ARBG to RGB

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).

Answer 2

if you don't need to know this pre-render, you could always use the win32 method of getpixel, I believe.

Note: typing on iPhone in the middle of Missouri with no inet access. Will look up real win32 example and see if there is a .net equivalent.

In case anyone cares, and doesn't want to use the (excellent) answer posted above, you can get the color value of a pixel in .Net via this link MSDN example