Image coding method based on secondary transform, and device therefor

An image encoding method performed by an image encoding apparatus, the method comprising:

• deriving prediction samples based on an intra prediction mode applied to a target block;
• deriving residual samples for the target block based on the prediction samples;
• deriving transform coefficients for the target block based on a primary transform for the residual samples;
• deriving modified transform coefficients based on a secondary transform of the transform coefficients;
• deriving quantized transform coefficients by performing quantization based on the modified transform coefficients;
• and
• encoding a transform index related to a transform kernel matrix,
• wherein the the deriving of modified transform coefficients comprises,
• determining a transform set and the transform kernel matrix;
• and
• performing a matrix operation of the the transform coefficients and the transform kernel matrix,
• wherein the transform index that represents at least one of first index information, second index information, and third index information, the first index information indicating the inverse secondary transform being not applied to the target block, the second index information indicating a first transform kernel matrix as the transform kernel matrix for the inverse secondary transform, and the third index information indicating a second transform kernel matrix as the transform kernel matrix for the inverse secondary transform,
• wherein the transform set is determined based on the intra prediction mode of the target block,
• wherein the intra prediction mode of the target block is mapped to one of the four transform sets.

A non-transitory computer readable storage medium storing encoded information generated by an encoding method, the method comprising:

• deriving prediction samples based on an intra prediction mode applied to a target block;
• deriving residual samples for the target block based on the prediction samples;
• deriving transform coefficients for the target block based on a primary transform for the residual samples;
• deriving modified transform coefficients based on a secondary transform of the transform coefficients;
• deriving quantized transform coefficients by performing quantization based on the modified transform coefficients;
• encoding a transform index related to a transform kernel matrix and residual information related to the quantized transform coefficients to generate the encoded information,
• wherein the the deriving of modified transform coefficients comprises,
• determining a transform set and the transform kernel matrix;
• and
• performing a matrix operation of the the transform coefficients and the transform kernel matrix,
• wherein the transform index that represents at least one of first index information, second index information, and third index information, the first index information indicating the inverse secondary transform being not applied to the target block, the second index information indicating a first transform kernel matrix as the transform kernel matrix for the inverse secondary transform, and the third index information indicating a second transform kernel matrix as the transform kernel matrix for the inverse secondary transform,
• wherein the transform set is determined based on the intra prediction mode of the target block,
• wherein the intra prediction mode of the target block is mapped to one of the four transform sets.