Secondary transform for video encoding and decoding
A method and apparatus for encoding/decoding a picture use a secondary transform for video encoding and decoding in INTRA mode and more particularly use a secondary transform for the transform units of a coding unit in intra sub partition mode. Different signaling and selection variants are proposed according to various parameters of the coding unit and transform unit.
1. A video encoding method comprising:
splitting an intra coding block into a plurality of transform blocks;
obtaining a decision whether a second transform is to be applied and a selection of an index of a selected transform in a list of transforms, the second transform being a Low-Frequency-Non-Separable Transform;
when the decision is that the second transform is to be applied:
performing a first transform on all of the plurality of transform blocks, wherein the first transform is a two-dimensional (2D) discrete cosine transform (DCT) 2 transform; and
performing the second transform on all of the plurality of transform blocks of the intra coding block using the selected transform; and
when the decision is that the second transform is not to be applied:
selecting a null value as the index for the second transform in the list of transforms; and
performing a first transform on all of the plurality of transform blocks, the first transform being selected in a set of transforms; and
encoding the intra coding block with signaling information representative of the index of the second transform.
2. The method of claim 1 , wherein the second transform is applied when:
the size of the intra coding block is greater than a threshold in at least one of width or height;
the size of the intra coding block is smaller than a threshold in width and height;
the size of a transform block of the plurality of transform blocks is greater than a threshold in at least one of width or height; or
the size of a transform block of the plurality of transform blocks is smaller than a threshold in width and height.
3. The method of claim 1 , wherein the plurality of transform blocks have different sizes, and wherein the second transform is performed based on a largest transform block of the plurality of transform blocks.
4. A video decoding method for a coding block that is split into a plurality of transform blocks, comprising:
obtaining information representative of an index of a second transform;
when the index is a null value:
performing an inverse of a first transform on all of the plurality of transform blocks of the coding block, the first transform being selected in a set of transforms; and
when the index is not a null value:
performing an inverse of the second transform on all of the plurality of transform blocks of the coding block using a second transform selected identified by the index in a list of second transforms, the second transform being a Low-Frequency-Non-Separable Transform; and
performing an inverse of a first transform on all of the plurality of transform blocks of the coding block, wherein the first transform is a two-dimensional (2D) discrete cosine transform (DCT) 2 transform; and
decoding the inversed transform blocks.
5. The method of claim 4 , wherein the second transform is to be applied when:
the size of the coding block is greater than a threshold in at least one of width or height;
the size of the coding block is smaller than a threshold in width and height;
the size of a transform block of the plurality of transform blocks is greater than a threshold in at least one of width or height; or
the size of a transform block of the plurality of transform blocks is smaller than a threshold in width and height.
6. The method of claim 4 , wherein the plurality of transform blocks have different sizes, and wherein the second transform is performed based on a largest transform block of the plurality of transform blocks.
7. A video encoding apparatus comprising a processor configured to:
split an intra coding block into a plurality of transform blocks;
obtain a decision whether a second transform is to be applied and a selection of an index of a selected transform in a list of transforms, the second transform being a Low-Frequency-Non-Separable Transform;
when the decision is that the second transform is to be applied:
perform a first transform on all of the plurality of transform blocks, the first transform being a two-dimensional (2D) discrete cosine transform (DCT) 2 transform; and
perform the second transform on all the transform blocks of the plurality of transform blocks of the intra coding block using the selected transform; and
when the decision is that the second transform is not to be applied:
selecting a null value as the index for the second transform in the list of transforms; and
performing a first transform on all of the plurality of transform blocks, the first transform being selected in a set of transforms; and
encode the intra coding block with signaling information representative of the index of the second transform.
8. A video decoding apparatus for a coding block that is split into a plurality of transform blocks, the apparatus comprising a processor configured to:
obtain information representative of an index of a second transform;
when the index is a null value:
perform an inverse of a first transform on all of the plurality of transform blocks of the coding block, the first transform being selected in a set of transforms; and
when the index is not a null value:
perform an inverse of the second transform on all the transform blocks of the plurality of transform blocks of the coding block using a second transform selected identified by the index in a list of second transforms, the second transform being a Low-Frequency-Non-Separable Transform; and
perform an inverse of a first transform on all of the plurality of transform blocks of the coding block, wherein the first transform is a two-dimensional (2D) discrete cosine transform (DCT) 2 transform; and
decode the inversed transform blocks.
