of encoded video in a video decoder to reconstruct coefficients of a set, the set comprising a first nonempty subset of coefficients and a second nonempty subset of coefficients, the method comprising:
for each coefficient of the second subset, determining a respective absolute value of that coefficient;
determining a first sum of the respective absolute values of the coefficients of the second subset;
determining, for a first coefficient belonging to the first subset, a parity of the first coefficient based on the first sum;
if the determined parity is odd, then determining that a flag which indicates whether the absolute value of the first coefficient is greater than zero or equal to zero is not present in the bitstream and decoding a first flag which indicates whether the absolute value of the first coefficient is greater than one or equal to one; and
if the determined parity is even, then decoding a flag which indicates whether the absolute value of the first coefficient is greater than zero or equal to zero and if it is greater than zero, determining that a flag which indicates whether the absolute value of the first coefficient is greater than one or equal to one is not present in the bitstream and decoding a flag which indicates whether the absolute value of the first coefficient is greater than two or equal to two.
2. The method claimed in claim 1 , further comprising, if the first flag is decoded to be 1,
determining that a flag which indicates whether the absolute value of the first coefficient is greater than two or equal to two is not present in the bitstream; and
decoding remaining level information for the first coefficient, wherein the remaining level information specifies the absolute value of the first coefficient minus three.
3. The method claimed in claim 1 , further comprising decoding remaining level information, wherein the remaining level information comprises a difference between a decoded level and a scalar, divided by two.
4. The method claimed in claim 1 , wherein the set further comprises a third nonempty subset and the second and third subsets contain at least one coefficient in common and each contains at least one coefficient that is not in the other subset, the method further comprising:
for each coefficient of the third subset, determining a respective absolute value of that coefficient;
determining a second sum of the respective absolute values of coefficients of the third subset; and
determining, for a second coefficient different from the first coefficient belonging to the first subset, a parity of the second coefficient based on the second sum.
5. The method claimed in claim 1 , further comprising determining a sign bit of one of the coefficients of the set based on a second sum of absolute values of coefficients from the set.
6. The method claimed in claim 1 , wherein determining the parity of the first coefficient further comprises determining that parity hiding is enabled for the set of coefficients.
7. The method claimed in claim 6 , wherein determining that parity hiding is enabled comprises determining that the set of coefficients includes more than a threshold number of non-zero coefficients.
8. A decoder for decoding a bitstream of encoded video to reconstruct coefficients of a set, the set comprising a first nonempty subset of coefficients and a second nonempty subset of coefficients, the decoder comprising:
a processor;
a memory; and
a decoding application stored in memory and containing instructions for configuring the processor to
for each coefficient of the second subset, determine a respective absolute value of that coefficient;
determine a first sum of the respective absolute values of the coefficients of the second subset;
determine, for a first coefficient belonging to the first subset, a parity of the first coefficient based on the first sum;
if the determined parity is odd, then determine that a flag which indicates whether the absolute value of the first coefficient is greater than zero or equal to zero is not present in the bitstream and decode a first flag which indicates whether the absolute value of the first coefficient is greater than one or equal to one; and
if the determined parity is even, then decode a flag which indicates whether the absolute value of the first coefficient is greater than zero or equal to zero and if it is greater than zero, determine that a flag which indicates whether the absolute value of the first coefficient is greater than one or equal to one is not present in the bitstream and decode a flag which indicates whether the absolute value of the first coefficient is greater than two or equal to two.
9. The decoder claimed in claim 8 , further comprising instructions that, when executed, cause the processor to, if the first flag is decoded to be 1,
determine that a flag which indicates whether the absolute value of the first coefficient is greater than two or equal to two is not present in the bitstream; and
decode remaining level information for the first coefficient, wherein the remaining level information specifies the absolute value of the first coefficient minus three.
10. The decoder claimed in claim 8 , further comprising instructions that, when executed, cause the processor to decode remaining level information, wherein the remaining level information comprises a difference between a decoded level and a scalar, divided by two.
11. The decoder claimed in claim 8 , wherein the set further comprises a third nonempty subset and the second and third subsets contain at least one coefficient in common and each contains at least one coefficient that is not in the other subset, the decoder further comprising instructions that, when executed, cause the processor to:
for each coefficient of the third subset, determine a respective absolute value of that coefficient;
determine a second sum of the respective absolute values of coefficients of the third subset;
determine, for a second coefficient different from the first coefficient belonging to the first subset, a parity of the second coefficient based on the second sum.
12. The decoder claimed in claim 8 , further comprising instructions that cause the processor to determine a sign bit of one of the coefficients of the set based on a second sum of absolute values of coefficients from the set.
13. The decoder claimed in claim 8 , wherein the instructions, when executed, cause the processor to determine that parity hiding is enabled for the set of coefficients.
14. The decoder claimed in claim 13 , wherein the instructions, when executed, cause the processor to determine that parity hiding is enabled by determining that the set of coefficients includes more than a threshold number of non-zero coefficients.
15. A non-transitory processor-readable medium storing processor-executable instructions for decoding a bitstream of encoded video in a video decoder to reconstruct coefficients of a set, the set comprising a first nonempty subset of coefficients and a second nonempty subset of coefficients, the instructions which, when executed, cause a processor to:
for each coefficient of the second subset, determine a respective absolute value of that coefficient;
determine a first sum of the respective absolute values of the coefficients of the second subset;
determine, for a first coefficient belonging to the first subset, a parity of the first coefficient based on the first sum;
if the determined parity is odd, then determine that a flag which indicates whether the absolute value of the first coefficient is greater than zero or equal to zero is not present in the bitstream and decode a first flag which indicates whether the absolute value of the first coefficient is greater than one or equal to one; and
if the determined parity is even, then decode a flag which indicates whether the absolute value of the first coefficient is greater than zero or equal to zero and if it is greater than zero, determine that a flag which indicates whether the absolute value of the first coefficient is greater than one or equal to one is not present in the bitstream and decode a flag which indicates whether the absolute value of the first coefficient is greater than two or equal to two.
16. The non-transitory processor-readable medium claimed in claim 15 , further comprising instructions which, when executed, cause the processor to, if the first flag is decoded to be 1,
determine that a flag which indicates whether the absolute value of the first coefficient is greater than two or equal to two is not present in the bitstream; and
decode remaining level information for the first coefficient, wherein the remaining level information specifies the absolute value of the first coefficient minus three.
17. The non-transitory processor-readable medium claimed in claim 15 , further comprising instructions which, when executed, cause the processor to decode remaining level information, wherein the remaining level information comprises a difference between a decoded level and a scalar, divided by two.
18. The non-transitory processor-readable medium claimed in claim 15 , wherein the set further comprises a third nonempty subset and the second and third subsets contain at least one coefficient in common and each contains at least one coefficient that is not in the other subset, further comprising instructions which, when executed, cause the processor to:
for each coefficient of the third subset, determine a respective absolute value of that coefficient;
determine a second sum of the respective absolute values of coefficients of the third subset;
determine, for a second coefficient different from the first coefficient belonging to the first subset, a parity of the second coefficient based on the second sum.
19. The non-transitory processor-readable medium claimed in claim 15 , wherein determining the parity of the first coefficient comprises determining that parity hiding is enabled for the set of coefficients.
20. The non-transitory processor-readable medium claimed in claim 15 , further comprising instructions which, when executed, cause the processor to determine a sign bit of one of the coefficients of the set based on a second sum of absolute values of coefficients from the set.