gistfile1.txt

With glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA), scaling both source and destination with the same factor:

Note that:
(src*a*k) + (dst*(1-a)*k) = ((src*a) + (dst*(1-a)))*k

But since k is a part of a, we have to do some trickery:

alpha = (1.0 - (1.0 - alpha)*factor)
alpha = 1.0 - (factor - alpha*factor)
alpha = 1.0 + (alpha-1)*factor


So the fragment shader code becomes:

color_out = vec4(color);
color_out.rgb = color_out.rgb*vec3(factor);
color_out.a = 1.0 + (color_out.a - 1.0)*factor;