Use restrictions for cross-component 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 chroma block of a video and a bitstream of the video, a parameter of a cross-component linear model for the chroma block according to a rule;
• and
• perform the conversion based on the determining,
• wherein the rule specifies that the parameter of the cross-component linear model is determined based on neighbouring chroma samples of the chroma block and down-sampled collocated luma samples derived based on variables representing neighbouring luma samples of a luma block corresponding to the chroma block,
• wherein the rule further specifies that when a color format of the chroma block is 4:2:0, the down-sampled collocated luma samples is derived further based on at least one of a value of a first syntax element and a value of a first variable, wherein the first syntax element is included in the bitstream indicating whether chroma sample positions are vertically shifted relative to corresponding luma sample position, and the first variable indicates whether the chroma block locates at a boundary of a coding tree unit comprising the chroma block,
• wherein the rule further specifies that when the color format of the chroma block is 4:2:2, the down-sampled collocated luma samples is derived without considering the value of the first syntax element,
• wherein the rule further specifies that when the value of the first variable indicates that the chroma block locates at the boundary of the coding tree unit comprising the chroma block, the down-sampled collocated luma samples is derived using a same filter, regardless of the value of the first syntax element, for the chroma block having the color format of 4:2:0 and the chroma block having the color format of 4:2:2, and
• wherein the rule further specifies that when the value of the first variable indicates that the chroma block does not locate at the boundary of the coding tree unit comprising the chroma block and the color format of the chroma block is 4:2:0, the down-sampled collocated luma samples are obtained using different filters in response to the first syntax element having different values.

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

• determine, for a conversion between a chroma block of a video and a bitstream of the video, a parameter of a cross-component linear model for the chroma block according to a rule;
• and
• perform the conversion based on the determining,
• wherein the rule specifies that the parameter of the cross-component linear model is determined based on neighbouring chroma samples of the chroma block and down-sampled collocated luma samples derived based on variables representing neighbouring luma samples of a luma block corresponding to the chroma block,
• wherein the rule further specifies that when a color format of the chroma block is 4:2:0, the down-sampled collocated luma samples is derived further based on at least one of a value of a first syntax element and a value of a first variable, wherein the first syntax element is included in the bitstream indicating whether chroma sample positions are vertically shifted relative to corresponding luma sample position, and the first variable indicates whether the chroma block locates at a boundary of a coding tree unit comprising the chroma block,
• wherein the rule further specifies that when the color format of the chroma block is 4:2:2, the down-sampled collocated luma samples is derived without considering the value of the first syntax element,
• wherein the rule further specifies that when the value of the first variable indicates that the chroma block locates at the boundary of the coding tree unit comprising the chroma block, the down-sampled collocated luma samples is derived using a same filter, regardless of the value of the first syntax element, for the chroma block having the color format of 4:2:0 and the chroma block having the color format of 4:2:2, and
• wherein the rule further specifies that when the value of the first variable indicates that the chroma block does not locate at the boundary of the coding tree unit comprising the chroma block and the color format of the chroma block is 4:2:0, the down-sampled collocated luma samples are obtained using different filters in response to the first syntax element having different values.

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 a parameter of a cross-component linear model for a chroma block of the video according to a rule;
• and
• generating the bitstream based on the determining,
• wherein the rule specifies that the parameter of the cross-component linear model is determined based on neighbouring chroma samples of the chroma block and down-sampled collocated luma samples derived based on variables representing neighbouring luma samples of a luma block corresponding to the chroma block,
• wherein the rule further specifies that when a color format of the chroma block is 4:2:0, the down-sampled collocated luma samples is derived further based on at least one of a value of a first syntax element and a value of a first variable, wherein the first syntax element is included in the bitstream indicating whether chroma sample positions are vertically shifted relative to corresponding luma sample position, and the first variable indicates whether the chroma block locates at a boundary of a coding tree unit comprising the chroma block,
• wherein the rule further specifies that when the color format of the chroma block is 4:2:2, the down-sampled collocated luma samples is derived without considering the value of the first syntax element,
• wherein the rule further specifies that when the value of the first variable indicates that the chroma block locates at the boundary of the coding tree unit comprising the chroma block, the down-sampled collocated luma samples is derived using a same filter, regardless of the value of the first syntax element, for the chroma block having the color format of 4:2:0 and the chroma block having the color format of 4:2:2, and
• wherein the rule further specifies that when the value of the first variable indicates that the chroma block does not locate at the boundary of the coding tree unit comprising the chroma block and the color format of the chroma block is 4:2:0, the down-sampled collocated luma samples are obtained using different filters in response to the first syntax element having different values.