Image decoding method for chroma component and device therefor
An image decoding method performed by a decoding device, according to the present document, comprises the steps of: deriving a chroma quantization parameter table on the basis of quantization parameter data; deriving a quantization parameter for combined chroma coding on the basis of the chroma quantization parameter table; deriving prediction samples of a current chroma block on the basis of prediction information; deriving transform coefficients of the current chroma block on the basis of residual information; deriving residual samples by dequantizing the transform coefficients on the basis of the quantization parameter; and generating a reconstructed picture on the basis of the prediction samples and the residual samples.
1. An image decoding method, performed by a decoding apparatus, comprising:
obtaining 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;
obtaining the quantization parameter data for the combined chroma coding based on the flag;
obtaining quantization parameter data for a chroma cb component and quantization parameter data for a chroma cr component;
deriving a chroma quantization parameter mapping table for the combined chroma coding based on the quantization parameter data for the combined chroma coding;
deriving 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;
deriving a quantization parameter for the combined chroma coding based on the chroma quantization parameter mapping table for the combined chroma coding;
deriving 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;
deriving prediction samples of the current chroma block based on the prediction information;
deriving transform coefficients of the current chroma block based on the residual information;
deriving 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
generating 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.
2. The image decoding method of claim 1 , wherein deriving the prediction samples of the current chroma block based on the prediction information comprises:
deriving a prediction mode of the current chroma block based on the prediction information; and
deriving the prediction samples based on the prediction mode.
3. The image decoding method of claim 1 , wherein the quantization parameter data for the combined chroma coding includes a syntax element representing an offset for deriving the quantization parameter for the combined chroma coding.
4. The image decoding method of claim 3 , wherein the flag is signaled through a picture parameter set (PPS).
5. The image decoding method of claim 1 , wherein:
when a value of the flag is 0, the flag represents that the quantization parameter data for the combined chroma coding is not present, and
when a value of the flag is 1, the flag represents that the quantization parameter data for the combined chroma coding is present.
6. The image decoding method of claim 1 , wherein the flag is signaled through a sequence parameter set (SPS).
7. The image decoding method of claim 1 , wherein the quantization parameter data for the combined chroma coding includes syntax elements for quantization parameter values of indices of the chroma quantization parameter mapping table for the combined chroma coding.
8. The image decoding method of claim 1 , wherein a value of the chroma type is not 0.
9. The image decoding method of claim 1 , wherein the quantization parameter for the combined chroma coding is derived based on a quantization parameter for an index identical with an index of a quantization parameter for a luma component corresponding to the current chroma block.
10. 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.
11. The image encoding method of claim 10 , wherein the prediction information includes prediction mode information representing a prediction mode of the current chroma block.
12. The image encoding method of claim 10 , wherein the flag is signaled through a sequence parameter set (SPS).
13. The image encoding method of claim 10 , wherein the quantization parameter data includes syntax elements for quantization parameter values of indices of the chroma quantization parameter mapping table.
14. The image encoding method of claim 10 , wherein the flag is signaled through a picture parameter set (PPS).
15. 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.