How to calculate an RGB colour by specifying an alpha blending amount?

Question

I\'m writing a colour picker that gets the pixel RGB values from wherever you point to on the screen. I want to also have the option of specifying that the colour I picked

Answer 1

I'm just wondering how come you know its alpha and not its r,g,b... Unless the blending mode used is in every case Normal, you won't be able to calculate the original value. And even if all is ideal, you will only be able to approximate, as some rounding has probably taken place already when going from original to result. Also, you will most likely have to round when going back from result to original.

If the blending mode is Normal, then it's not too difficult :-)

opacity*original + (1-opacity)*background = resulting pixel

original R = (resulting R - ((1-opacity)*background R)) / opacity.
original G = (resulting G - ((1-opacity)*background G)) / opacity.
original B = (resulting B - ((1-opacity)*background B)) / opacity.

in which opacity is (alpha/100).