Image coding method based on secondary transform, and device therefor

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
• encoding 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 transform coefficients are one-dimensionally arranged according to a row-first direction for applying the non-separable secondary transform, based on an intra prediction mode applied to the target block being a horizontal mode in a left direction with respect to a top-left diagonal direction.

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
• encoding residual information based on the modified transform coefficients 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 transform coefficients are one-dimensionally arranged according to a row-first direction for applying the non-separable secondary transform, based on an intra prediction mode applied to the target block being a horizontal mode in a left direction with respect to a top-left diagonal direction.