Method and apparatus for video decoding using a nominal directional mode and an angular offset

1. A method for video decoding in a decoder, comprising:

decoding coding information of a block to be reconstructed from a coded video bitstream, the coding information indicating intra prediction information for the block; and

when the block is coded with a directional mode,

determining the directional mode based on a nominal mode and an angular offset, the decoded coding information indicating the nominal mode and the angular offset,

determining a non-separable transform for the block based on the nominal mode, wherein the non-separable transform is a non-separable secondary transform, and

reconstructing the block based on the determined directional mode and the non-separable transform, wherein the reconstructing further includes applying the non-separable secondary transform only to a first N transform coefficients that are consecutively arranged along a scanning order used for entropy coding, the first transform coefficients of the block, wherein N is an arbitrary positive integer, and not applying the non-separable secondary transform to remaining transform coefficients of the block.

2. The method of claim 1 , wherein the determining the non-separable transform further comprises determining a transform set mode that is associated with the nominal mode, the determined transform set mode indicating a set of one or more non-separable transforms that includes the non-separable transform.

3. The method of claim 2 , wherein

the decoded coding information further indicates a non-separable transform index; and

the reconstructing the block further includes:

determining the non-separable transform in the set of one or more non-separable transforms, based on the non-separable transform index; and

reconstructing the block based on the directional mode and the determined non-separable transform.

4. The method of claim 1 , wherein the non-separable secondary transform does not apply to a PAETH mode.

5. The method of claim 1 , wherein the reconstructing the block based on the directional mode and the non-separable secondary transform further comprises:

applying the non-separable secondary transform only to the first transform coefficients in the block, each of the first transform coefficients having a coordinate (x, y), and a sum of the respective x and y coordinates being less than a threshold value.

6. The method of claim 1 , wherein a horizontal transform and a vertical transform in a primary transform for the block are included in a subset of a set of line graph transforms.

7. The method of claim 1 , wherein

the block includes the first transform coefficients obtained with the non-separable secondary transform and second transform coefficients obtained without the non-separable secondary transform; and

the reconstructing the block further includes entropy decoding the first transform coefficients and the second transform coefficients separately.

8. The method of claim 1 , wherein

non-directional modes include a DC mode, a PAETH mode, a SMOOTH mode, a SMOOTH_V mode, a SMOOTH_H mode, recursive filtering modes, and a chroma from luma (CfL) mode, the DC mode, the PAETH mode, the SMOOTH mode, the SMOOTH_V mode, and the SMOOTH_H mode being based on averaging of neighboring samples of the block, and

when the block is coded with one of the non-directional modes,

determining a set of one or more non-separable transforms associated with the one of the non-directional modes, one of (a) at least another one of the non-directional modes and (b) a non-directional nominal mode being associated with the set of one or more non-separable transforms,

determining a particular non-separable transform in the set of one or more non-separable transforms based on a non-separable transform index indicated by the coding information, and

reconstructing the block based on the non-directional mode and the particular non-separable transform.

9. The method of claim 8 , wherein the one of the non-directional modes and the one of the at least another one of the non-directional modes and the non-directional nominal mode include one of (a) the recursive filtering modes and one of the DC mode and the SMOOTH mode, (b) the SMOOTH mode, the SMOOTH_H mode, and the SMOOTH_V mode, (c) the SMOOTH mode, the SMOOTH_H mode, the SMOOTH_V mode, and the PAETH mode, (d) the recursive filtering modes, the SMOOTH mode, and the PAETH mode, (e) a vertical mode for the nominal mode and the SMOOTH_V mode, (f) a horizontal mode for the nominal mode and the SMOOTH_H mode, and (v) the CfL mode and one of the DC mode, the SMOOTH mode, and the PAETH mode.

10. An apparatus for video decoding, comprising:

processing circuitry configured to

decode coding information of a block to be reconstructed from a coded video bitstream, the coding information indicating intra prediction information for the block; and

when the block is coded with a directional mode,

determine the directional mode based on a nominal mode and an angular offset, the decoded coding information indicating the nominal mode and the angular offset,

determine a non-separable transform for the block based on the nominal mode, wherein the non-separable transform is a non-separable secondary transform, and

reconstruct the block based on the determined directional mode and the non-separable transform,

wherein the processing circuitry is further configured to apply the non-separable secondary transform only to a first N transform coefficients that are consecutively arranged along a scanning order used for entropy coding the first transform coefficients of the block, wherein N is an arbitrary positive integer, and not apply the non-separable secondary transform to remaining transform coefficients of the block.

11. The apparatus of claim 10 , wherein the processing circuitry is further configured to determine a transform set mode that is associated with the nominal mode, the determined transform set mode indicating a set of one or more non-separable transforms that includes the non-separable transform.

12. The apparatus of claim 11 , wherein

the decoded coding information further indicates a non-separable transform index; and

the processing circuitry is further configured to:

determine the non-separable transform in the set of one or more non-separable transforms, based on the non-separable transform index; and

reconstruct the block based on the directional mode and the determined non-separable transform.

13. The apparatus of claim 10 , wherein the processing circuitry is further configured to:

apply the non-separable secondary transform only to the first transform coefficients in the block, each of the first transform coefficients having a coordinate (x, y), and a sum of the respective x and y coordinates being less than a threshold value.

14. The apparatus of claim 10 , wherein a horizontal transform and a vertical transform in a primary transform for the block are included in a subset of a set of line graph transforms.

15. The apparatus of claim 10 , wherein

the block includes the first transform coefficients obtained with the non-separable secondary transform and second transform coefficients obtained without the non-separable secondary transform; and

the processing circuitry is configured to entropy decode the first transform coefficients and the second transform coefficients separately.

16. The apparatus of claim 10 , wherein

non-directional modes include a DC mode, a PAETH mode, a SMOOTH mode, a SMOOTH_V mode, a SMOOTH_H mode, recursive filtering modes, and a chroma from luma (CfL) mode, the DC mode, the PAETH mode, the SMOOTH mode, the SMOOTH_V mode, and the SMOOTH_H mode being based on averaging of neighboring samples of the block, and

when the block is coded with one of the non-directional modes, the processing circuitry is configured to:

determine a set of one or more non-separable transforms associated with the one of the non-directional modes, one of (a) at least another one of the non-directional modes and (b) a non-directional nominal mode being associated with the set of one or more non-separable transforms,

determine a particular non-separable transform in the set of one or more non-separable transforms based on a non-separable transform index indicated by the coding information, and

reconstruct the block based on the non-directional mode and the particular non-separable transform.

17. The apparatus of claim 16 , wherein the one of the non-directional modes and the one of the at least another one of the non-directional modes and the non-directional nominal mode include one of (a) the recursive filtering modes and one of the DC mode and the SMOOTH mode, (b) the SMOOTH mode, the SMOOTH_H mode, and the SMOOTH_V mode, (c) the SMOOTH mode, the SMOOTH_H mode, the SMOOTH_V mode, and the PAETH mode, (d) the recursive filtering modes, the SMOOTH mode, and the PAETH mode, (e) a vertical mode for the nominal mode and the SMOOTH_V mode, (f) a horizontal mode for the nominal mode and the SMOOTH_H mode, and (v) the CfL mode and one of the DC mode, the SMOOTH mode, and the PAETH mode.

18. The method of claim 1 , wherein the non-separable secondary transform does not apply to a recursive filtering mode.