Methods and apparatus for low-complexity MTS

1. A method for video decoding, the method comprising:

determining whether at least one parameter of a block is less than or equal to a threshold;

signaling, in response to determining the at least one parameter of the block is less than or equal to the threshold, one of a horizontal transform and a vertical transform;

splitting, in response to determining that the at least one parameter of the block is greater than the threshold, the block into sub-blocks;

applying a first signaling scheme on a luma component and a second signaling scheme on a chroma component;

performing ones of transforms on the sub-blocks; and

decoding a video stream by using the sub-blocks upon which the ones of the transforms are performed,

wherein a maximum block size of the first signaling scheme is different than a maximum block size of the second signaling scheme.

2. The method according to claim 1 ,

wherein signaling the at least one of the horizontal transform and the vertical transform is further in response to determining whether at least one signaled transform type selected from multiple candidate transform types is absent symmetry/anti-symmetry characteristics, and

wherein the at least one signaled transform type is at least one of a discrete cosine transform (DCT)-4, a DCT-5, a DCT-6, a DCT-7, a DCT-8, a discrete sine transform (DST)-4, a DST-5, a DST-6, a DST-7, and a DST-8, and

wherein a DCT-1, a DCT-2, a DCT-3, a DST-1, a DST-2, and a DST-3 comprise the symmetry/anti-symmetry characteristics.

3. The method according to claim 1 , wherein signaling the at least one of the horizontal transform and the vertical transform is further in response to determining whether at least one signaled transform type selected from multiple candidate transform types is other than a recursive transform.

4. The method according to claim 1 , wherein signaling the one of the horizontal transform and the vertical transform comprises signaling a vertical transform selection before signaling a horizontal transform selection.

5. The method according to claim 1 , wherein the threshold is that the parameter is at least one of a width and a height of the block having an integer less than or equal to 16.

6. The method according to claim 1 , wherein splitting the block into the sub-blocks comprises determining different transform types respective to ones of the sub-blocks depending on relative locations of the ones of the sub-blocks with the block.

7. The method according to claim 1 , further comprising:

signaling, in response to splitting the block into the sub-blocks, ones of individual transform indices for each of the sub-blocks respectively,

wherein each of the transform indices specify ones of transform selections.

8. The method according to claim 1 , further comprising:

signaling, in response to splitting the block into the sub-blocks, a transform index as shared by a plurality of the sub-blocks respectively.

9. The method according to claim 1 ,

wherein the maximum block size of the second signaling scheme is one of 2, 4, 8, and 16.

10. The method according to claim 1 , further comprising:

using a transform selection on a luma component; and

re-using, in response to determining that the luma component shares a block partitioning tree with a chroma component co-located with the luma component, the transform selection of a luma component on the chroma component.

11. An apparatus comprising:

at least one memory configured to store computer program code;

at least one hardware processor configured to access the computer program code and operate as instructed by the computer program code, the computer program code including:

determining code configured to cause the processor to determine whether at least one parameter of a block is less than or equal to a threshold;

signaling code configured to cause the processor to signal, in response to determining the at least one parameter of the block is less than or equal to the threshold, one of a horizontal transform and a vertical transform;

splitting code configured to cause the processor to split, in response to determining that the at least one parameter of the block is greater than the threshold, the block into sub-blocks;

applying code configured to cause the processor to apply a first signaling scheme on a luma component and a second signaling scheme on a chroma component;

transform code configured to cause the processor to perform ones of transforms on the sub-blocks; and

decoding code configured to cause the processor to perform decoding of a video stream by using the sub-blocks upon which the ones of the transforms are performed,

wherein a maximum block size of the first signaling scheme is different than a maximum block size of the second signaling scheme.

12. The apparatus according to claim 11 ,

wherein signaling code is further configured to cause the processor to signal the at least one of the horizontal transform and the vertical transform in further response to determining whether at least one signaled transform type selected from multiple candidate transform types is absent symmetry/anti-symmetry characteristics, and

wherein the at least one signaled transform type is at least one of a discrete cosine transform (DCT)-4, a DCT-5, a DCT-6, a DCT-7, a DCT-8, a discrete sine transform (DST)-4, a DST-5, a DST-6, a DST-7, and a DST-8, and

wherein a DCT-1, a DCT-2, a DCT-3, a DST-1, a DST-2, and a DST-3 comprise the symmetry/anti-symmetry characteristics.

13. The apparatus according to claim 11 , wherein the signaling code is further configured to cause the processor to signal the at least one of the horizontal transform and the vertical transform is in further response to determining whether at least one signaled transform type selected from multiple candidate transform types is other than a recursive transform.

14. The apparatus according to claim 11 , wherein signaling code is further configured to cause the processor to signal the one of the horizontal transform and the vertical transform comprises signaling a vertical transform selection before signaling a horizontal transform selection.

15. The apparatus according to claim 11 , wherein the threshold is that the parameter is at least one of a width and a height of the block having an integer less than or equal to 16.

16. The apparatus according to claim 11 , wherein splitting code is further configured to cause the processor to split the block into the sub-blocks and determine different transform types respective to ones of the sub-blocks depending on relative locations of the ones of the sub-blocks with the block.

17. The apparatus according to claim 11 , wherein the signaling code is further configured to cause the processor to signal, in response to splitting the block into the sub-blocks, ones of individual transform indices for each of the sub-blocks respectively,

wherein each of the transform indices specify ones of transform selections.

18. The apparatus according to claim 11 , wherein the signaling code is further configured to cause the processor to signal, in response to splitting the block into the sub-blocks, a transform index as shared by a plurality of the sub-blocks respectively.

19. The apparatus according to claim 11 , further comprising:

using code configured to cause the processor to use a transform selection on a luma component; and

re-using code configured to cause the processor to re-use, in response to determining that the luma component shares a block partitioning tree with a chroma component co-located with the luma component, the transform selection of a luma component on the chroma component.

20. A non-transitory computer readable medium storing a program causing a computer to execute a process, the process comprising:

determining whether at least one parameter of a block is less than or equal to a threshold;

signaling, in response to determining the at least one parameter of the block is less than or equal to the threshold, one of a horizontal transform and a vertical transform;

splitting, in response to determining that the at least one parameter of the block is greater than the threshold, the block into sub-blocks;

applying a first signaling scheme on a luma component and a second signaling scheme on a chroma component;

performing ones of transforms on the sub-blocks; and

decoding a video stream by using the sub-blocks upon which the ones of the transforms are performed,

wherein a maximum block size of the first signaling scheme is different than a maximum block size of the second signaling scheme.