Image coding method based on secondary transform, and device therefor
An image decoding method according to the present document comprises the steps of: deriving transform coefficients through inverse quantization on the basis of quantized transform coefficients for a target block; deriving modified transform coefficients on the basis of an inverse reduced secondary transform (RST) for the transform coefficients; and generating, on the basis of an inverse primary transform for the modified transform coefficients, a restoration picture based on residual samples for the target block, wherein the modified transform coefficients derived according to the inverse RST are two-dimensionally arranged according to the order of a row priority direction or a column priority direction according to an intra prediction mode to be applied to the target block.
1. An image decoding method performed by a decoding apparatus, the method comprising:
obtaining residual information from a bitstream;
deriving quantized transform coefficients for a target block based on the residual information;
deriving transform coefficients for the target block by dequantizing the quantized transform coefficients;
deriving modified transform coefficients for the target block based on an inverse non-separable 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,
wherein the inverse non-separable transform is performed through a matrix operation of the transform coefficients for a top-left area of the target block and a transform kernel matrix,
wherein the transform kernel matrix is selected based on a transform index from a transform set,
wherein the transform set is derived based on a mapping relationship according to an intra prediction mode applied to the target block,
wherein the modified transform coefficients are two-dimensionally arranged according to a row-first direction based on the intra prediction mode being a horizontal mode in a left direction with respect to a top-left diagonal direction, and
wherein the modified transform coefficients are two-dimensionally arranged according to a column-first direction based on the intra prediction mode being a vertical mode in a right direction with respect to the top-left diagonal direction.
2. An image encoding method performed by an 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 for the target block based on a non-separable transform for the transform coefficients;
deriving quantized transform coefficients for the target block by quantizing the modified transform coefficients; and
encoding residual information based on the quantized transform coefficients,
wherein the non-separable transform is performed through a matrix operation of the transform coefficients for a top-left area of the target block and a transform kernel matrix,
wherein the transform kernel matrix is selected from a transform set,
wherein the transform set is derived based on a mapping relationship according to an intra prediction mode applied to the target block,
wherein the modified transform coefficients are two-dimensionally arranged according to a row-first direction based on the intra prediction mode being a horizontal mode in a left direction with respect to a top-left diagonal direction, and
wherein the modified transform coefficients are two-dimensionally arranged according to a column-first direction based on the intra prediction mode being a vertical mode in a right direction with respect to the top-left diagonal direction.
3. A non-transitory computer readable storage medium storing a bitstream generated by the image encoding method of the claim 2 .
4. A transmission method of data for an image, the method comprising:
obtaining a bitstream for the image, wherein the bitstream is generated based on 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 for the target block based on a non-separable transform for the transform coefficients, deriving quantized transform coefficients for the target block by quantizing the modified transform coefficients, and encoding residual information based on the quantized transform coefficients; and
transmitting the data comprising the bitstream,
wherein the non-separable transform is performed through a matrix operation of the transform coefficients for a top-left area of the target block and a transform kernel matrix,
wherein the transform kernel matrix is selected from a transform set,
wherein the transform set is derived based on a mapping relationship according to an intra prediction mode applied to the target block,
wherein the modified transform coefficients are two-dimensionally arranged according to a row-first direction based on the intra prediction mode being a horizontal mode in a left direction with respect to a top-left diagonal direction, and
wherein the modified transform coefficients are two-dimensionally arranged according to a column-first direction based on the intra prediction mode being a vertical mode in a right direction with respect to the top-left diagonal direction.