Quantization parameter derivation for cross-channel residual encoding and decoding

An apparatus comprising:

• at least one processor;
• and
• at least one memory including computer program code;
• the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform:
• receive a block that contains quantized residual coefficients based on at least two color channels of a video presentation or an image;
• obtain a first quantization parameter;
• obtain an offset defining a relationship between the first quantization parameter and a second quantization parameter for the block;
• determine a value of the second quantization parameter by using the first quantization parameter and the offset;
• and
• reconstruct residual samples by dequantizing the quantized residual coefficients using the second quantization parameter, if the block has been encoded by using a cross-channel residual coding mode.

A method comprising:

• obtaining a block that contains quantized residual coefficients based on at least two color channels of a video presentation or an image;
• determining a first quantization parameter;
• determining an offset defining a relationship between the first quantization parameter and a second quantization parameter for the block;
• determining a value of the second quantization parameter by using the first quantization parameter and the offset;
• determining whether the block has been encoded by using a cross-channel residual coding mode;
• and
• quantizing residual samples to obtain quantized residual coefficients by using the second quantization parameter, if the block has been encoded by using the cross-channel residual coding mode.

An apparatus comprising

• at least one processor;
• and
• at least one memory including computer program code;
• the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform:
• obtain a block that contains quantized residual coefficients based on at least two color channels of a video presentation or an image;
• determine a first quantization parameter;
• determine an offset defining a relationship between the first quantization parameter and a second quantization parameter for the block;
• determine a value of the second quantization parameter by using the first quantization parameter and the offset;
• determine whether the block has been encoded by using a cross-channel residual coding mode;
• and
• quantize residual samples to obtain quantized residual coefficients by using the second quantization parameter, if the block has been encoded by using the cross-channel residual coding mode.