Coupled primary and secondary transform

1. A method of controlling residual coding for decoding or encoding of a video sequence, the method being performed by at least one processor, and the method comprising:

determining whether a secondary transform is used, based on a primary transform that is set;

based on the secondary transform being determined to be used:

setting a flag indicating whether the secondary transform is used to indicate that the secondary transform is used;

determining the secondary transform, based on the set primary transform and an intra prediction mode of a current block of the video sequence; and

performing the residual coding of a luma coding block of the current block and a chroma coding block of the current block, using the set primary transform and the determined secondary transform;

determining whether a first intra prediction direction of the luma coding block is aligned with a second intra prediction direction of the chroma coding block, each of the luma coding block and the chroma coding block having a same coordinate of the current block; and

based on the first intra prediction direction being determined to be not aligned with the second intra prediction direction, setting the primary transform to a default transform.

2. The method of claim 1 , wherein the determining whether the first intra prediction direction of the luma coding block is aligned with the second intra prediction direction of the chroma coding block comprises determining that the first intra prediction direction is aligned with the second intra prediction direction, based on both the first intra prediction direction and the second intra prediction direction corresponding to one of angular prediction modes, a planar mode and a DC mode.

3. The method of claim 1 , wherein the determining whether the first intra prediction direction of the luma coding block is aligned with the second intra prediction direction of the chroma coding block comprises determining that the first intra prediction direction is aligned with the second intra prediction direction, based on both the first intra prediction direction and the second intra prediction direction corresponding to a same intra prediction mode.

4. The method of claim 1 , wherein the default transform is one of a discrete cosine transform (DCT)-2, an identity transform that scales an input by a pre-defined constant, a discrete sine transform (DST)-7 and a DCT-8.

5. The method of claim 1 , further comprising, based on the secondary transform being determined to not be used, determining that the flag indicates that the secondary transform is not used.

6. The method of claim 1 , wherein the determining whether the secondary transform is used comprises, based on the set primary transform being one of a discrete cosine transform (DCT)-2, a discrete sine transform (DST)-7, a DCT-8, a DST-4, a DCT-4 and an identity transform that scales an input by a pre-defined constant, determining that the secondary transform is not used.

7. An apparatus for controlling residual coding for decoding or encoding of a video sequence, the apparatus comprising:

at least one memory configured to store computer program code; and

at least one processor configured to access the at least one memory and operate according to the computer program code, the computer program code comprising:

first determining code configured to cause the at least one processor to determine whether a secondary transform is used, based on a primary transform that is set;

first setting code configured to cause the at least one processor to, based on the secondary transform being determined to be used, set a flag indicating whether the secondary transform is used to indicate that the secondary transform is used;

second determining code configured to cause the at least one processor to, based on the secondary transform being determined to be used, determine the secondary transform, based on the set primary transform and an intra prediction mode of a current block of the video sequence;

performing code configured to cause the at least one processor to perform the residual coding of a luma coding block of the current block and a chroma coding block of the current block, using the set primary transform and the determined secondary transform;

third determining code configured to cause the at least one processor to determine whether a first intra prediction direction of the luma coding block is aligned with a second intra prediction direction of the chroma coding block, each of the luma coding block and the chroma coding block having a same coordinate of the current block; and

second setting code configured to cuase the at least one processor to, based on the first intra prediction direction being determined to be not aligned with the second intra prediction direction, set the primary transform to a default transform.

8. The apparatus of claim 7 , wherein the third determining code is further configured to cause the at least one processor to determine that the first intra prediction direction is aligned with the second intra prediction direction, based on both the first intra prediction direction and the second intra prediction direction corresponding to one of angular prediction modes, a planar mode and a DC mode.

9. The apparatus of claim 7 , wherein the third determining code is further configured to cause the at least one processor to determine that the first intra prediction direction is aligned with the second intra prediction direction, based on both the first intra prediction direction and the second intra prediction direction corresponding to a same intra prediction mode.

10. The apparatus of claim 7 , wherein the default transform is one of a discrete cosine transform (DCT)-2, an identity transform that scales an input by a pre-defined constant, a discrete sine transform (DST)-7 and a DCT-8.

11. The apparatus of claim 7 , further comprising fourth determining code configured to cause the at least one processor to, based on the secondary transform being determined to not be used, determine that the flag indicates that the secondary transform is not used.

12. The apparatus of claim 7 , wherein the first determining code is further configured to cause the at least one processor to, based on the set primary transform being one of a discrete cosine transform (DCT)-2, a discrete sine transform (DST)-7, a DCT-8, a DST-4, a DCT-4 and an identity transform that scales an input by a pre-defined constant, determine that the secondary transform is not used.

13. A non-transitory computer-readable storage medium storing instructions that cause at least one processor to:

determine whether a secondary transform is used, based on a primary transform that is set;

based on the secondary transform being determined to be used:

set a flag indicating whether the secondary transform is used to indicate that the secondary transform is used;

determine the secondary transform, based on the set primary transform and an intra prediction mode of a current block of a video sequence; and

perform residual coding of a luma coding block of the current block and a chroma coding block of the current block, using the set primary transform and the determined secondary transform;

determine whether a first intra prediction direction of the luma coding block is aligned with a second intra prediction direction of the chroma coding block, each of the luma coding block and the chroma coding block having a same coordinate of the current block; and

based on the first intra prediction direction being determined to be not aligned with the second intra prediction direction, set the primary transform to a default transform.

14. The non-transitory computer-readable storage medium of claim 13 , wherein the instructions further cause the at least one processor to determine that the first intra prediction direction is aligned with the second intra prediction direction, based on both the first intra prediction direction and the second intra prediction direction corresponding to one of angular prediction modes, a planar mode and a DC mode.

15. The non-transitory computer-readable storage medium of claim 13 , wherein the instructions further cause the at least one processor to determine that the first intra prediction direction is aligned with the second intra prediction direction, based on both the first intra prediction direction and the second intra prediction direction corresponding to a same intra prediction mode.

16. The non-transitory computer-readable storage medium of claim 13 , wherein the default transform is one of a discrete cosine transform (DCT)-2, an identity transform that scales an input by a pre-defined constant, a discrete sine transform (DST)-7 and a DCT-8.

17. The non-transitory computer-readable storage medium of claim 13 , wherein the instructions further cause the at least one processor to, based on the set primary transform being one of a discrete cosine transform (DCT)-2, a discrete sine transform (DST)-7, a DCT-8, a DST-4, a DCT-4 and an identity transform that scales an input by a pre-defined constant, determine that the secondary transform is not used.