vec4 FUNCNAME(vec2 tc) { vec4 first = INPUT1(tc); vec4 second = INPUT2(tc); vec4 result = vec4(PREFIX(strength_first)) * first + vec4(PREFIX(strength_second)) * second; // Clamping alpha at some stage, either here or in AlphaDivisionEffect, // is actually very important for some use cases. Consider, for instance, // the case where we have additive blending (strength_first = strength_second = 1), // and add two 50% gray 100% opaque (0.5, 0.5, 0.5, 1.0) pixels. Without // alpha clamping, we'd get (1.0, 1.0, 1.0, 2.0), which would then in // conversion to postmultiplied be divided back to (0.5, 0.5, 0.5)! // Clamping alpha to 1.0 fixes the problem, and we get the expected result // of (1.0, 1.0, 1.0). Similarly, adding (0.5, 0.5, 0.5, 0.5) to itself // yields (1.0, 1.0, 1.0, 1.0) (100% white 100% opaque), which makes sense. // // The classic way of doing additive blending with premultiplied alpha // is to give the additive component alpha=0, but this also doesn't make // sense in a world where we could end up postmultiplied; just consider // the case where we have first=(0, 0, 0, 0) (ie., completely transparent) // and second=(0.5, 0.5, 0.5, 0.5) (ie., white at 50% opacity). // Zeroing out the alpha of second would yield (0.5, 0.5, 0.5, 0.0), // which has undefined RGB values in postmultiplied storage; certainly // e.g. (0, 0, 0, 0) would not be an expected output. Also, it would // break the expectation that A+B = B+A. // // Note that we do _not_ clamp RGB, since it might be useful to have // out-of-gamut colors. We could choose to do the alpha clamping in // AlphaDivisionEffect instead, though; I haven't thought a lot about // if that would be better or not. result.a = clamp(result.a, 0.0, 1.0); return result; }