Image coding method and apparatus therefor

A method for image encoding, the method performed by an encoding apparatus and comprising:

• deriving residual samples for a target block based on prediction samples for the target block;
• deriving transform coefficients for the target block based on a first-order transform for the residual samples;
• deriving modified transform coefficients based on a second-order transform for the transform coefficients;
• encoding image information including residual information derived based on the modified transform coefficients.
• wherein the deriving the modified transform coefficient includes deriving a transform kernel to be applied to the second-order transform,
• wherein, based on that a horizontal length and a vertical length of the target block are both greater than or equal to 8, and the horizontal length or the vertical length is 8, the transform kernel is set to a 32×64 matrix, and
• wherein, based on that both the horizontal length and the vertical length of the target block are 8, a 16×64 matrix sampled from the 32×64 matrix is applied to the second-order transform of the target block.

A computer-readable digital storage medium, wherein a bitstream generated according to a predetermined method is stored in the computer-readable digital storage medium, the method comprising:

• deriving residual samples for a target block based on prediction samples for the target block;
• deriving transform coefficients for the target block based on a first-order transform for the residual samples;
• deriving modified transform coefficients based on a second-order transform for the transform coefficients;
• encoding image information including residual information derived based on the modified transform coefficients to generate the bitstream,
• wherein the deriving the modified transform coefficient includes deriving a transform kernel to be applied to the second-order transform,
• wherein, based on that a horizontal length and a vertical length of the target block are both greater than or equal to 8, and the horizontal length or the vertical length is 8, the transform kernel is set to a 32×64 matrix, and
• wherein, based on that both the horizontal length and the vertical length of the target block are 8, a 16×64 matrix sampled from the 32×64 matrix is applied to the second-order transform of the target block.