LUMA mapping with chroma scaling (LMCS) in video coding

1. A method of coding video data, the method comprising:

determining that chroma related syntax elements of the video data are present for luma mapping with chroma scaling (LMCS) for a first coding unit (CU) of the video data;

coding a first syntax element of the video data when the chroma related syntax elements are present for LMCS for the first CU, the first syntax element is indicative of a first value for determining a scaling parameter for chroma scaling in LMCS;

coding a chroma block of the first CU based on the scaling parameter for chroma scaling;

coding a second syntax element in a picture header for a picture that includes a second CU having a luma block, the second syntax element being an identification of an adaptation parameter set (APS);

coding a third value for a third syntax element for the luma block, the third syntax element specifying a number of bits used for representation of a fourth syntax element for LMCS, wherein the second syntax element is in the picture header and the third value of the third syntax element is within a range of 0 to a bit depth —2; and

coding the luma block using LMCS.

2. The method of claim 1 , further comprising:

parsing a flag indicating that the chroma related syntax elements are present, wherein determining that the chroma related syntax elements are present comprises determining that the chroma related syntax elements are present based on the flag.

3. The method of claim 1 , further comprising:

signaling a flag indicating that the chroma related syntax elements are present based on the determination that the chroma related syntax elements are present.

4. The method of claim 1 , wherein coding the first syntax element comprises parsing the first syntax element based on the determination that the chroma related syntax elements are present.

5. The method of claim 1 , wherein coding the first syntax element comprises signaling the first syntax element only when the chroma related syntax elements are present.

6. The method of claim 1 , wherein coding the first syntax element comprises coding the first syntax element in an LMCS syntax structure in the APS.

7. The method of claim 1 , wherein the first syntax element specifies an absolute codeword value of the first value that is used for determining the scaling parameter for chroma scaling.

8. The method of claim 1 , further comprising:

coding at least one flag in the APS specifying that cross-component filters for a chroma component are not signaled when a sequence parameter set (SPS) indicates that cross component adaptive loop filter (CCALF) is disabled.

9. The method of claim 1 , further comprising:

determining that the chroma related syntax elements are not present for LMCS for a third CU; and

bypassing coding of the first syntax element when the chroma related syntax elements are not present for LMCS for the third CU.

10. The method of claim 1 , wherein coding the chroma block of the first CU based on the scaling parameter for chroma scaling comprises decoding the chroma block, and wherein decoding the chroma block comprises:

inverse scaling a scaled residual chroma block based on the scaling parameter to generate a residual chroma block;

determining a chroma prediction block; and

adding the residual chroma block to the chroma prediction block to reconstruct the chroma block.

11. The method of claim 1 , wherein coding the chroma block based on the scaling parameter for chroma scaling comprises encoding the chroma block, and wherein encoding the chroma block comprises:

determining a chroma prediction block;

subtracting the chroma prediction block from the chroma block to generate a residual block;

scaling the residual block to generate a scaled residual block based on the scaling parameter; and

signaling information indicative of the scaled residual block.

12. A device for coding video data, the device comprising:

memory configured to store the video data; and

processing circuitry configured to:

determine that chroma related syntax elements of the video data are present for luma mapping with chroma scaling (LMCS) for a first coding unit (CU) of the video data;

code a first syntax element of the video data when the chroma related syntax elements are present for LMCS for the first CU, the first syntax element is indicative of a first value for determining a scaling parameter for chroma scaling in LMCS;

code a chroma block of the first CU based on the scaling parameter for chroma scaling;

code a second syntax element in a picture header for a picture that includes a second CU having a luma block, the second syntax element being an identification of an adaptation parameter set (APS);

code a third value for a third syntax element for the luma block, the third syntax element specifying a number of bits used for representation of a fourth syntax element for LMCS, wherein the second syntax element is in the picture header and the third value of the third syntax element is within a range of 0 to a bit depth —2; and

code the luma block using LMCS.

13. The device of claim 12 , wherein the processing circuitry is configured to

parse a flag indicating that the chroma related syntax elements are present,

wherein to determine that the chroma related syntax elements are present, the processing circuitry is configured to determine that the chroma related syntax elements are present based on the flag.

14. The device of claim 12 , wherein the processing circuitry is configured to:

signal a flag indicating that the chroma related syntax elements are present based on the determination that the chroma related syntax elements are present.

15. The device of claim 12 , wherein to code the first syntax element, the processing circuitry is configured to parse the first syntax element based on the determination that the chroma related syntax elements are present.

16. The device of claim 12 , wherein to code the first syntax element, the processing circuitry is configured to signal the first syntax element only when the chroma related syntax elements are present.

