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 already has an alpha value. I'm just wondering how I can calculate the resulting colour.
For example:
The resulting pixel colour is 240,247,249 but I know that the original colour had a 10% opacity and was on top of a white (255,255,255) background. What's the calculation to work out the original RGB values?
Thanks in advance.
i'm just wondering how come you know its alpha and not it 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. and you will most likely have to round when going back from result to original.
If the blending mode is Normal, than I can't see what's your problem.
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).
The standard blending equation is:
out = alpha * new + (1 - alpha) * old
Where out, new and old are RGB colors, and alpha is a floating point number in the range [0,1].
So, you have (for red):
240 = 0.1 * newR + 0.9 * 255
Solving for newR, we get:
newR = (240 - 0.9 * 255) / 0.1
which evaluates to 105. Repeat for the other components, and you're done.