Image decoding method using chroma quantization parameter table, and device therefor

1 . A decoding apparatus for image decoding, the decoding apparatus comprising:

a memory; and

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

obtain a flag representing whether one chroma quantization parameter table is applied or not;

obtain at least one chroma quantization parameter data based on the flag;

derive at least one chroma quantization parameter table based on the at least one chroma quantization parameter data;

derive at least one chroma quantization parameter for chroma components based on the at least one chroma quantization parameter table;

derive residual samples for the chroma components based on the at least one chroma quantization parameter; and

generate a reconstructed picture based on the residual samples,

wherein the flag equal to 0 represents that a first chroma quantization parameter table for a Cb component, a second chroma quantization parameter table for a Cr component, and a third chroma quantization parameter table for a joint CbCr component are signaled,

wherein the first chroma quantization parameter table for the Cb component is derived based on first chroma quantization parameter data, and the second chroma quantization parameter table for the Cr component is derived based on second chroma quantization parameter data,

wherein a first chroma quantization parameter for the Cb component is derived based on the first chroma quantization parameter table, and a second chroma quantization parameter for the Cr component is derived based on the second chroma quantization parameter table, and

wherein the at least one processor configured to derive the residual samples for the chroma components based on the at least one chroma quantization parameter comprises the at least one processor configured to:

derive transform coefficients for the chroma components based on received residual information;

derive residual samples for the Cb component by dequantizing transform coefficients for the Cb component based on the first chroma quantization parameter; and

derive residual samples for the Cr component by dequantizing transform coefficients for the Cr component based on the second chroma quantization parameter.

2 . An encoding apparatus for image encoding, the encoding apparatus comprising:

a memory; and

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

derive residual samples for chroma components;

generate a flag representing whether one chroma quantization parameter table is applied or not;

generate at least one chroma quantization parameter data for the chroma components based on the flag; and

encode the at least one chroma quantization parameter data and the flag,

wherein the flag equal to 0 represents that a first chroma quantization parameter table for a Cb component, a second chroma quantization parameter table for a Cr component, and a third chroma quantization parameter table for a joint CbCr component are signaled,

wherein the first chroma quantization parameter table for the Cb component is derived based on first chroma quantization parameter data, and the second chroma quantization parameter table for the Cr component is derived based on second chroma quantization parameter data,

wherein a first chroma quantization parameter for the Cb component is derived based on the first chroma quantization parameter table, and a second chroma quantization parameter for the Cr component is derived based on the second chroma quantization parameter table, and

wherein the at least one processor configured to derive the residual samples for the chroma components comprises the at least one processor configured to:

derive quantized residual samples for the Cb component by quantizing residual samples for the Cb component based on the first chroma quantization parameter;

derive quantized residual samples for the Cr component by quantizing residual samples for the Cr component based on the second chroma quantization parameter; and

derive transform coefficients based on transforming quantized chroma components.

3 . An apparatus for transmitting data for an image, the apparatus comprising:

at least one processor configured to obtain a bitstream of image information including a flag and at least one chroma quantization parameter data for chroma components; and

a transmitter configured to transmit the data including the bitstream of the image information including the flag and the at least one chroma quantization parameter data,

wherein residual samples for the chroma components is derived,

wherein the flag equal to 0 represents that a first chroma quantization parameter table for a Cb component, a second chroma quantization parameter table for a Cr component, and a third chroma quantization parameter table for a joint CbCr component are signaled,

wherein the first chroma quantization parameter table for the Cb component is derived based on first chroma quantization parameter data, and the second chroma quantization parameter table for the Cr component is derived based on second chroma quantization parameter data,

wherein a first chroma quantization parameter for the Cb component is derived based on the first chroma quantization parameter table, and a second chroma quantization parameter for the Cr component is derived based on the second chroma quantization parameter table, and

wherein deriving the residual samples for the chroma components comprises:

deriving quantized residual samples for the Cb component by quantizing residual samples for the Cb component based on the first chroma quantization parameter;

deriving quantized residual samples for the Cr component by quantizing residual samples for the Cr component based on the second chroma quantization parameter; and

deriving transform coefficients based on transforming quantized chroma components.