Video 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 reduced secondary transform (RST) using a predetermined matrix kernel matrix for the transform coefficients;
• deriving quantized transform coefficients by performing quantization based on the modified transform coefficients, and
• generating flag information related to whether or not a transform index is present and the transform index, wherein the transform index is related to whether the inverse RST is applied to the target block and which one of the transform kernel matrices applied to the target block.

A non-transitory computer readable storage medium storing encoded information causing a decoding apparatus to perform an image decoding method, the method comprising:

• deriving transform coefficients through dequantization based on quantized transform coefficients for a target block;
• deriving modified transform coefficients based on an inverse reduced secondary transform (RST) using a preset transform kernel matrix for the transform coefficients;
• and
• deriving residual samples for the target block based on an inverse primary transform for the modified transform coefficients,
• wherein the deriving the modified transform coefficients comprises:
• deriving flag information related to whether or not a transform index is received, wherein the transform index is related to whether the inverse RST is applied to the target block and which one of the transform kernel matrices applied to the target block;
• deriving the transform index based on the flag information;
• and
• performing a matrix operation of predetermined transform coefficients in the target block and the transform kernel matrix based on the transform index.