Sample padding for cross-component adaptive loop filtering

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 unit of a video and a bitstream of the video, that a mirrored padding process for padding an unavailable luma sample which is unavailable due to a virtual boundary is used during an application of a cross-component adaptive loop filtering (CC-ALF) tool to the video unit, in a case that the virtual boundary is applied to the video unit;
• and
• perform the conversion based on the determination,
• wherein the mirrored padding process includes padding a corresponding sample of the unavailable luma sample and considering the corresponding sample as unavailable even when the corresponding sample is available for the virtual boundary, whereby the corresponding sample is located in a filter support region in the CC-ALF tool, and
• wherein the unavailable luma sample is denoted as located at row M in the video unit and the corresponding sample to be padded is denoted as located at row N in the video unit, and wherein d (C,M)=d (N,C), whereby d (x,y) denotes a distance between row x and row y, wherein C represents a row where a representative luma sample is located in the video unit, where C, M and N are integers and M is not equal to N, and wherein the representative luma sample is a collocated luma sample of a chroma sample to be filtered in the video unit.

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

• determine, for a conversion between a video unit of a video and a bitstream of the video, that a mirrored padding process for padding an unavailable luma sample which is unavailable due to a virtual boundary is used during an application of a cross-component adaptive loop filtering (CC-ALF) tool to the video unit, in a case that the virtual boundary is applied to the video unit;
• and
• perform the conversion based on the determination,
• wherein the mirrored padding process includes padding a corresponding sample of the unavailable luma sample and considering the corresponding sample as unavailable even when the corresponding sample is available for the virtual boundary, whereby the corresponding sample is located in a filter support region in the CC-ALF tool, and
• wherein the unavailable luma sample is denoted as located at row M in the video unit and the corresponding sample to be padded is denoted as located at row N in the video unit, and wherein d (C,M)=d (N,C), whereby d (x,y) denotes a distance between row x and row y, wherein C represents a row where a representative luma sample is located in the video unit, where C, M and N are integers and M is not equal to N, and wherein the representative luma sample is a collocated luma sample of a chroma sample to be filtered in the video unit.

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 mirrored padding process for padding an unavailable luma sample which is unavailable due to a virtual boundary is used during an application of a cross-component adaptive loop filtering (CC-ALF) tool to a video unit of a video, in a case that the virtual boundary is applied to the video unit;
• and
• generating the bitstream based on the determining,
• wherein the mirrored padding process includes padding a corresponding sample of the unavailable luma sample and considering the corresponding sample as unavailable even when the corresponding sample is available for the virtual boundary, whereby the corresponding sample is located in a filter support region in the CC-ALF tool, and
• wherein the unavailable luma sample is denoted as located at row M in the video unit and the corresponding sample to be padded is denoted as located at row N in the video unit, and wherein d (C,M)=d (N,C), whereby d (x,y) denotes a distance between row x and row y, wherein C represents a row where a representative luma sample is located in the video unit, where C, M and N are integers and M is not equal to N, and wherein the representative luma sample is a collocated luma sample of a chroma sample to be filtered in the video unit.