Luma mapping-based video or image coding

1. An image decoding apparatus, comprising:

an entropy decoder for obtaining image information including prediction related information and luma mapping with chroma scaling (LMCS) related information from a bitstream;

a predictor for generating prediction luma samples for a current block in a current picture based on the prediction related information, for deriving LMCS codewords based on the LMCS related information, and for generating mapped prediction luma samples based on the prediction luma samples and the LMCS codewords; and

an adder for generating reconstructed luma samples for the current block based on the mapped prediction luma samples,

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

wherein each of the APS s 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 apparatus 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 apparatus 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 apparatus of claim 3 , wherein a value of the LMCS related APS ID information is in a range of 0 to 3.

5. The apparatus 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 apparatus 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 apparatus 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 apparatus of claim 1 , wherein the predetermined range is 0 to 3.

9. The apparatus of claim 1 , wherein the image information comprises residual information,

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.

10. The apparatus of claim 9 , 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.

11. The apparatus of claim 9 , wherein the chroma residual scaling factor is a single chroma residual scaling factor.

12. The apparatus of claim 1 , wherein the generating the mapped prediction luma samples comprises:

deriving a forward mapping scaling factor for the prediction luma samples; and

generating the mapped prediction luma samples based on the forward mapping scaling factor,

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

13. The apparatus 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.

14. The apparatus of claim 13 , wherein the inverse mapping scaling factor is derived using a derived piecewise index based on the reconstructed luma samples.

15. An image encoding apparatus, the method comprising:

a predictor for generating prediction luma samples for a current block in a current picture, for deriving luma mapping with chroma scaling (LMCS) codewords, and for generating mapped prediction luma samples based on the LMCS codewords;

a residual processor for deriving LMCS related information based on the LMCS codewords, for generating residual luma samples for the current block based on the mapped prediction luma samples, and for deriving residual information based on the residual luma samples; and

an entropy encoder for encoding image information including the LMCS related information and the residual information,

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

wherein each of the APS s 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.

16. The apparatus of claim 15 , 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.

17. The apparatus of claim 16 , wherein the predetermined range is 0 to 3.

18. The apparatus of claim 16 , 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.

19. The apparatus of claim 18 , 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.

20. A transmitting apparatus of data for an image, comprising:

a processor configured to obtain a bitstream, wherein the bitstream is generated based on generating prediction luma samples for a current block in a current picture, deriving luma mapping with chroma scaling (LMCS) codewords, deriving LMCS related information based on the LMCS codewords, generating mapped prediction luma samples based on the LMCS codewords, generating residual luma samples for the current block based on the mapped prediction luma samples, deriving residual information based on the residual luma samples, and encoding image information including the LMCS related information and the residual information; and

a transmitter configured to transmit the data comprising the bitstream,

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

wherein each of the APS s 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.