Context modeling of side information for reduced secondary transforms in video

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:

• perform a conversion between a current video region of a video and a bitstream of the video,
• wherein the performance of the conversion includes configuring, only based on a partition type of the current video region, a context model for coding a first bin, the first bin and a second bin included in a bin string corresponding to a first index of a secondary transform tool applied to the current video region,
• wherein the first index indicates an applicability of the secondary transform tool and/or a kernel information of the secondary transform tool, and
• 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
• wherein the secondary transform tool includes applying, during decoding, an inverse secondary transform to an output of dequantization to the video block before applying an inverse primary transform.

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

• perform a conversion between a current video region of a video and a bitstream of the video,
• wherein the performance of the conversion includes configuring, only based on a partition type of the current video region, a context model for coding a first bin, the first bin and a second bin included in a bin string corresponding to a first index of a secondary transform tool applied to the current video region,
• wherein the first index indicates an applicability of the secondary transform tool and/or a kernel information of the secondary transform tool, and
• 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
• wherein the secondary transform tool includes applying, during decoding, an inverse secondary transform to an output of dequantization to the video 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:

• generating the bitstream of the video for a current video region of the video,
• wherein the generating the bitstream includes configuring, only based on a partition type of the current video region, a context model for coding a first bin, the first bin and a second bin included in a bin string corresponding to a first index of a secondary transform tool applied to the current video region,
• wherein the first index indicates an applicability of the secondary transform tool and/or a kernel information of the secondary transform tool, and
• 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
• wherein the secondary transform tool includes applying, during decoding, an inverse secondary transform to an output of dequantization to the video block before applying an inverse primary transform.