Syntax signaling and parsing based on colour component

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, for a conversion between a current chroma block of a video and a bitstream of the video, whether a first side information about a secondary transform tool is included in the bitstream for the current chroma block by checking whether one or more conditions is satisfied;
• and
• perform the conversion at least based on the determine,
• wherein the secondary transform tool includes applying, during encoding, a forward secondary transform to an output of a forward primary transform applied to a residual of a video block prior to quantization, or applying, during decoding, an inverse secondary transform to an output of dequantization of the video block before applying an inverse primary transform,
• wherein the checking whether the one or more conditions is satisfied comprises checking whether a tree type of the current chroma block is dual tree chroma,
• wherein, in response to the tree type of the current chroma block being dual tree chroma, whether the first side information is included in the bitstream for the current chroma block is decoupled from whether a coding mode of a corresponding luma block is a first coding mode, and
• wherein, in the first coding mode, prediction samples of a block are derived by performing a boundary downsampling operation on reference samples of the block based on a size of the block, followed by a matrix vector multiplication operation, and selectively followed by an upsampling operation.

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

• determine, for a conversion between a current chroma block of a video and a bitstream of the video, whether a first side information about a secondary transform tool is included in the bitstream for the current chroma block by checking whether one or more conditions is satisfied;
• and
• perform the conversion at least based on the determine,
• wherein the secondary transform tool includes applying, during encoding, a forward secondary transform to an output of a forward primary transform applied to a residual of a video block prior to quantization, or applying, during decoding, an inverse secondary transform to an output of dequantization of the video block before applying an inverse primary transform,
• wherein the checking whether the one or more conditions is satisfied comprises checking whether a tree type of the current chroma block is dual tree chroma,
• wherein, in response to the tree type of the current chroma block being dual tree chroma, whether the first side information is included in the bitstream for the current chroma block is decoupled from whether a coding mode of a corresponding luma block is a first coding mode, and
• wherein, in the first coding mode, prediction samples of a block are derived by performing a boundary downsampling operation on reference samples of the block based on a size of the block, followed by a matrix vector multiplication operation, and selectively followed by an upsampling operation.

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, for a current chroma block of a video, whether a first side information about a secondary transform tool is included in the bitstream for the current chroma block by checking whether one or more conditions is satisfied;
• and
• generating the bitstream at least based on the determining,
• wherein the secondary transform tool includes applying, during encoding, a forward secondary transform to an output of a forward primary transform applied to a residual of a video block prior to quantization, or applying, during decoding, an inverse secondary transform to an output of dequantization of the video block before applying an inverse primary transform,
• wherein the checking whether the one or more conditions is satisfied comprises checking whether a tree type of the current chroma block is dual tree chroma,
• wherein, in response to the tree type of the current chroma block being dual tree chroma, whether the first side information is included in the bitstream for the current chroma block is decoupled from whether a coding mode of a corresponding luma block is a first coding mode, and
• wherein, in the first coding mode, prediction samples of a block are derived by performing a boundary downsampling operation on reference samples of the block based on a size of the block, followed by a matrix vector multiplication operation, and selectively followed by an upsampling operation.