Clamp alpha in MixEffect.

[movit] / mix_effect.frag

diff --git a/mix_effect.frag b/mix_effect.frag
index f76881ba2519012fae38f65f3981f10ed009c6d4..450ca21f5ed285f349f330f431740fb6d75785b0 100644 (file)
--- a/mix_effect.frag
+++ b/mix_effect.frag
@@ -1,5 +1,33 @@
 vec4 FUNCNAME(vec2 tc) {
        vec4 first = INPUT1(tc);
        vec4 second = INPUT2(tc);
 vec4 FUNCNAME(vec2 tc) {
        vec4 first = INPUT1(tc);
        vec4 second = INPUT2(tc);

-       return vec4(PREFIX(strength_first)) * first + vec4(PREFIX(strength_second)) * second;
+       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;

 }
 }