Video or image coding based on luma mapping and chroma scaling
According to disclosure of the present document, an LMCS procedure can be used for blocks having various block tree structures, and memories used in the LMCS procedure can be reduced (limited).
1. An image decoding method performed by a decoding apparatus, the method comprising:
obtaining image information including prediction mode information and luma mapping with chroma scaling (LMCS) related information from a bitstream;
deriving an inter prediction mode for a current block in a current picture based on the prediction mode information;
generating predicted luma samples based on the inter prediction mode;
generating mapped predicted luma samples based on a mapping procedure for luma samples; and
generating modified reconstructed luma samples based on an inverse mapping procedure for luma samples,
wherein:
the image information includes LMCS adaptation parameter sets (APSs);
an LMCS APS includes an LMCS data field;
the LMCS data field includes the LMCS related information;
LMCS codewords used in the mapping procedure and the inverse mapping procedure are derived based on the LMCS data field;
a maximum number of the LMCS APSs is a predetermined value; and
the predetermined value is 4.
2. The image decoding method of claim 1 , wherein in the mapping procedure for the luma samples:
a mapping index is derived based on the predicted luma samples; and
the mapped predicted luma samples are generated using a first mapping value based on the mapping index.
3. The image decoding method of claim 1 , wherein:
the LMCS related information includes information on bins for inverse mapping; and
a minimum bin index and a maximum bin index are derived based on the information on the bins, and
wherein in the inverse mapping procedure for the luma samples:
an inverse mapping index is derived based on mapping values based on bin indices of from the minimum bin index to the maximum bin index; and
the modified reconstructed luma samples are generated using a second mapping value based on the inverse mapping index.
4. The image decoding method of claim 1 , wherein:
the image information includes a sequence parameter set (SPS); and
the SPS includes a linear LMCS enabled flag indicating whether or not a linear LMCS is enabled.
5. The image decoding method of claim 1 , wherein:
the image information includes header information;
the header information includes LMCS related APS ID information; and
the LMCS related APS ID information represents an ID of the LMCS APS.
6. The image decoding method of claim 5 , wherein based on a value of a second LMCS enabled flag being 1, the LMCS related APS ID information is incorporated into the header information.
7. The image decoding method of claim 1 , wherein based on a value of type information being 1, the LMCS APS incorporates the LMCS data field including LMCS parameters.
8. The image decoding method of claim 1 , wherein:
the image information includes residual information;
residual chroma samples are generated based on the residual information;
a piecewise index is identified based on the LMCS related information;
a chroma residual scaling factor is derived based on the piecewise index; and
scaled residual chroma samples are generated based on the residual chroma samples and the chroma residual scaling factor.
9. The image decoding method of claim 8 , wherein regardless of a block tree structure of the current block, a chroma residual scaling enabled flag indicating whether or not chroma residual scaling is applied for the current block is signaled.
10. The image decoding method of claim 9 , wherein the chroma residual scaling factor is a single chroma residual scaling factor.
11. The image decoding method of claim 1 , wherein the generating the mapped predicted luma samples comprises:
deriving a forward mapping scaling factor for the predicted luma samples; and
generating the mapped predicted luma samples based on the forward mapping scaling factor, and
wherein the forward mapping scaling factor is a single factor for the predicted luma samples.
12. The image decoding method of claim 1 , further comprising:
deriving an inverse mapping scaling factor for the reconstructed luma samples; and
generating modified reconstructed luma samples based on the reconstructed luma samples and the inverse mapping scaling factor,
wherein the inverse mapping scaling factor is a single factor for the reconstructed luma samples.
13. The image decoding method of claim 11 , wherein the inverse mapping scaling factor is derived using a piecewise index derived based on the reconstructed luma samples.
14. An image encoding method performed by an encoding apparatus, the method comprising:
deriving an inter prediction mode;
generating predicted luma samples based on the inter prediction mode;
generating luma mapping with chroma scaling (LMCS) related information;
generating mapped predicted luma samples based on a mapping procedure for luma samples;
generating modified reconstructed luma samples based on an inverse mapping procedure for luma samples;
generating residual information based on the mapped predicted luma samples and the modified reconstructed luma samples; and
encoding image information including the LMCS related information and the residual information,
wherein:
the image information includes LMCS adaptation parameter sets (APSs);
an LMCS APS includes an LMCS data field;
the LMCS data field includes the LMCS related information;
LMCS codewords used in the mapping procedure and the inverse mapping procedure are derived based on the LMCS data field;
a maximum number of the LMCS APSs is a predetermined value; and
the predetermined value is 4.
15. A non-transitory computer-readable storage medium storing a bitstream generated by a method, the method comprising:
deriving an inter prediction mode;
generating predicted luma samples based on the inter prediction mode;
generating luma mapping with chroma scaling (LMCS) related information;
generating mapped predicted luma samples based on a mapping procedure for luma samples;
generating modified reconstructed luma samples based on an inverse mapping procedure for luma samples;
generating residual information based on the mapped predicted luma samples and the modified reconstructed luma samples; and
encoding image information including the LMCS related information and the residual information to output the bitstream,
wherein:
the image information includes LMCS adaptation parameter sets (APSs);
an LMCS APS includes an LMCS data field;
the LMCS data field includes the LMCS related information;
LMCS codewords used in the mapping procedure and the inverse mapping procedure are derived based on the LMCS data field;
a maximum number of the LMCS APSs is a predetermined value; and
the predetermined value is 4.