Mapping table derivation for fixed filter sets in video coding

1 . A method of decoding a block of video data, the method comprising:

applying a first stage adaptive loop filter (ALF) to a reconstructed sample of a reconstructed block for a color component, wherein applying the first stage ALF comprises:

selecting a first set of filters from a plurality of sets of fixed filters based on a quantization parameter of the block of video data and a second characteristic of the block of video data;

determining a first class index for the reconstructed sample;

selecting a filter from a first set of filters based on the first class index; and

applying the filter from the first set of filters to the reconstructed sample to determine a first intermediate sample value;

applying a second stage ALF to the reconstructed sample of the reconstructed block for the color component, wherein applying the second stage ALF comprises:

determining a second class index for the reconstructed sample;

selecting a second filter from a second set of filters based on the second class index;

applying the second filter to the reconstructed sample to determine a first sample modification value;

determining a second sample modification value based on the first intermediate sample value; and

determining a filtered reconstructed sample based on the reconstructed sample, the first sample modification value, and the second sample modification value.

2 . The method of claim 1 , wherein the second set of filters comprises signaled filters determined based on syntax signaled in the video data.

3 . The method of claim 1 , wherein determining the first class index comprises:

determining an activity value for the reconstructed sample;

determining a direction for the reconstructed sample; and

determining the first class index based on the activity value and the direction.

4 . The method of claim 1 , wherein determining the filtered reconstructed sample based on the reconstructed sample, the first sample modification value, and the second sample modification value comprises adding the first sample modification value and the second sample modification value to the reconstructed sample.

5 . The method of claim 1 , wherein determining the second sample modification value based on the first intermediate sample value comprises clipping the second sample modification value to determine a clipped sample modification value and adding the first sample modification value and the clipped sample modification value to the reconstructed sample.

6 . The method of claim 1 , wherein determining the second sample modification value based on the first intermediate sample value comprises determining a difference between the reconstructed sample and the first intermediate sample value.

7 . The method of claim 1 , further comprising:

adding a predicted sample value to a residual sample value to determine the reconstructed sample; and

applying a deblocking filter to a sum of the predicted sample value and the residual sample value to determine the reconstructed sample.

8 . The method of claim 1 , wherein the second characteristic of the block comprises a slice type for a slice that includes the block.

9 . The method of claim 1 , wherein the second characteristic of the block comprises a prediction mode for a prediction block of the block.

10 . The method of claim 1 , wherein the second characteristic of the block comprises one of a temporal layer ID for a layer that includes the block, a transform type for a residual block of the reconstructed block, a multiple transform selection index for the residual block of the reconstructed block, or a hash hit percentage for the reconstructed block.

11 . The method of claim 1 , wherein selecting the first set of filters from the plurality of sets of fixed filters based on the quantization parameter of the block of video data and the second characteristic of the block of video data comprises:

determining a mapping table based on the second characteristic of the block; and

selecting a set of filters from the mapping based on the quantization parameter.

12 . The method of claim 1 , further comprising:

outputting a decoded picture of the video data, wherein the decoded picture comprises the filtered reconstructed sample.

13 . The method of claim 1 , wherein the method of decoding is performed as part of a video encoding process.

14 . 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:

select a first set of filters from a plurality of sets of fixed filters based on a quantization parameter of the block of video data and a second characteristic of the block of video data;

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 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.

15 . The device of claim 14 , wherein the one or more processors are further configured to:

add a predicted sample value to a residual sample value to determine the reconstructed sample; and

apply a deblocking filter to a sum of the predicted sample value and the residual sample value to determine the reconstructed sample.

16 . The device of claim 14 , wherein the second characteristic of the block comprises a slice type for a slice that includes the block.

17 . The device of claim 14 , wherein the second characteristic of the block comprises a prediction mode for a prediction block of the block.

18 . The device of claim 14 , wherein the second characteristic of the block comprises one of a temporal layer ID for a layer that includes the block, a transform type for a residual block of the reconstructed block, a multiple transform selection index for the residual block of the reconstructed block, or a hash hit percentage for the reconstructed block.

19 . The device of claim 14 , wherein to select the first set of filters from the plurality of sets of fixed filters based on the quantization parameter of the block of video data and the second characteristic of the block of video data, the one or more processors are further configured to:

determine a mapping table based on the second characteristic of the block; and

select a set of filters from the mapping based on the quantization parameter.

20 . The device of claim 15 , wherein the device comprises a video encoder.