Threshold is the obvious dithering technique (or "quantization algorithm") that makes similar sections appear as a solid. If you want to perform a statistical analysis to decided which colors to use beforehand, you could then adjust each pixel according to which of those chosen colors it is closest to. This would give a cartoon look (i.e., looks like cell shading rather than interspersed dots), which I believe is what you are after. Not sure if there is a name for the algorithm which combines closest match and statistical analysis, but if not then just create it and give it any name you like (e.g., Smart Threshold). :)
Submit an article or tip