Virtual boundary processing for adaptive loop filtering

A method for decoding an image of a video sequence, the method comprising:

• obtaining a set of sample values associated with the image, the set of sample values comprising a current sample value and sample values neighboring the current sample value, wherein the obtained set of sample values is included within a first block of sample values;
• determining a relative location of the current sample value with respect to a virtual boundary, wherein the virtual boundary is defined with respect to a block boundary between the first block of sample values and a second block of sample values, wherein the virtual boundary is parallel with the block boundary and separated from the block boundary by at least one row or column of sample values included in the first block of sample values;
• and
• as a result of determining that the current sample is at the virtual boundary or is one row or column away from the virtual boundary:
• reducing a filter strength for adaptive loop filtering of the current sample value by increasing a right shift value, wherein an increase of the right shift value by N reduces a contribution from adaptive loop filtering by a factor of 2{circumflex over ( )}N;
• and
• adaptive loop filtering the current sample value based on the increased right shift value.

A decoder, the decoder comprising:

• memory;
• and
• processing circuitry, wherein the decoder is configured to perform a method comprising:
• obtaining a set of sample values associated with the image, the set of sample values comprising a current sample value and sample values neighboring the current sample value, wherein the obtained set of sample values is included within a first block of sample values;
• determining a relative location of the current sample value with respect to a virtual boundary, wherein the virtual boundary is defined with respect to a block boundary between the first block of sample values and a second block of sample values, wherein the virtual boundary is parallel with the block boundary and separated from the block boundary by at least one row or column of sample values included in the first block of sample values;
• and
• as a result of determining that the current sample is at the virtual boundary or is one row or column away from the virtual boundary:
• reducing a filter strength for adaptive loop filtering of the current sample value by increasing a right shift value, wherein an increase of the right shift value by N reduces a contribution from adaptive loop filtering by a factor of 2{circumflex over ( )}N;
• and
• adaptive loop filtering the current sample value based on the increased right shift value.

A method for encoding an image of a video sequence, the method comprising:

• obtaining a set of sample values associated with the image, the set of sample values comprising a current sample value and sample values neighboring the current sample value, wherein the obtained set of sample values is included within a first block of sample values;
• determining a relative location of the current sample value with respect to a virtual boundary, wherein the virtual boundary is defined with respect to a block boundary between the first block of sample values and a second block of sample values, wherein the virtual boundary is parallel with the block boundary and separated from the block boundary by at least one row or column of sample values included in the first block of sample values;
• and
• as a result of determining that the current sample is at the virtual boundary or is one row or column away from the virtual boundary:
• reducing a filter strength for adaptive loop filtering of the current sample value by increasing a right shift value, wherein an increase of the right shift value by N reduces a contribution from adaptive loop filtering by a factor of 2{circumflex over ( )}N, and
• adaptive loop filtering the current sample value based on the increased right shift value.

An encoder, the encoder comprising:

• memory;
• and
• processing circuitry, wherein the encoder is configured to perform a method comprising:
• obtaining a set of sample values associated with the image, the set of sample values comprising a current sample value and sample values neighboring the current sample value, wherein the obtained set of sample values is included within a first block of sample values;
• determining a relative location of the current sample value with respect to a virtual boundary, wherein the virtual boundary is defined with respect to a block boundary between the first block of sample values and a second block of sample values, wherein the virtual boundary is parallel with the block boundary and separated from the block boundary by at least one row or column of sample values included in the first block of sample values;
• and
• as a result of determining that the current sample is at the virtual boundary or is one row or column away from the virtual boundary:
• reducing a filter strength for adaptive loop filtering of the current sample value by increasing a right shift value, wherein an increase of the right shift value by N reduces a contribution from adaptive loop filtering by a factor of 2{circumflex over ( )}N, and
• adaptive loop filtering the current sample value based on the increased right shift value.