Signaling of reshaping information in video processing

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, that a scaling process is applied on chroma residual samples of the current chroma video block;
• and
• perform the conversion by applying the scaling process on the chroma residual samples based on at least one scaling factor varScale,
• wherein in the scaling process, the chroma residual samples are scaled before being used to reconstruct the current chroma video block, and
• wherein the at least one scaling factor varScale is derived based on a variable invAvgLuma which is derived using specific luma samples by an offset-based average operation, and the variable invAvgLuma is calculated depending on a color format of the video, the color format of the video being 4:4:4 format, 4:2:0 format, or 4:2:2 format,
• wherein the calculated value of invAvgLuma is dependent on position variables used for determining the specific luma samples, and the values of the position variables are determined using the color format of the video, different values of the position variables corresponding to different color formats including 4:4:4 format, 4:2:0 format, and 4:2:2 format, and
• wherein the scaling process is based on a first piecewise linear model, and wherein an index identifies a piece to which the variable invAvgLuma belongs, and the at least one scaling factor varScale is derived based on the index.

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, that a scaling process is applied on chroma residual samples of the current chroma video block;
• and
• perform the conversion by applying the scaling process on the chroma residual samples based on at least one scaling factor varScale,
• wherein in the scaling process, the chroma residual samples are scaled before being used to reconstruct the current chroma video block, and
• wherein the at least one scaling factor varScale is derived based on a variable invAvgLuma which is derived using specific luma samples by an offset-based average operation, and the variable invAvgLuma is calculated depending on a color format of the video, the color format of the video being 4:4:4 format, 4:2:0 format, or 4:2:2 format,
• wherein the calculated value of invAvgLuma is dependent on position variables used for determining the specific luma samples, and the values of the position variables are determined using the color format of the video, different values of the position variables corresponding to different color formats including 4:4:4 format, 4:2:0 format, and 4:2:2 format, and
• wherein the scaling process is based on a first piecewise linear model, and wherein an index identifies a piece to which the variable invAvgLuma belongs, and the at least one scaling factor varScale is derived based on the index.

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 that a scaling process is applied on chroma residual samples of a current chroma video block of a video region of a video;
• and
• generating the bitstream by applying the scaling process on the chroma residual samples based on at least one scaling factor varScale,
• wherein in the scaling process, the chroma residual samples are scaled before being used to reconstruct the current chroma video block, and
• wherein the at least one scaling factor varScale is derived based on a variable invAvgLuma which is derived using specific luma samples by an offset-based average operation, and the variable invAvgLuma is calculated depending on a color format of the video, the color format of the video being 4:4:4 format, 4:2:0 format, or 4:2:2 format,
• wherein the calculated value of invAvgLuma is dependent on position variable sused for determining the specific luma samples, and the values of the position variables are determined using the color format of the video, different values of the position variables corresponding to different color formats including 4:4:4 format, 4:2:0 format, and 4:2:2 format, and
• wherein the scaling process is based on a first piecewise linear model, and wherein an index identifies a piece to which the variable invAvgLuma belongs, and the at least one scaling factor varScale is derived based on the index.