Transform-based image coding method and device for same

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

• deriving prediction samples for a coding block by performing prediction;
• deriving residual samples for the coding block based on the prediction samples;
• deriving transform coefficients by performing a transform on the residual samples, the transform including at least one of a primary transform or a non-separable transform;
• and
• encoding, into a bitstream, prediction information on the prediction and residual information on the transform coefficients;
• wherein based on a size of the coding block being less than or equal to a size of a predetermined maximum transform block, a Low Frequency Non-Separable Transform (LFNST) index is encoded into the bitstream;
• and
• wherein based on a tree type of the coding block being a dual tree chroma and a color format being 4:2:0, a width and height of a block to which the non-separable transform is applied are derived as half the width and height of the coding block, respectively.

A non-transitory computer-readable digital storage medium that stores instructions that when executed by one or more processors cause the one or more processors to perform operations comprising:

• deriving prediction samples for a coding block by performing prediction;
• deriving residual samples for the coding block based on the prediction samples;
• deriving transform coefficients by performing a transform on the residual samples, the transform including at least one of a primary transform or a non-separable transform;
• and
• encoding prediction information on the prediction and residual information on the transform coefficients to generate a bitstream;
• wherein based on a size of the coding block being less than or equal to a size of a predetermined maximum transform block, a Low Frequency Non-Separable Transform (LFNST) index is encoded into the bitstream;
• and
• wherein based on a tree type of the coding block being a dual tree chroma and a color format being 4:2:0, a width and height of a block to which the non-separable transform is applied are derived as half the width and height of the coding block, respectively.

A transmission method for a bitstream, comprising:

• generating the bitstream by:
• deriving prediction samples for a coding block by performing prediction, deriving residual samples for the coding block based on the prediction samples, deriving transform coefficients by performing a transform on the residual samples, the transform including at least one of a primary transform or a non-separable transform, and encoding prediction information on the prediction and residual information on the transform coefficients;
• and
• transmitting the bitstream to an image decoding apparatus;
• wherein based on a size of the coding block being less than or equal to a size of a predetermined maximum transform block, a Low Frequency Non-Separable Transform (LFNST) index is encoded into the bitstream;
• and
• wherein based on a tree type of the coding block being a dual tree chroma and a color format being 4:2:0, a width and height of a block to which the non-separable transform is applied are derived as half the width and height of the coding block, respectively.