Image decoding method for chroma component and device therefor

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

• deriving prediction samples of a current chroma block based on inter prediction or intra prediction;
• deriving residual samples of the current chroma block based on the prediction samples;
• generating quantization parameter data for combined chroma coding of the residual samples based on a chroma type of the current chroma block;
• generating a flag representing whether the quantization parameter data for the combined chroma coding is present;
• generating quantization parameter data for a chroma cb component and quantization parameter data for a chroma cr component;
• and
• encoding image information including prediction information and residual information for the current chroma block, the quantization parameter data for the combined chroma coding, the quantization parameter data for the chroma cb component, the quantization parameter data for the chroma cr component and the flag,
• wherein the quantization parameter data for the combined chroma coding includes a syntax element for a starting quantization parameter of a 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 a 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.

A non-transitory computer-readable storage medium storing a bitstream generated by a method, the method comprising:

• deriving prediction samples of a current chroma block based on inter prediction or intra prediction;
• deriving residual samples of the current chroma block based on the prediction samples;
• generating quantization parameter data for combined chroma coding of the residual samples based on a chroma type of the current chroma block;
• generating a flag representing whether the quantization parameter data for the combined chroma coding is present;
• generating quantization parameter data for a chroma cb component and quantization parameter data for a chroma cr component;
• encoding image information including prediction information and residual information for the current chroma block, the quantization parameter data for the combined chroma coding, the quantization parameter data for the chroma cb component, the quantization parameter data for the chroma cr component and the flag;
• and
• generating the bitstream including the image information,
• wherein the quantization parameter data for the combined chroma coding includes a syntax element for a starting quantization parameter of a 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 a 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.