Converting ARBG to RGB with alpha blending

Question

Let's say that we have an ARGB color:

Color argb = Color.FromARGB(127, 69, 12, 255); //Light Urple. 

When this is painted on top of an existing color, the colors will blend. So when it is blended with white, the resulting color is Color.FromARGB(255, 162, 133, 255);

The solution should work like this:

Color blend = Color.White;  Color argb = Color.FromARGB(127, 69, 12, 255); //Light Urple.       Color rgb = ToRGB(argb, blend); //Same as Color.FromARGB(255, 162, 133, 255); 

What is ToRGB's implementation?

Answer 1

It's called alpha blending.

In psuedocode, assuming the background color (blend) always has 255 alpha. Also assumes alpha is 0-255.

alpha=argb.alpha() r = (alpha/255)*argb.r() + (1 - alpha/255)*blend.r() g = (alpha/255)*argb.g() + (1 - alpha/255)*blend.g() b = (alpha/255)*argb.b() + (1 - alpha/255)*blend.b() 

note: you probably need to be a bit (more) careful about floating-point/int math and rounding issues, depending on language. Cast intermediates accordingly

Edited to add:

If you don't have a background color with an alpha of 255, the algebra gets alot more complicated. I've done it before and it's a fun exercise left to the reader (if you really need to know, ask another question :).

In other words, what color C blends into some background the same as blending A, then blending B. This is sort of like calculating A+B (which isn't the same as B+A).