LUMA MAPPING-BASED VIDEO OR IMAGE CODING

1 . An image decoding method comprising:

obtaining image information including prediction related information, luma mapping with chroma scaling (LMCS) related information and residual information from a bitstream;

generating predicted luma samples for a current block in a current picture based on the prediction related information;

deriving LMCS codewords based on the LMCS related information;

generating mapped predicted luma samples based on the predicted luma samples and the LMCS codewords;

deriving residual luma samples based on the residual information; and

generating reconstructed luma samples for the current block based on the mapped predicted luma samples and the residual luma samples,

wherein the image information includes adaptation parameter sets (APSs), wherein each of the APSs includes type information specifying whether the APS is an LMCS APS,

wherein an LMCS data field included in the APS of which the type information specifies the LMCS APS includes the LMCS related information,

wherein the LMCS codewords are derived based on the LMCS data field, and

wherein an ID value of the APS of which the type information specifies the LMCS APS is in a predetermined range.

2 . The image decoding method of claim 1 , wherein the APS comprises the LMCS data field including LMCS parameters based on that a value of the type information is 1.

3 . The image decoding method of claim 1 , wherein the image information comprises header information,

wherein the header information includes LMCS related APS ID information, and

wherein the LMCS related APS ID information represents an ID of the LMCS APS for the current picture or the current block.

4 . The image decoding method of claim 3 , wherein a value of the LMCS related APS ID information is in a range of 0 to 3.

5 . The image decoding method of claim 3 , wherein the image information comprises a sequence parameter set (SPS), and

wherein the SPS includes a first LMCS available flag representing whether the LMCS data field is available.

6 . The image decoding method of claim 5 , wherein the header information comprises a second LMCS available flag representing whether the LMCS data field in a picture is available based on that a value of the first LMCS available flag is 1.

7 . The image decoding method of claim 6 , wherein the LMCS related APS ID information is included in the header information based on that a value of the second LMCS available flag is 1.

8 . The image decoding method of claim 1 , wherein the predetermined range is 0 to 3.

9 . The image decoding method of claim 1 , wherein a maximum number of LMCS APSs among the APSs is a predetermined value.

10 . The image decoding method of claim 9 , wherein the predetermined value is 4.

11 . The image decoding method of claim 1 , wherein residual chroma samples are generated based on the residual information,

wherein a piecewise index is identified based on the LMCS related information,

wherein a chroma residual scaling factor is derived based on the piecewise index, and

wherein scaled residual chroma samples are generated based on the residual chroma samples and the chroma residual scaling factor.

12 . The image decoding method of claim 11 , wherein in case that the current block is a single-tree structure or a dual-tree structure, a chroma residual scaling available flag representing whether chroma residual scaling is applied for the current block is signaled.

13 . The image decoding method of claim 11 , wherein the chroma residual scaling factor is a single chroma residual scaling factor.

14 . 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,

wherein the forward mapping scaling factor is a single factor for the predicted luma samples.

15 . The image decoding method of claim 1 , further comprising:

deriving an inverse mapping scaling factor for the reconstructed luma samples; and

generating a 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.

16 . The image decoding method of claim 15 , wherein the inverse mapping scaling factor is derived using a derived piecewise index based on the reconstructed luma samples.

17 . An image encoding method comprising:

generating predicted luma samples for a current block in a current picture;

generating prediction related information;

deriving luma mapping with chroma scaling (LMCS) codewords;

deriving LMCS related information based on the LMCS codewords;

generating mapped predicted luma samples based on the LMCS codewords;

generating residual luma samples for the current block based on the mapped predicted luma samples;

deriving residual information based on the residual luma samples; and

encoding image information including the prediction related information, the LMCS related information and the residual information,

wherein the image information includes adaptation parameter sets (APSs),

wherein each of the APSs includes type information specifying whether the APS is an LMCS APS,

wherein an LMCS data field included in the APS of which type information specifies the LMCS APS includes the LMCS related information,

wherein the LMCS data field includes information on the LMCS codewords, and

wherein an ID value of the APS of which the type information specifies the LMCS APS is in a predetermined range.

18 . The image encoding method of claim 17 , wherein the image information comprises header information,

wherein the header information includes LMCS related APS ID information,

wherein the LMCS related APS ID information represents an ID of the LMCS APS for the current picture or the current block, and

wherein a value of the LMCS related APS ID information is in the predetermined range.

19 . The image encoding method of claim 18 , wherein the predetermined range is 0 to 3.

20 . A method for transmitting image information, the method comprising:

obtaining a bitstream for the image information; and

transmitting the bitstream,

wherein predicted luma samples for a current block in a current picture are generated,

wherein prediction related information is generated,

wherein luma mapping with chroma scaling (LMCS) codewords are derived,

wherein LMCS related information is derived based on the LMCS codewords,

wherein mapped predicted luma samples are generated based on the LMCS codewords,

wherein residual luma samples for the current block are generated based on the mapped predicted luma samples,

wherein residual information is derived based on the residual luma samples,

wherein the image information including the prediction related information, the LMCS related information and the residual information is encoded,

wherein the image information includes adaptation parameter sets (APSs),

wherein each of the APSs includes type information specifying whether the APS is an LMCS APS,

wherein an LMCS data field included in the APS of which type information specifies the LMCS APS includes the LMCS related information,

wherein the LMCS data field includes information on the LMCS codewords, and

wherein an ID value of the APS of which the type information specifies the LMCS APS is in a predetermined range.