Methods for simplifying adaptive loop filter in video coding

A video decoding method, comprising:

• selecting, from a plurality of pixels of an 8×8 pixel window, a subset of pixels for classifying a video block associated with the 8×8 pixel window, wherein the subset of pixels is selected by skipping, in a diagonal direction, alternate lines of the plurality of pixels of the 8×8 pixel window;
• obtaining a sum of diagonal variations using the subset of pixels that is selected by skipping, in the diagonal direction, alternate lines of the plurality of pixels of the 8×8 pixel window;
• determining a diagonal gradient based on the sum of diagonal variations;
• classifying the video block for adaptive loop filtering (ALF) based on the determined diagonal gradient;
• performing ALF on the video block based on the classification of the video block;
• and
• decoding a frame comprising the video block.

A video encoding device comprising:

• a processor configured to:
• select, from a plurality of pixels of an 8×8 pixel window, a subset of pixels for classifying a video block associated with the 8×8 pixel window, wherein the subset of pixels is selected by skipping, in a diagonal direction, alternate lines of the plurality of pixels of the 8×8 pixel window;
• obtain a sum of diagonal variations using the subset of pixels that is selected by skipping, in the diagonal direction, alternate lines of the plurality of pixels of the 8×8 pixel window;
• determine a diagonal gradient based on the sum of diagonal variations;
• classify the video block for adaptive loop filtering (ALF) based on the determined diagonal gradient;
• perform ALF on the video block based on the classification of the video block;
• and
• generate a residual based on the video block.

A video encoding method, comprising:

• selecting, from a plurality of pixels of an 8×8 pixel window, a subset of pixels for classifying a video block associated with the 8×8 pixel window, wherein the subset of pixels is selected by skipping, in a diagonal direction, alternate lines of the plurality of pixels of the 8×8 pixel window;
• obtaining a sum of diagonal variations using the subset of pixels that is selected by skipping, in the diagonal direction, alternate lines of the plurality of pixels of the 8×8 pixel window;
• determining a diagonal gradient based on the sum of diagonal variations;
• classifying the video block for adaptive loop filtering (ALF) based on the determined diagonal gradient;
• performing ALF on the video block based on the classification of the video block;
• and
• generating a residual based on the video block.