Color Similarity/distance In RGBA Color Space
Answer : Finally, I've found it! After thorough testing and experimentation my conclusions are: The correct way is to calculate maximum possible difference between the two colors. Formulas with any kind of estimated average/typical difference had room for discontinuities. I was unable to find a working formula that calculates the distance without blending RGBA colors with some backgrounds. There is no need to take every possible background color into account. It can be simplified down to blending maximum and minimum separately for each of R/G/B channels: blend the channel in both colors with channel =0 as the background, measure squared difference blend the channel in both colors with channel =max as the background, measure squared difference take higher of the two. Fortunately blending with "white" and "black" is trivial when you use premultiplied alpha. The complete formula for premultiplied alpha color space is: rgb *= a // colors must b...