Performing chroma deblocking for blocks which use joint chroma coding

A deblocking method for deblocking a chroma block edge between a first chroma block of a first image block and a second chroma block of a second image block in an image encoding or an image decoding, comprising:

• performing a decision process for the chroma block edge;
• and
• performing a filtering process for the chroma block edge at least based on a threshold parameter (tC) determined in the decision process;
• wherein the decision process comprises:
• determining a first chroma quantization parameter (QpCp) for the first chroma block, wherein the first chroma quantization parameter (QpCp) is determined based on a first luma quantization parameter (QpYP) of a first luma block of the first image block and a chroma quantization parameter (QP) mapping table for the first chroma block;
• determining a second chroma quantization parameter (QpCq) for the second chroma block, wherein the second chroma quantization parameter (QpCq) is determined based on a second luma quantization parameter (QpYQ) of a second luma block of the second image block and a chroma QP mapping table for the second chroma block;
• determining a third chroma quantization parameter (QpC) based on the first chroma quantization parameter (QpCp) for the first chroma block and the second chroma quantization parameter (QpCq) for the second chroma block;
• and
• determining the threshold parameter (tC) based on the third chroma quantization parameter (QpC).