Encoder, a decoder and corresponding methods of chroma qantization control

A decoder or encoder, comprising:

• one or more processors;
• and
• a non-transitory computer-readable storage medium coupled to the one or more processors and storing programming for execution by the one or more processors, wherein the programming, when executed by the one or more processors, causes the decoder or encoder to:
• obtain a luminance QP for a luma component of a coding unit;
• obtain a QP index (QPi) based at least in a part on the luminance QP;
• obtain a chrominance QP (QPc) for a chroma component of the coding unit based on the QP index by using a preset lookup table, wherein the maximum difference between the QP index and the chrominance QP is equal to 3.