Selection of matrices for reduced secondary transform in video 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:

• make a first determination, during a conversion between a current chroma block of a video and a bitstream of the video, that the current chroma block is coded with a cross-component linear model intra prediction mode;
• make a second determination, that a pre-defined intra prediction mode is utilized in a secondary transform process for the current chroma block in a case that a coding mode of a luma block covering a luma sample located to correspond to a chroma sample at a center of the current chroma block is a specific coding mode;
• determine a transform set for the secondary transform process based on the pre-defined intra prediction mode;
• and
• perform the conversion based on the transform set,
• wherein, during the secondary transform process:
• for encoding, a forward secondary transform is applied to an output of a forward primary transform applied to a residual of the current chroma block prior to quantization, or
• for decoding, an inverse secondary transform is applied to an output of dequantization of the current chroma block before applying an inverse primary transform.

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

• make a first determination, during a conversion between a current chroma block of a video and a bitstream of the video, that the current chroma block is coded with a cross-component linear model intra prediction mode;
• make a second determination, that a pre-defined intra prediction mode is utilized in a secondary transform process for the current chroma block in a case that a coding mode of a luma block covering a luma sample located to correspond to a chroma sample at a center of the current chroma block is a specific coding mode;
• determine a transform set for the secondary transform process based on the pre-defined intra prediction mode;
• and
• perform the conversion based on the transform set,
• wherein, during the secondary transform process:
• for encoding, a forward secondary transform is applied to an output of a forward primary transform applied to a residual of the current chroma block prior to quantization, or
• for decoding, an inverse secondary transform is applied to an output of dequantization of the current chroma block before applying an inverse primary transform.

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:

• making a first determination that a current chroma block of the video is coded with a cross-component linear model intra prediction mode;
• making a second determination, that a pre-defined intra prediction mode is utilized in a secondary transform process for the current chroma block in a case that a coding mode of a luma block covering a luma sample located to correspond to a chroma sample at a center of the current chroma block is a specific coding mode;
• determining a transform set for the secondary transform process based on the pre-defined intra prediction mode;
• and
• generating the bitstream based on the transform set,
• wherein, during the secondary transform process:
• for encoding, a forward secondary transform is applied to an output of a forward primary transform applied to a residual of the current chroma block prior to quantization, or
• for decoding, an inverse secondary transform is applied to an output of dequantization of the current chroma block before applying an inverse primary transform.