Chroma intra mode derivation in screen content coding

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:

• derive, for a conversion between a current chroma block of a video and a bitstream of the video, a chroma prediction mode for the current chroma block based on a luma coding information of a first luma block corresponding to the current chroma block;
• and
• perform the conversion based on the derived chroma prediction mode,
• wherein in response to the first luma block being coded with a differential coding mode in which differences between quantized residuals derived with a luma prediction mode and predictions of the quantized residuals are included in the bitstream, the chroma prediction mode is identical to the luma prediction mode,
• wherein the chroma prediction mode being identical to the luma prediction mode is further based on a value of a syntax element intra_chroma_pred_mode, and wherein the syntax element intra_chroma_pred_mode is used to specify a mapping relation between a chroma component intra mode and a luma component intra mode,
• wherein in response to the first luma block being coded with a first prediction mode and/or a variance of the first prediction mode, the chroma prediction mode is derived as a DC mode, and
• wherein in the first prediction mode, reconstructed samples are represented by a set of representative color values, and the set of representative color values comprises at least one of 1) palette predictors, 2) escape samples, or 3) palette information included in the bitstream.

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

• derive, for a conversion between a current chroma block of a video and a bitstream of the video, a chroma prediction mode for the current chroma block based on a luma coding information of a first luma block corresponding to the current chroma block;
• and
• perform the conversion based on the derived chroma prediction mode,
• wherein in response to the first luma block being coded with a differential coding mode in which differences between quantized residuals derived with a luma prediction mode and predictions of the quantized residuals are included in the bitstream, the chroma prediction mode is identical to the luma prediction mode,
• wherein the chroma prediction mode being identical to the luma prediction mode is further based on a value of a syntax element intra_chroma_pred_mode, and wherein the syntax element intra_chroma_pred_mode is used to specify a mapping relation between a chroma component intra mode and a luma component intra mode,
• wherein in response to the first luma block being coded with a first prediction mode and/or a variance of the first prediction mode, the chroma prediction mode is derived as a DC mode, and
• wherein in the first prediction mode, reconstructed samples are represented by a set of representative color values, and the set of representative color values comprises at least one of 1) palette predictors, 2) escape samples, or 3) palette information included in the bitstream.

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:

• deriving, for a current chroma block of a video, the chroma prediction mode for the current chroma block based on a luma coding information of a first luma block corresponding to the current chroma block;
• and
• generating the bitstream based on the derived chroma prediction mode,
• wherein in response to the first luma block being coded with a differential coding mode in which differences between quantized residuals derived with a luma prediction mode and predictions of the quantized residuals are included in the bitstream, the chroma prediction mode is identical to the luma prediction mode,
• wherein the chroma prediction mode being identical to the luma prediction mode is further based on a value of a syntax element intra_chroma_pred_mode, and wherein the syntax element intra_chroma_pred_mode is used to specify a mapping relation between a chroma component intra mode and a luma component intra mode,
• wherein in response to the first luma block being coded with a first prediction mode and/or a variance of the first prediction mode, the chroma prediction mode is derived as a DC mode, and
• wherein in the first prediction mode, reconstructed samples are represented by a set of representative color values, and the set of representative color values comprises at least one of 1) palette predictors, 2) escape samples, or 3) palette information included in the bitstream.