a memory; and

at least one processor connected to the memory, the at least one processor configured to:

obtain image information including prediction information and residual information for a current chroma block, and a flag representing whether quantization parameter data for combined chroma coding is present, based on a chroma type;

obtain the quantization parameter data for the combined chroma coding based on the flag;

obtain quantization parameter data for a chroma cb component and quantization parameter data for a chroma cr component;

derive a chroma quantization parameter mapping table for the combined chroma coding based on the quantization parameter data for the combined chroma coding;

derive a chroma quantization parameter mapping table for the chroma cb component and a chroma quantization parameter mapping table for the chroma cr component based on the quantization parameter data for the chroma cb component and the quantization parameter data for the chroma cr component;

derive a quantization parameter for the combined chroma coding based on the chroma quantization parameter mapping table for the combined chroma coding;

derive a quantization parameter for the chroma cb component and a quantization parameter for the chroma cr component based on the chroma quantization parameter mapping table for the chroma cb component and the chroma quantization parameter mapping table for the chroma cr component;

derive prediction samples of the current chroma block based on the prediction information;

derive transform coefficients of the current chroma block based on the residual information;

derive residual samples by dequantizing the transform coefficients based on a quantization parameter related to the current chroma block among the quantization parameter for the combined chroma coding, the quantization parameter for the chroma cb component and the quantization parameter for the chroma cr component; and

generate a reconstructed picture based on the prediction samples and the residual samples,

wherein the quantization parameter data for the combined chroma coding includes a syntax element for a starting quantization parameter of the chroma quantization parameter mapping table for the combined chroma coding and a syntax element for a number of quantization parameters in the chroma quantization parameter mapping table for the combined chroma coding,

wherein the quantization parameter for the combined chroma coding is derived based on a quantization parameter corresponding to a quantization parameter of a luma component in the chroma quantization parameter mapping table for the combined chroma coding.