Restricted upsampling process in matrix-based intra prediction

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 video block of a video and a bitstream of the video, that a first intra mode is applied on the video block of the video, wherein a process in the first intra mode includes a matrix vector multiplication operation and followed by an upsampling operation to generate prediction samples for the video block of the video;
• and
• perform the conversion based on the prediction samples;
• wherein inputs to the upsampling operation includes neighboring reference samples of the video block of the video, and
• wherein inputs to the upsampling operation includes:
• a set of left neighboring reference samples selected from left neighboring reference samples of the video block, and
• a set of above neighboring reference samples selected from above neighboring reference samples of the video block.

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

• determine, for a conversion between a video block of a video and a bitstream of the video, that a first intra mode is applied on the video block of the video, wherein process in the first intra mode includes a matrix vector multiplication operation and followed by an upsampling operation to generate prediction samples for the video block of the video;
• and
• perform the conversion based on the prediction samples;
• wherein inputs to the upsampling operation includes neighboring reference samples of the video block of the video, and
• wherein inputs to the upsampling operation includes:
• a set of left neighboring reference samples selected from left neighboring reference samples of the video block, and
• a set of above neighboring reference samples selected from above neighboring reference samples of the video block.

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 first intra mode is applied on a video block of the video, wherein process in the first intra mode includes a matrix vector multiplication operation and followed by an upsampling operation to generate prediction samples for the video block of the video;
• and
• generating the bitstream based on the determining;
• wherein inputs to the upsampling operation includes neighboring reference samples of the video block of the video, and
• wherein inputs to the upsampling operation includes:
• a set of left neighboring reference samples selected from left neighboring reference samples of the video block, and
• a set of above neighboring reference samples selected from above neighboring reference samples of the video block.