obtaining image information;

deriving a quantization parameter for joint chroma coding based on the image information;

deriving residual samples based on the quantization parameter for the joint chroma coding; and

generating a reconstructed picture based on the residual samples,

wherein the obtaining the image information comprises:

obtaining a flag representing whether quantization parameter data for joint chroma coding is present, based on a chroma array type; and

obtaining the quantization parameter data for the joint chroma coding based on the flag,

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

wherein the chroma quantization parameter mapping table is derived based on the quantization parameter data,

wherein the quantization parameter for the joint chroma coding is derived based on the chroma quantization parameter mapping table, and

wherein the flag representing whether the quantization parameter data for the joint chroma coding is present is included in an SPS (Sequence Parameter Set) of the image information.