Method and apparatus for identity transform in multiple transform selection

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 to replace a transform type of Multiple Transform Selection (MTS) by using Identity transform (IDT), based on determining whether either one or both of a block height and a block width meet a predetermined condition; and

based on determining that either one or both of the block height and the block width meet the predetermined condition, replacing the transform type of MTS by using the IDT and keeping a syntax of the MTS unchanged,

wherein the IDT is a linear transform process using an N×N transform core in which each coefficient of a main diagonal of the N×N transform core is non-zero and each coefficient of the N×N transform core, other than the main diagonal, is zero, and wherein N is an integer.

2. The method of claim 1 , wherein the transform type of the MTS is discrete cosine transform DCT-4 or DCT-8.

3. The method of claim 1 , wherein the transform type of the MTS is applied as a horizontal transform and the predetermined condition is that the block width is smaller than a predetermined threshold.

4. The method of claim 3 , wherein the predetermined threshold is one from among 2, 4, 8, 16, 32, 64, 128, and 156.

5. The method of claim 1 , wherein the transform type of the MTS is applied as a vertical transform and the predetermined condition is that the block height is smaller than a predetermined threshold.

6. The method of claim 5 , wherein the predetermined threshold is one from among 2, 4, 8, 16, 32, 64, 128, and 156.

7. The method of claim 1 , wherein the predetermined condition is that the transform type of the MTS is used on a horizontal direction or a vertical direction, but is not used on both the horizontal direction and the vertical direction.

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

at least one non-transitory computer readable medium that stores computer program code; and

at least one processor configured to access the at least one non-transitory computer readable medium and operate according to the computer program code, the computer program code comprising:

determining code configured to cause the at least one processor to determine whether to replace a transform type of Multiple Transform Selection (MTS) by using Identity transform (IDT), based on determining whether either one or both of a block height and a block width meet a predetermined condition; and

replacing code configured to cause the at least one processor to, based on determining that either one or both of the block height and the block width meet the predetermined condition, replace the transform type of MTS by using the IDT and keeping a syntax of the MTS unchanged,

wherein the IDT is a linear transform process using an N×N transform core in which each coefficient of a main diagonal of the N×N transform core is non-zero and each coefficient of the N×N transform core, other than the main diagonal, is zero, and wherein N is an integer.

9. The apparatus of claim 8 , wherein the transform type of the MTS is discrete cosine transform DCT-4 or DCT-8.

10. The apparatus of claim 8 , wherein the transform type of the MTS is applied as a horizontal transform and the predetermined condition is that the block width is smaller than a predetermined threshold.

11. The apparatus of claim 10 , wherein the predetermined threshold is one from among 2, 4, 8, 16, 32, 64, 128, and 156.

12. The apparatus of claim 8 , wherein the transform type of the MTS is applied as a vertical transform and the predetermined condition is that the block height is smaller than a predetermined threshold.

13. The apparatus of claim 12 , wherein the predetermined threshold is one from among 2, 4, 8, 16, 32, 64, 128, and 156.

14. The apparatus of claim 8 , wherein the predetermined condition is that the transform type of the MTS is used on a horizontal direction or a vertical direction, but is not used on both the horizontal direction and the vertical direction.

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

determine whether to replace a transform type of Multiple Transform Selection (MTS) by using Identity transform (IDT), based on determining whether either one or both of a block height and a block width meet a predetermined condition; and

based on determining that either one or both of the block height and the block width meet the predetermined condition, replace the transform type of MTS by using the IDT and keeping a syntax of the MTS unchanged,

wherein the IDT is a linear transform process using an N×N transform core in which each coefficient of a main diagonal of the N×N transform core is non-zero and each coefficient of the N×N transform core, other than the main diagonal, is zero, and wherein N is an integer.

16. The non-transitory computer-readable storage medium of claim 15 , wherein the transform type of the MTS is discrete cosine transform DCT-4 or DCT-8.

17. The non-transitory computer-readable storage medium of claim 15 , wherein the transform type of the MTS is applied as a horizontal transform and the predetermined condition is that the block width is smaller than a predetermined threshold.

18. The non-transitory computer-readable storage medium of claim 17 , wherein the predetermined threshold is one from among 2, 4, 8, 16, 32, 64, 128, and 156.

19. The non-transitory computer-readable storage medium of claim 15 , wherein the transform type of the MTS is applied as a vertical transform and the predetermined condition is that the block height is smaller than a predetermined threshold.

20. The non-transitory computer-readable storage medium of claim 19 , wherein the predetermined threshold is one from among 2, 4, 8, 16, 32, 64, 128, and 156.