For you color geeks, I need some help.


King Bowser
So I'm doing a few calculations here and there. And I want to find a formula for transparency:

Let's say I have a 255,127,0 color. With 40% opacity (I think that's 102 in the alpha channel). I put that layer on a 15,15,15 background with 100% opacity (255 alpha). What is the formula to get the output?
(for reference, the output in this case is 132, 81, 30, but I want to find a consistent way to calculate that)

Probably this will help too.

Right now, I'm too trying to figure this, but if someone somewhere had already done this sort of calculations (most definitely), then I would be happy to see what they came up with.

EDIT: also if it is possible, I want to know how to directly modify the alpha channel of a pixel in photoshop.