Transform-based image coding method and device
An image decoding method according to the present disclosure includes deriving transform coefficients through dequantization based on the quantized transform coefficients for the target block; deriving modified transform coefficients based on an inverse reduced secondary transform (RST) for the transform coefficients; deriving residual samples for the target block based on an inverse primary transform for the modified transform coefficients; and generating a reconstructed samples based on the residual samples, and prediction samples derived based on an intra prediction mode for the target block, wherein the inverse RST is performed based on a transform kernel matrix selected from a transform set including a plurality of transform kernel matrices, the transform set is determined based on a mapping relationship according to the intra prediction mode applied to the target block, and a plurality of intra prediction modes including the intra prediction mode of the target block are mapped to one transform set.
1. An image decoding method performed by a decoding apparatus, the method comprising:
deriving transform coefficients for a target block from residual information;
deriving modified transform coefficients based on an inverse non-separable secondary transform for the transform coefficients;
deriving residual samples for the target block based on an inverse primary transform for the modified transform coefficients; and
generating a reconstructed picture based on the residual samples for the target block and prediction samples derived based on an intra prediction mode for the target block,
wherein the inverse non-separable secondary transform derives a larger number of output data than input data through a matrix operation of the input data for a top-left area of the target block and a transform kernel matrix,
wherein the input data for the top-left area is in a top-left 4×4 area of the target block and the output data for the top-left area is in the top-left 4×4 area of the target block or a top-left 8×8 area of the target block;
wherein the inverse non-separable secondary transform is performed based on the transform kernel matrix selected from a transform set including a plurality of transform kernel matrices, and
wherein the transform set is determined based on a mapping relationship according to the intra prediction mode applied to the target block.
2. The image decoding method of claim 1 , wherein:
a first intra prediction mode for a first target block to which the inverse non-separable secondary transform is applied and a second intra prediction mode for a second target block to which the inverse non-separable secondary transform is applied are different from each other; and
the transform sets mapped to the first intra prediction mode and the second intra prediction mode are same.
3. The image decoding method of claim 1 , wherein a plurality of intra prediction modes including the intra prediction mode of the target block are mapped to one transform set, and
wherein five or more intra prediction modes are mapped to one transform set.
4. The image decoding method of claim 1 , wherein the modified transform coefficients are mapped into the target block based on a diagonal scan order.
5. The image decoding method of claim 1 , further comprising obtaining information on whether the inverse non-separable secondary transform is applied,
wherein the information is included in a sequence parameter set (SPS) in a bitstream.
6. An image encoding method performed by an image encoding apparatus, the method comprising:
deriving residual samples for a target block;
deriving transform coefficients for the target block based on a primary transform for the residual samples;
deriving modified transform coefficients based on a non-separable secondary transform for the transform coefficients; and
generating residual information based on the modified transform coefficients,
wherein the non-separable secondary transform derives a smaller number of output data than input data through a matrix operation of the input data for a top-left area of the target block and a transform kernel matrix,
wherein the input data for the top-left area is in a top-left 4×4 area of the target block or a top-left 8×8 area of the target block and the output data for the top-left area is in the top-left 4×4 area of the target block,
wherein the non-separable secondary transform is performed based on the transform kernel matrix selected from a transform set including a plurality of transform kernel matrices, and
wherein the transform set is determined based on a mapping relationship according to an intra prediction mode applied to the target block.
7. The image encoding method of claim 6 , wherein:
the target block includes a first block and a second block;
a first intra prediction mode for a first target block to which the non-separable secondary transform is applied and a second intra prediction mode for a second target block to which the non-separable secondary transform is applied are different from each other; and
and the transform sets mapped to the first intra prediction mode and the second intra prediction mode are same.
8. The image encoding method of claim 6 , wherein a plurality of intra prediction modes including the intra prediction mode of the target block are mapped to one transform set, and
wherein five or more intra prediction modes are mapped to one transform set.
9. The image encoding method of claim 6 , further comprising encoding information on whether the inverse non-separable secondary transform is applied,
wherein the information is included in a sequence parameter set (SPS) in a bitstream.
10. A non-transitory computer readable storage medium storing a bitstream generated by a method, the method comprising:
deriving residual samples for a target block;
deriving transform coefficients for the target block based on a primary transform for the residual samples;
deriving modified transform coefficients based on a non-separable secondary transform for the transform coefficients; and
generating residual information based on the modified transform coefficients, and
encoding image information including the residual information to generate the bitstream,
wherein the non-separable secondary transform derives a smaller number of output data than input data through a matrix operation of the input data for a top-left area of the target block and a transform kernel matrix,
wherein the input data for the top-left area is in a top-left 4×4 area of the target block or a top-left 8×8 area of the target block and the output data for the top-left area is in the top-left 4×4 area of the target block,
wherein the non-separable secondary transform is performed based on the transform kernel matrix selected from a transform set including a plurality of transform kernel matrices, and
wherein the transform set is determined based on a mapping relationship according to an intra prediction mode applied to the target block.
11. The image decoding method of claim 1 , wherein a range of values of matrix coefficients in the transform kernel matrix is from −128 to 127.
12. The image encoding method of claim 6 , wherein a range of values of matrix coefficients in the transform kernel matrix is from −128 to 127.
13. The non-transitory computer readable storage medium of claim 10 , wherein a range of values of matrix coefficients in the transform kernel matrix is from −128 to 127.