Adaptive loop filter with fixed filters

A device for decoding video data, the device comprising:

• a memory configured to store video data;
• one or more processors implemented in circuitry and configured to:
• apply a first stage adaptive loop filter (ALF) to a reconstructed sample of a reconstructed block for a color component, wherein to apply the first stage ALF, the one or more processors are further configured to:
• determine a first class index for the reconstructed sample;
• select a filter from a first set of filters based on the first class index;
• and
• apply the filter from the first set of filters to the reconstructed sample to determine a first intermediate sample value;
• apply a second stage ALF to the reconstructed sample of the reconstructed block for the color component, wherein to apply the second stage ALF, the one or more processors are further configured to:
• determine a second class index for the reconstructed sample;
• select a second filter from a second set of filters based on the second class index;
• apply the second filter to the reconstructed sample to determine a first sample modification value;
• determine a second sample modification value based on a difference between the reconstructed sample and the first intermediate sample value;
• and
• determine a filtered reconstructed sample based on the reconstructed sample, the first sample modification value, and the second sample modification value.

A non-transitory computer-readable storage medium storing instructions that when executed by one or more processors cause the one or more processors to:

• apply a first stage adaptive loop filter (ALF) to a reconstructed sample of a reconstructed block for a color component, wherein to apply the first stage ALF, the one or more processors are further configured to:
• determine a first class index for the reconstructed sample;
• select a filter from a first set of filters based on the first class index;
• and
• apply the filter from the first set of filters to the reconstructed sample to determine a first intermediate sample value;
• apply a second stage ALF to the reconstructed sample of the reconstructed block for the color component, wherein to apply the second stage ALF, the one or more processors are further configured to:
• determine a second class index for the reconstructed sample;
• select a second filter from a second set of filters based on the second class index;
• apply the second filter to the reconstructed sample to determine a first sample modification value;
• determining a second sample modification value based on a difference between the reconstructed sample and the first intermediate sample value;
• and
• determine a filtered reconstructed sample based on the reconstructed sample, the first sample modification value, and the second sample modification value.