Color averaging in an image?

Question

Image processing

The question is. There is an image at the entrance. For example, this:

The task is to replace colors that are close to each other with their average value.
Those. make the image simpler. Is there an algorithm suitable for this task?
Here is an example of an enlarged area of ​​the image

Answer 1

Posterization, Quantization , Isohelium is what they call it