9. A non-transitory computer-readable storage medium having stored instructions that, when executed by a processor, cause the processor to:
split an intra coding block into a plurality of transform blocks;
obtain a decision whether a second transform is to be applied and a selection of an index of a selected transform in a list of transforms, the second transform being a Low-Frequency-Non-Separable Transform;
when the decision is that the second transform is to be applied:
perform a first transform on all of the plurality of transform blocks, the first transform being a two-dimensional (2D) discrete cosine transform (DCT) 2 transform; and
perform the second transform on all the transform blocks of the plurality of transform blocks of the intra coding block using the selected transform; and
when the decision is that the second transform is not to be applied:
selecting a null value as the index for the second transform in the list of transforms; and
performing a first transform on all of the plurality of transform blocks, the first transform being selected in a set of transforms; and
encode the intra coding block with signaling information representative of the index of the second transform.
10. A non-transitory computer-readable storage medium having stored instructions that, when executed by a processor on a coding block that is split into a plurality of transform blocks, cause the processor to:
obtain information representative of an index of a second transform;
when the index is a null value:
perform an inverse of a first transform on all of the plurality of transform blocks of the coding block, the first transform being selected in a set of transforms; and
when the index is not a null value:
perform an inverse of the second transform on all the transform blocks of the plurality of transform blocks of the coding block using a second transform selected identified by the index in a list of second transforms, the second transform being a Low-Frequency-Non-Separable Transform; and
perform an inverse of a first transform on all of the plurality of transform blocks of the coding block, wherein the first transform is a two-dimensional (2D) discrete cosine transform (DCT) 2 transform; and
decode the inversed transform blocks.
11. The apparatus of claim 7 , wherein the second transform is applied when:
the size of the intra coding block is greater than a threshold in at least one of width or height;
the size of the intra coding block is smaller than a threshold in width and height;
the size of a transform block of the plurality of transform blocks is greater than a threshold in at least one of width or height; or
the size of a transform block of the plurality of transform blocks is smaller than a threshold in width and height.
12. The apparatus of claim 7 , wherein the plurality of transform blocks have different sizes, and wherein the second transform is performed based on a largest transform block of the plurality of transform blocks.
13. The apparatus of claim 8 , wherein the second transform is applied when:
the size of the intra coding block is greater than a threshold in at least one of width or height;
the size of the intra coding block is smaller than a threshold in width and height;
the size of a transform block of the plurality of transform blocks is greater than a threshold in at least one of width or height; or
the size of a transform block of the plurality of transform blocks is smaller than a threshold in width and height.
14. The apparatus of claim 8 , wherein the plurality of transform blocks have different sizes, and wherein the second transform is performed based on a largest transform block of the plurality of transform blocks.
15. The non-transitory computer-readable storage medium of claim 9 , wherein the second transform is applied when:
the size of the intra coding block is greater than a threshold in at least one of width or height;
the size of the intra coding block is smaller than a threshold in width and height;
the size of a transform block of the plurality of transform blocks is greater than a threshold in at least one of width or height; or
the size of a transform block of the plurality of transform blocks is smaller than a threshold in width and height.
16. The non-transitory computer-readable storage medium of claim 9 , wherein the plurality of transform blocks have different sizes, and wherein the second transform is performed based on a largest transform block of the plurality of transform blocks.
17. The non-transitory computer-readable storage medium of claim 10 , wherein the second transform is applied when:
the size of the intra coding block is greater than a threshold in at least one of width or height;
the size of the intra coding block is smaller than a threshold in width and height;
the size of a transform block of the plurality of transform blocks is greater than a threshold in at least one of width or height; or
the size of a transform block of the plurality of transform blocks is smaller than a threshold in width and height.
18. The non-transitory computer-readable storage medium of claim 10 , wherein the plurality of transform blocks have different sizes, and wherein the second transform is performed based on a largest transform block of the plurality of transform blocks.