Transform-based image coding method, and device therefor

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

• deriving prediction samples for a current block;
• deriving residual samples for the current block based on the prediction samples;
• deriving transform coefficients for the current block based on a transform for the residual samples;
• and
• encoding residual information derived through quantization of the transform coefficients and a multiple transform selection (MTS) index related to transform kernels for the transform,
• wherein the MTS index is encoded into a bitstream based on a tree type of the current block, a partition type of the current block, a low frequency non-separable transform (LFNST) index related to an LFNST kernel for the current block, and a variable related to whether at least one significant coefficient is present in a second region other than a first region within the current block, the first region being a 16×16 region including a top-left sample position of the current block,
• wherein the variable is set to an initial value equal to 1 in a coding unit syntax,
• wherein based on the at least one significant coefficient being present in the second region other than the first region, the variable is changed to 0 in a residual coding syntax,
• wherein the LFNST index and the MTS index are encoded into the coding unit syntax, and
• wherein the MTS index is encoded immediately after the LFNST index is encoded into the coding unit syntax.

A method for transmitting data for image information comprising:

• obtaining a bitstream of the image information including residual information, wherein the bitstream is generated by deriving prediction samples for a current block, deriving residual samples for the current block based on the prediction samples;
• deriving transform coefficients for the current block based on a transform for the residual samples, and encoding residual information derived through quantization of the transform coefficients and a multiple transform selection (MTS) index related to transform kernels for the transform;
• and
• transmitting the data comprising the bitstream of the image information,
• wherein the MTS index is encoded into a bitstream based on a tree type of the current block, a partition type of the current block, a low frequency non-separable transform (LFNST) index related to an LFNST kernel for the current block, and a variable related to whether at least one significant coefficient is present in a second region other than a first region within the current block, the first region being a 16×16 region including a top-left sample position of the current block,
• wherein the variable is set to an initial value equal to 1 in a coding unit syntax,
• wherein based on the at least one significant coefficient being present in the second region other than the first region, the variable is changed to 0 in a residual coding syntax,
• wherein the LFNST index and the MTS index are encoded into the coding unit syntax, and
• wherein the MTS index is encoded immediately after the LFNST index is encoded into the coding unit level syntax.