Coding and decoding of transform skipped blocks
Video encoder and decoder and methods therein for transform skipped encoding and decoding of blocks of pixels. The methods comprises a modified use of an existing context model associated with transform coefficients, for deriving contexts for a bitmask indicating which residual values (i.e. not transform coefficients), in a transform skipped residual block, that are greater than one. The methods are applicable to video coding schemes such as HEVC.
1. A method performed by a video decoder operable according to a video decoding scheme applying residual decoding comprising use of a context model for deriving contexts to be used for decoding flags, in a bitmask, which flags indicate, respectively, whether an absolute value of a non-zero transform coefficients in a transformed residual block is greater than one, where the context model comprises a plurality of states, each state corresponding to a context and a probability, where a procedure for deriving contexts for a number of flags related to transform coefficients is started in a first state, A, and where, after decoding a flag which corresponds to an absolute value of a transform coefficient that is greater than one, the context model goes to a second state, D, which is used for decoding remaining flags in said bitmask, the method comprising:
for a transform-skipped residual block comprising residual values and not transform coefficients, deriving contexts to be used for decoding flags, in a bitmask, that indicate, respectively, whether an absolute value of a non-zero residual value in the transform-skipped block is greater than one, by use of the context model, starting in the second state D.
2. The method of claim 1 , wherein the decoding scheme further applies reverse scan order for transform-skipped blocks.
3. The method of claim 1 , wherein the decoding scheme is an extension of HEVC.
4. A method performed by a video encoder operable according to a video encoding scheme applying residual encoding comprising use of a context model for associating contexts to flags in a bitmask, which flags indicate, respectively, whether an absolute value of a non-zero transform coefficients in a transformed residual block is greater than one, where the context model comprises a plurality of states, each state corresponding to a context and a probability, where a procedure for deriving contexts to be associated with a number of flags related to transform coefficients is started in a first state, A, and where, after encoding a flag which corresponds to a transform coefficient greater than one, the context model goes to a second state, D, which is used for encoding remaining flags in said bitmask, the method comprising:
for a transform-skipped residual block comprising residual values and not transform coefficients, deriving contexts to be associated with flags in a bit map, which flags indicate, respectively, whether an absolute value of a non-zero residual value in the transform-skipped residual block is greater than one, by use of the context model, starting in the second state D.
5. The method of claim 4 , wherein the encoding scheme further applies reverse scan order for transform-skipped blocks.
6. The method of claim 4 , wherein the encoding scheme is HEVC.
7. A video decoder operable according to a video decoding scheme applying residual decoding comprising use of a context model for deriving contexts to be used for decoding flags in a bitmask, which flags indicate, respectively, whether an absolute value of a non-zero transform coefficients in a transformed residual block is greater than one, where the context model comprises a plurality of states, each state corresponding to a context and a probability, where a procedure for deriving contexts for a number of flags related to transform coefficients is started in a first state, A, and where, after decoding a flag which corresponds to an absolute value of a transform coefficient that is greater than one, the context model goes to a second state, D, which is used for decoding remaining flags in said bitmask;
the video decoder comprising processing circuitry and a memory comprising instructions that when executed by the processing circuitry causes the video decoder to:
for a transform-skipped residual block comprising residual values and not transform coefficients, derive contexts to be used for decoding flags, in a bitmask, which flags indicate, respectively, whether an absolute value of a non-zero residual value in the transform-skipped block is greater than one, by use of the context model, starting in the second state D.
8. The video decoder of claim 7 , wherein the decoding scheme further applies reverse scan order for transform-skipped blocks.
9. A user equipment comprising the video decoder of claim 7 .
10. A video encoder operable according to a video encoding scheme applying residual encoding comprising use of a context model for associating contexts to flags in a bitmask, which flags indicate, respectively, whether an absolute value of a non-zero transform coefficients in a transformed residual block is greater than one, where the context model comprises a plurality of states, each state corresponding to a context and a probability, where a procedure for deriving contexts to be associated with a number of flags related to transform coefficients is started in a first state, A, and where, after encoding a flag which corresponds to a transform coefficient greater than one, the context model goes to a second state, D, which is used for encoding remaining flags in said bitmask,
the video encoder comprising processing circuitry and a memory comprising instructions that when executed by the processing circuitry cause the video decoder to:
for a transform-skipped residual block comprising residual values and not transform coefficients, derive contexts to be associated with flags in a bitmask, which flags indicate, respectively, whether an absolute value of a non-zero residual value in the transform-skipped residual block is greater than one, by use of the context model, starting in the second state D.
11. The video encoder of claim 10 , wherein the encoding scheme further applies reverse scan order for transform-skipped blocks.
12. A user equipment comprising the video encoder of claim 10 .