Simplified transform coding tools
Devices, systems and methods for digital video coding, which includes using multiple transforms, are described. In a representative aspect, a method for video processing includes selecting, as part of a multiple transform selection (MTS) process, a common transform as both a vertical transform and a horizontal transform, and applying, as part of performing a conversion between a current block of a video and a bitstream representation of the video, the vertical transform and the horizontal transform to the current block.
1. A method of processing video data, comprising:
determining, for a current transform block of a video whether to apply a transform coefficient zero-out based on an implicit selected transform of a multiple transform selection process for a primary transform; and
performing, based on the determining, a conversion between the current transform block and a bitstream of the video,
wherein the implicit selected transform of the multiple transform selection process is in an absence of one or more transform indices,
wherein the transform coefficient zero-out is applied when (a) a size of the current transform block is M×32 or 32×N and (b) the current transform block is coded with the implicit selected transform,
wherein M and N are positive integers, M≤32, and N≤32, and
wherein one or more transform matrices applied to the current transform block comprise at least one of a horizontal transform matrix or a vertical transform matrix that is based on a set of representative coefficients of the current transform block.
2. The method of claim 1 , wherein in response to the transform coefficient zero-out being applied to the current transform block with a size M×32, when M=32 only left 16 columns and top 16 rows of transform coefficients of the current transform block have non-zero coefficients.
3. The method of claim 1 , wherein in response to the transform coefficient zero-out being applied to the current transform block with a size 32×N, when N=32 only left 16 columns and top 16 rows of transform coefficients of the current transform block have non-zero coefficients.
4. The method of claim 1 , wherein the set of representative coefficients comprises partial of all coefficients of the current transform block.
5. The method of claim 1 , wherein the one or more transform matrices is based on a function of the set of representative coefficients, and an output of the function is a number of the set of representative coefficients.
6. The method of claim 5 , wherein the horizontal transform matrix comprises a first transform matrix and the vertical transform matrix comprises a second transform matrix when the number is even, and wherein the horizontal transform matrix comprises a third transform matrix and the vertical transform matrix comprises a fourth transform matrix when the number is odd.
7. The method of claim 6 , wherein the first transform matrix is identical to the second transform matrix and the third transform matrix is identical to the fourth transform matrix.
8. The method of claim 6 , wherein each of the first transform matrix, the second transform matrix, the third transform matrix, and the fourth transform matrix is one of a Discrete Cosine Transform (DCT) of Type II (DCT-II) or a Discrete Sine Transform (DST) of Type VII (DST-VII).
9. The method of claim 7 , wherein the first transform matrix is different from the third transform matrix.
10. The method of claim 8 , wherein the first transform matrix and the second transform matrix are the Discrete Sine Transform (DST) of Type VII (DST-VII), and the third transform matrix and the fourth transform matrix are the Discrete Cosine Transform (DCT) of Type II (DCT-II).
11. The method of claim 1 , wherein the conversion includes encoding the current transform block into the bitstream.
12. The method of claim 1 , wherein the conversion includes decoding the current transform block from the bitstream.
13. An apparatus for processing video data comprising a processor and a non-transitory memory with instructions thereon, wherein the instructions upon execution by the processor, cause the processor to:
determine, for a current transform block of a video, whether to apply a transform coefficient zero-out based on an implicit selected transform of a multiple transform selection process for a primary transform; and
perform, based on determining whether to apply the transform coefficient zero-out, a conversion between the current transform block and a bitstream of the video,
wherein the implicit selected transform of the multiple transform selection process is in an absence of one or more transform indices,
wherein the transform coefficient zero-out is applied when (a) a size of the current transform block is M×32 or 32×N and (b) the current transform block is coded with the implicit selected transform,
wherein M and N are positive integers, M≤32, and N≤32, and
wherein one or more transform matrices applied to the current transform block comprises at least one of a horizontal transform matrix or a vertical transform matrix that is based on a set of representative coefficients of the current transform block.
14. The apparatus of claim 13 , wherein in response to the transform coefficient zero-out being applied to the current transform block with a size M×32, when M=32 only left 16 columns and top 16 rows of transform coefficients of the current transform block have non-zero coefficients.
15. The apparatus of claim 13 , wherein in response to the transform coefficient zero-out being applied to the current transform block with a size 32×N, when N=32 only left 16 columns and top 16 rows of transform coefficients of the current transform block have non-zero coefficients.
16. The apparatus of claim 13 , wherein the set of representative coefficients comprises partial of all coefficients of the current transform block.
17. The apparatus of claim 13 , wherein the one or more transform matrices is based on a function of the set of representative coefficients, and an output of the function is a number of the set of representative coefficients.
18. The apparatus of claim 17 , wherein the horizontal transform matrix comprises a first transform matrix and the vertical transform matrix comprises a second transform matrix when the number is even, and wherein the horizontal transform matrix comprises a third transform matrix and the vertical transform matrix comprises a fourth transform matrix when the number is odd.
19. A non-transitory computer-readable storage medium storing instructions that cause a processor to:
determine, for a current transform block of a video, whether to apply a transform coefficient zero-out based on an implicit selected transform of a multiple transform selection process for a primary transform; and
perform, based on determining whether to apply the transform coefficient zero-out, a conversion between the current transform block and a bitstream of the video,
wherein the implicit selected transform of the multiple transform selection process is in an absence of one or more transform indices,
wherein the transform coefficient zero-out is applied when (a) a size of the current transform block is M×32 or 32×N and (b) the current transform block is coded with the implicit selected transform,
wherein M and N are positive integers, M≤32, and N≤32, and
wherein one or more transform matrices applied to the current transform block comprise at least one of a horizontal transform matrix or a vertical transform matrix that is based on a set of representative coefficients of the current transform block.
20. A method for storing a bitstream of a video, comprising:
determining, for a current transform block of the video, whether to apply a transform coefficient zero-out based on an implicit selected transform of a multiple transform selection process for a primary transform;
generating the bitstream of the video based on the determining; and
storing the bitstream in a non-transitory computer-readable recording medium,
wherein the implicit selected transform of the multiple transform selection process is in an absence of one or more transform indices,
wherein the transform coefficient zero-out is applied when (a) a size of the current transform block is M×32 or 32×N and (b) the current transform block is coded with the implicit selected transform,
wherein M and N are positive integers, M≤32, and N≤32, and
wherein one or more transform matrices applied to the current transform block comprise at least one of a horizontal transform matrix or a vertical transform matrix that is based on a set of representative coefficients of the current transform block.