Selective use of cross component mode in video coding

1. A method of processing video data, comprising:

determining, during a conversion between a current chroma video block of a video region of a video and a bitstream of the video, whether to enable or disable a scaling process applied on chroma residual samples of the current chroma video block based on a syntax element indicative of selectively enabling or disabling the scaling process in the bitstream of the video if included; and

performing the conversion by applying the scaling process on the chroma residual samples based on at least one scaling factor in response to the scaling process being enabled, and refraining from applying the scaling process on the chroma residual samples in response to the scaling process being disabled,

wherein the syntax element is included in a picture header associated with the current video block,

wherein in response to a value of the syntax element being equal to one, the application of the scaling process for blocks below a picture level is enabled, and in response to the value of the syntax element being equal to zero, the application of the scaling process for blocks below the picture level is disabled, and

wherein in response to the syntax element being not included in the bitstream, it is inferred that the application of the scaling process for blocks below the picture level is disabled, and a value of the syntax element is inferred to be zero.

2. The method of claim 1 , wherein the at least one scaling factor is derived based on an averaged luma variable computed based on neighboring luma samples of a video unit of a video region of the video which is determined based on covers a luma sample corresponding to a top-left sample of the current chroma video block.

3. The method of claim 2 , wherein the at least one scaling factor is derived by:

checking an availability of one or more neighboring luma blocks of the video unit;

determining, based on the availability of the one or more neighboring luma blocks, whether to retrieve the neighboring luma samples of the video unit, wherein the one or more neighboring luma blocks comprise the neighboring luma samples; and

deriving the at least one scaling factor based on the averaged luma variable computed using the neighboring luma samples by a rounding-based average operation in case that the neighboring samples are available.

4. The method of claim 3 , wherein the neighboring luma samples are located in predefined positions neighboring to the video unit.

5. The method of claim 4 , wherein the neighboring luma samples located in the predefined positions neighboring to the video unit include reconstructed luma samples external to the video unit.

6. The method of claim 4 , wherein the neighboring luma samples located in the predefined positions neighboring to the video unit include reconstructed luma samples adjacent to the video unit.

7. The method of claim 6 , wherein the reconstructed luma samples adjacent to the video unit includes at least one of one or more left neighboring sample columns or one or more above neighboring sample rows of the video unit.

8. The method of claim 7 , wherein in response to the one or more left neighboring sample columns and the one or more above neighboring sample rows of the video unit being determined to be unavailable, the at least one scaling factor is derived by setting the averaged luma variable equal to a default value.

9. The method of claim 8 , wherein the default value equals to 1<<(bitDepth−1), where bitDepth is a bit depth of the video.

10. The method of claim 2 , wherein the scaling process is based on a piecewise linear model, and wherein an index identifying a piece to which the averaged luma variable belongs, and the at least one scaling factor is derived based on the index.

11. The method of claim 1 , wherein the conversion includes encoding the current chroma video block into the bitstream.

12. The method of claim 1 , wherein the conversion includes decoding the current chroma video block from the bitstream.

13. An apparatus for processing video data comprising a processor and a non-transitory memory with instructions thereon, wherein the instructions upon execution by the processor, cause the processor to:

determine, during a conversion between a current chroma video block of a video region of a video and a bitstream of the video, whether to enable or disable a scaling process applied on chroma residual samples of the current chroma video block based on a syntax element indicative of selectively enabling or disabling the scaling process in the bitstream of the video if included; and

perform the conversion by applying the scaling process on the chroma residual samples based on at least one scaling factor in response to the scaling process being enabled, and refraining from applying the scaling process on the chroma residual samples in response to the scaling process being disabled,

wherein the syntax element is included in a picture header associated with the current video block,

wherein in response to a value of the syntax element being equal to one, the application of the scaling process for blocks below a picture level is enabled, and in response to the value of the syntax element being equal to zero, the application of the scaling process for blocks below the picture level is disabled, and

wherein in response to the syntax element being not included in the bitstream, it is inferred that the application of the scaling process for blocks below the picture level is disabled, and a value of the syntax element is inferred to be zero.

14. A non-transitory computer-readable storage medium storing instructions that cause a processor to:

determine, during a conversion between a current chroma video block of a video region of a video and a bitstream of the video, whether to enable or disable a scaling process applied on chroma residual samples of the current chroma video block based on a syntax element indicative of selectively enabling or disabling the scaling process in the bitstream of the video if included; and

perform the conversion by applying the scaling process on the chroma residual samples based on at least one scaling factor in response to the scaling process being enabled, and refraining from applying the scaling process on the chroma residual samples in response to the scaling process being disabled,

wherein the syntax element is included in a picture header associated with the current video block,

wherein in response to a value of the syntax element being equal to one, the application of the scaling process for blocks below a picture level is enabled, and in response to the value of the syntax element being equal to zero, the application of the scaling process for blocks below the picture level is disabled, and

wherein in response to the syntax element being not included in the bitstream, it is inferred that the application of the scaling process for blocks below the picture level is disabled, and a value of the syntax element is inferred to be zero.

15. A non-transitory computer-readable recording medium storing a bitstream of a video which is generated by a method performed by a video processing apparatus, wherein the method comprises:

determining whether to enable or disable a scaling process applied on chroma residual samples of the current chroma video block based on a syntax element indicative of selectively enabling or disabling the scaling process in the bitstream of the video if included; and

generating the bitstream by applying the scaling process on the chroma residual samples based on at least one scaling factor in response to the scaling process being enabled, and refraining from applying the scaling process on the chroma residual samples in response to the scaling process being disabled,

wherein the syntax element is included in a picture header associated with the current video block,

wherein in response to a value of the syntax element being equal to one, the application of the scaling process for blocks below a picture level for blocks is enabled, and in response to the value of the syntax element being equal to zero, the application of the scaling process for blocks below the picture level is disabled, and

wherein in response to the syntax element being not included in the bitstream, it is inferred that the application of the scaling process for blocks below the picture level is disabled, and a value of the syntax element is inferred to be zero.

16. The apparatus of claim 13 , wherein the at least one scaling factor is derived based on an averaged luma variable computed based on neighboring luma samples of a video unit of the video which is determined based on a luma sample corresponding to a top-left sample of the current chroma video block, and wherein the at least one scaling factor is derived by:

checking an availability of one or more neighboring luma blocks of the video unit;

determining, based on the availability of the one or more neighboring luma blocks, whether to retrieve the neighboring luma samples of the video unit, wherein the one or more neighboring luma blocks comprise the neighboring luma samples; and

deriving the at least one scaling factor based on the averaged luma variable computed using the neighboring luma samples by a rounding-based average operation in case that the neighboring samples are available.

17. The non-transitory computer-readable storage medium of claim 14 , wherein the at least one scaling factor is derived based on an averaged luma variable computed based on neighboring luma samples of a video unit of the video which is determined based on a luma sample corresponding to a top-left sample of the current chroma video block, and wherein the at least one scaling factor is derived by:

checking an availability of one or more neighboring luma blocks of the video unit;

determining, based on the availability of the one or more neighboring luma blocks, whether to retrieve the neighboring luma samples of the video unit, wherein the one or more neighboring luma blocks comprise the neighboring luma samples; and

deriving the at least one scaling factor based on the averaged luma variable computed using the neighboring luma samples by a rounding-based average operation in case that the neighboring samples are available.