Filter shape switch for adaptive loop filter in video coding

1 . A method for processing video data comprising:

determining to apply a filter to a video unit, wherein the filter employs an extended tap for a first sample in the video unit and a spatial tap for a second sample in the video unit; and

performing a conversion between a visual media data and a bitstream based on the video unit.

2 . The method of claim 1 , wherein the extended tap is applied to only luma components of the first sample.

3 . The method of claim 1 , wherein a shape of the spatial tap comprises a cross shape.

4 . The method of claim 1 , wherein a shape of the extended tap comprises a diamond shape.

5 . The method of claim 1 , wherein a shape of the extended tap comprises a cross shape.

6 . The method of claim 1 , wherein the extended tap is a first extended tap, wherein the filter employs a second extended tap, and wherein the first extended tap receives different input than the second extended tap.

7 . The method of claim 1 , wherein the filter comprises 20 spatial taps and 5 extended taps.

8 . The method of claim 1 , wherein the filter comprises 20 spatial taps and 9 extended taps.

9 . The method of claim 1 , wherein the filter comprises 20 spatial taps and 2 extended taps.

10 . The method of claim 1 , wherein a first syntax element is signaled to indicate whether at least one extended tap inside the filter is enabled.

11 . The method of claim 10 , wherein the first syntax element is binarized by unary code, or truncated unary code, or fixed-length code, or exponential Golomb code, or truncated exponential Golomb code.

12 . The method of claim 1 , further comprising using an intermediate filtering result as an input for the extended tap, wherein the intermediate filtering result comprises result of an offline-trained filter of an adaptive loop filter.

13 . The method of claim 1 , wherein an applied filter shape is different for a first coding tree unit (CTU) or coding tree block (CTB) than for a second CTU or CTB.

14 . The method of claim 1 , wherein a syntax element structure contains indications of shapes of the extended tap and/or the spatial tap.

15 . The method of claim 14 , wherein the syntax element structure signals:

a number of enabled filters with a selected filter shape candidate;

class merging results of the filters with the selected filter shape candidate; or

coefficients and parameters of filters with the selected filter shape candidate.

16 . The method of claim 1 , wherein the conversion includes encoding the visual media data into the bitstream.

17 . The method of claim 1 , wherein the conversion includes decoding the visual media data from the bitstream.

18 . An apparatus for processing video data comprising:

one or more processors; and

one or more non-transitory memories coupled to the one or more processor and storing instructions thereon that, when executed by the one or more processors, cause the apparatus to be configured to:

determine to apply a filter to a video unit, wherein the filter employs an extended tap for a first sample in the video unit and a spatial tap for a second sample in the video unit; and

perform a conversion between a visual media data and a bitstream based on the video unit.

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

determine to apply a filter to a video unit, wherein the filter employs an extended tap for a first sample in the video unit and a spatial tap for a second sample in the video unit; and

perform a conversion between a visual media data and a bitstream based on the video unit.

20 . A non-transitory computer-readable recording medium storing a bitstream of a video that is generated by a method performed by a video processing apparatus, wherein the method comprises:

determining to apply a filter to a video unit, wherein the filter employs an extended tap for a first sample in the video unit and a spatial tap for a second sample in the video unit; and

generating the bitstream based on the determining.