Implicit selection of transform candidates

1. A method of processing video data, comprising:

making a decision, for a conversion between a current block of a video and a bitstream of the video, based on a set of representative coefficients of the current block and/or one or more neighbouring blocks of the current block and in an absence of one or more transform indices, regarding one or more transform matrices applied to the current block in a transform process, wherein the one or more transform matrices comprises at least one of a horizontal transform matrix or a vertical transform matrix; and

performing the conversion based on the decision,

wherein the horizontal transform matrix or the vertical transform matrix is determined based on a parity of a number of representative coefficients in the set of representative coefficients, and

wherein the current block applies an implicit selected transform for a primary transform, wherein the implicit selected transform is not applied to the current block when the current block is an intra-coded block with a derived tree (DT), wherein the implicit selected transform is applied under at least one condition including that a height and a width of the current block is no greater than a threshold, and wherein the threshold is a positive integer.

2. The method of claim 1 , wherein the set of representative coefficients comprises partial of all coefficients of the current block and/or the one or more neighbouring blocks.

3. 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.

4. The method of claim 1 , wherein the horizontal transform matrix is identical to the vertical transform matrix.

5. The method of claim 4 , wherein the horizontal transform matrix and the vertical 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).

6. The method of claim 1 , wherein the horizontal transform matrix is different from the vertical transform matrix.

7. The method of claim 6 , wherein the horizontal transform matrix is a Discrete Sine Transform (DST) of Type VII (DST-VII), and the vertical transform matrix is a Discrete Cosine Transform (DCT) of Type II (DCT-II).

8. The method of claim 1 , wherein the current block is coded with a specific mode, where the specific mode comprises an intra mode or an inter mode.

9. The method of claim 1 , further comprising:

performing a conversion between a second block of the video which does not apply an implicit selected transform and the bitstream, wherein a default Discrete Cosine Transform (DCT) of Type II (DCT-II) is used for the second block.

10. The method of claim 1 , further comprising performing a conversion between a third block in a specific color format of the video and the bitstream, wherein the third block is coded with an intra block copy mode which uses reference samples in a same video region as the third block and a transform skip mode is allowed to be applied to the third block.

11. The method of claim 1 , wherein the conversion includes encoding the current block into the bitstream.

12. The method of claim 1 , wherein the conversion includes decoding the current 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:

make a decision, for a conversion between a current block of a video and a bitstream of the video, based on a set of representative coefficients of the current block and/or one or more neighbouring blocks of the current block and in an absence of one or more transform indices, regarding one or more transform matrices applied to the current block in a transform process, wherein the one or more transform matrices comprises at least one of a horizontal transform matrix or a vertical transform matrix; and

perform the conversion based on the decision,

wherein the horizontal transform matrix or the vertical transform matrix is determined based on a parity of a number of representative coefficients in the set of representative coefficients, and

wherein the current block applies an implicit selected transform for a primary transform, wherein the implicit selected transform is not applied to the current block when the current block is an intra-coded block with a derived tree (DT), wherein the implicit selected transform is applied under at least one condition including that a height and a width of the current block is no greater than a threshold, and wherein the threshold is a positive integer.

14. The apparatus of claim 13 , wherein the set of representative coefficients comprises partial of all coefficients of the current block and/or the one or more neighbouring blocks, and 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,

wherein the horizontal transform matrix is a Discrete Sine Transform (DST) of Type VII (DST-VII), and the vertical transform matrix is a Discrete Cosine Transform (DCT) of Type II (DCT-II).

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

make a decision, for a conversion between a current block of a video and a bitstream of the video, based on a set of representative coefficients of the current block and/or one or more neighbouring blocks of the current block and in an absence of one or more transform indices, regarding one or more transform matrices applied to the current block in a transform process, wherein the one or more transform matrices comprises at least one of a horizontal transform matrix or a vertical transform matrix; and

perform the conversion based on the decision,

wherein the horizontal transform matrix or the vertical transform matrix is determined based on a parity of a number of representative coefficients in the set of representative coefficients, and

wherein the current block applies an implicit selected transform for a primary transform, wherein the implicit selected transform is not applied to the current block when the current block is an intra-coded block with a derived tree (DT), wherein the implicit selected transform is applied under at least one condition including that a height and a width of the current block is no greater than a threshold, and wherein the threshold is a positive integer.

16. The non-transitory computer-readable storage medium of claim 15 , wherein the set of representative coefficients comprises partial of all coefficients of the current block and/or the one or more neighbouring blocks, 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,

wherein the horizontal transform matrix is a Discrete Sine Transform (DST) of Type VII (DST-VII), and the vertical transform matrix is a Discrete Cosine Transform (DCT) of Type II (DCT-II).

17. A non-transitory computer-readable recording medium storing a bitstream of a video which is generated by a method performed by a video processing apparatus, wherein the method comprises:

making a decision based on a set of representative coefficients of a current block of the video and/or one or more neighbouring blocks of the current block and in an absence of one or more transform indices, regarding one or more transform matrices applied to the current block in a transform process, wherein the one or more transform matrices comprises at least one of a horizontal transform matrix or a vertical transform matrix; and

generating the bitstream based on the decision,

wherein the horizontal transform matrix or the vertical transform matrix is determined based on a parity of a number of representative coefficients in the set of representative coefficients, and

wherein the current block applies an implicit selected transform for a primary transform, wherein the implicit selected transform is not applied to the current block when the current block is an intra-coded block with a derived tree (DT), wherein the implicit selected transform is applied under at least one condition including that a height and a width of the current block is no greater than a threshold, and wherein the threshold is a positive integer.

18. The non-transitory computer-readable recording medium of claim 17 , wherein the set of representative coefficients comprises partial of all coefficients of the current block and/or the one or more neighbouring blocks, and 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,

wherein the horizontal transform matrix is a Discrete Sine Transform (DST) of Type VII (DST-VII), and the vertical transform matrix is a Discrete Cosine Transform (DCT) of Type II (DCT-II).

19. The apparatus of claim 13 , wherein the horizontal transform matrix is identical to the vertical transform matrix.