Padding process at unavailable sample locations in 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 current block of a video and a bitstream of the video, that one or more above-left neighboring samples outside the current block are unavailable for a coding tool applied to the current block;
• pad the one or more above-left neighboring samples with above neighboring samples outside the current block, in case that left neighboring samples outside the current block are available and the above neighboring samples are available;
• and
• perform the conversion based on the padded one or more above-left neighboring samples, wherein the padded one or more above-left neighboring samples are used for a classification operation in an adaptive loop filter process of the coding tool, wherein the padded one or more above-left neighboring samples are used to determine a classification filter index in the classification operation, and the classification filter index is utilized to determine a filtering coefficient set,
• wherein during the classification operation, a classification of an M×N region of the current block is determined based on gradients of a subset of samples of a P×Q region including the M×N region, wherein the current block is a coding tree block, where the M×N region is a 4*4 region and the P×Q region is an 8*8 region, and
• wherein when there is a sample in a 4×4 block located at a left boundary of the current block, left-most two columns of samples in an 8×8 region are padded, or when there is a sample in a 4×4 block located at a top boundary of the current block, top-most two lines of samples in a 8×8 region are padded.

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

• determine, for a conversion between a current block of a video and a bitstream of the video, that one or more above-left neighboring samples outside the current block are unavailable for a coding tool applied to the current block;
• pad the one or more above-left neighboring samples with above neighboring samples outside the current block, in case that left neighboring samples outside the current block are available and the above neighboring samples are available;
• and
• perform the conversion based on the padded one or more above-left neighboring samples,
• wherein the padded one or more above-left neighboring samples are used for a classification operation in an adaptive loop filter process of the coding tool, wherein the padded one or more above-left neighboring samples are used to determine a classification filter index in the classification operation, and the classification filter index is utilized to determine a filtering coefficient set,
• wherein during the classification operation, a classification of an M×N region of the current block is determined based on gradients of a subset of samples of a P×Q region including the M×N region, wherein the current block is a coding tree block, where the M×N region is a 4*4 region and the P×Q region is an 8*8 region, and
• wherein when there is a sample in a 4×4 block located at a left boundary of the current block, left-most two columns of samples in an 8×8 region are padded, or when there is a sample in a 4×4 block located at a top boundary of the current block, top-most two lines of samples in a 8×8 region are padded.

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 block of a video, that one or more above-left neighboring samples outside the current block are unavailable for a coding tool applied to the current block;
• padding the one or more above-left neighboring samples with above neighboring samples outside the current block, in case that left neighboring samples outside the current block are available and the above neighboring samples are available;
• and
• generating the bitstream based on the padded one or more above-left neighboring samples,
• wherein the padded one or more above-left neighboring samples are used for a classification operation in an adaptive loop filter process of the coding tool, wherein the padded one or more above-left neighboring samples are used to determine a classification filter index in the classification operation, and the classification filter index is utilized to determine a filtering coefficient set,
• wherein during the classification operation, a classification of an M×N region of the current block is determined based on gradients of a subset of samples of a P×Q region including the M×N region, wherein the current block is a coding tree block, where the M×N region is a 4*4 region and the P×Q region is an 8*8 region, and
• wherein when there is a sample in a 4×4 block located at a left boundary of the current block, left-most two columns of samples in an 8×8 region are padded, or when there is a sample in a 4×4 block located at a top boundary of the current block, top-most two lines of samples in a 8×8 region are padded.