Cg Shaders Alpha Blending

Question

I want to use Alpha Blending (SRC_ALPHA, ONE_MINUS_SRC_ALPHA), which basically is this:

frag_out.aaaa * frag.out + ((1.0, 1.0, 1.0, 1.0) - frag_out.aaaa) * pixel_color

Let's say the pixel color already on screen is (1.0, 1.0, 1.0, 1.0) The color I am currently rendering is (1.0, 1.0, 1.0, 0.4)

When I render this the resulting color on screen has an alpha 0.76 (even though it was fully opaque BEFORE)

Indeed:

(0.4, 0.4, 0.4, 0.4) * (1.0, 1.0, 1.0, 0.4) + (0.6, 0.6, 0.6, 0.6) * (1.0, 1.0, 1.0, 1.0) = (1.0, 1.0, 1.0, 0.76)

So basically the color on the screen was opaque (white) and after I put a transparent sprite on top the screen become transparent (0.76 alpha) and I am able to look through the background. It would be perfect if I could blend it like this:

(0.4, 0.4, 0.4, 1.0) * (1.0, 1.0, 1.0, 0.4) + (0.6, 0.6, 0.6, 0.6) * (1.0, 1.0, 1.0, 1.0) = (1.0, 1.0, 1.0, 1.0)

Is it possible to achieve this? If yes, how?

Answer 1

This is perfectly possible when using OpenGL 3.0 and above since the blend functions can there be set separately for RGB and A. For your case this would be

glBlendFuncSeparate(GL_SOURCE_ALPHA, GL_ONE_MINUS_SOURCE_ALPHA, GL_ONE, GL_ONE);

The last parameter specifying the blend factor for destination alpha might have to be adapted since you didn't specify the behavior for destination alpha < 1. For more details have a look at the function documentation.