17. The device of claim 12 , wherein to code the first syntax element, the processing circuitry is configured to code the first syntax element in an LMCS syntax structure in the APS.

18. The device of claim 12 , wherein the first syntax element specifies an absolute codeword value of the first value that is used for determining the scaling parameter for chroma scaling.

19. The device of claim 12 , wherein the processing circuitry is configured to:

code at least one flag in the APS specifying that cross-component filters for a chroma component are not signaled when a sequence parameter set (SPS) indicates that cross component adaptive loop filter (CCALF) is disabled.

20. The device of claim 12 , wherein the processing circuitry is configured to:

determine that the chroma related syntax elements are not present for LMCS for a third CU; and

bypass coding of the first syntax element when the chroma related syntax elements are not present for LMCS for the third CU.

21. The device of claim 12 , wherein to code the chroma block of the first CU based on the scaling parameter for chroma scaling, the processing circuitry is configured to decode the chroma block, and wherein to decode the chroma block, the processing circuitry is configured to:

inverse scale a scaled residual chroma block based on the scaling parameter to generate a residual chroma block;

determine a chroma prediction block; and

add the residual chroma block to the chroma prediction block to reconstruct the chroma block.

22. The device of claim 12 , wherein to code the chroma block based on the scaling parameter for chroma scaling, the processing circuitry is configured to encode the chroma block, and wherein to encode the chroma block, the processing circuitry is configured to:

determine a chroma prediction block;

subtract the chroma prediction block from the chroma block to generate a residual block;

scale the residual block to generate a scaled residual block based on the scaling parameter; and

signal information indicative of the scaled residual block.

23. The device of claim 12 , wherein the device comprises one or more of a camera, a computer, a mobile device, a broadcast receiver device, or a set-top box.

24. A non-transitory computer-readable storage medium storing instructions thereon that when executed cause one or more processors to:

determine that chroma related syntax elements of the video data are present for luma mapping with chroma scaling (LMCS) for a first coding unit (CU) of the video data;

code a first syntax element of the video data when the chroma related syntax elements are present for LMCS for the first CU, the first syntax element is indicative of a first value for determining a scaling parameter for chroma scaling in LMCS;

code a chroma block of the first CU based on the scaling parameter for chroma scaling;

code a second syntax element in a picture header for a picture that includes a second CU having a luma block, the second syntax element being an identification of an adaptation parameter set (APS);

code a third value for a third syntax element for the luma block, the third syntax element specifying a number of bits used for representation of a fourth syntax element for LMCS, wherein the second syntax element is in the picture header and the third value of the third syntax element is within a range of 0 to a bit depth —2; and

code the luma block using LMCS.

25. The non-transitory computer-readable storage medium of claim 24 , further comprising instructions that cause the one or more processors to at least one of:

parse a flag indicating that the chroma related syntax elements are present, wherein the instructions that cause the one or more processors to determine that the chroma related syntax elements are present comprise instructions that cause the one or more processors to determine that the chroma related syntax elements are present based on the flag, or

signal a flag indicating that the chroma related syntax elements are present based on the determination that the chroma related syntax elements are present.

26. The non-transitory computer-readable storage medium of claim 24 , wherein the instructions that cause the one or more processors to code the first syntax element comprise instructions that cause the one or more processors to at least one of:

parse the first syntax element based on the determination that the chroma related syntax elements are present, or

signal the first syntax element only when the chroma related syntax elements are present.

27. The non-transitory computer-readable storage medium of claim 24 , wherein the instructions that cause the one or more processors to code the first syntax element comprise instructions that cause the one or more processors to code the first syntax element in an LMCS syntax structure in the APS, and wherein the first syntax element specifies an absolute codeword value of the first value that is used for determining the scaling parameter for chroma scaling.

28. A device for coding video data, the device comprising:

means for determining that chroma related syntax elements of the video data are present for luma mapping with chroma scaling (LMCS) for a first coding unit (CU) of the video data;

means for coding a first syntax element of the video data when the chroma related syntax elements are present for LMCS for the first CU, the first syntax element is indicative of a first value for determining a scaling parameter for chroma scaling in LMCS;

means for coding a chroma block of the first CU based on the scaling parameter for chroma scaling;

means for coding a second syntax element in a picture header for a picture that includes a second CU having a luma block, the second syntax element being an identification of an adaptation parameter set (APS);

means for coding a third value for a third syntax element for the luma block, the third syntax element specifying a number of bits used for representation of a fourth syntax element for LMCS, wherein the second syntax element is in the picture header and the third value of the third syntax element is within a range of 0 to a bit depth —2; and

means for coding the luma block using LMCS.