X-Git-Url: https://git.sesse.net/?p=movit;a=blobdiff_plain;f=mix_effect.frag;h=450ca21f5ed285f349f330f431740fb6d75785b0;hp=f76881ba2519012fae38f65f3981f10ed009c6d4;hb=95edbfccb0843da3cc105dadc5bc6d8e102f6071;hpb=ef7665d0d3854b3464800d8d7fef9a90f14d9a9f;ds=sidebyside

diff --git a/mix_effect.frag b/mix_effect.frag
index f76881b..450ca21 100644
--- a/mix_effect.frag
+++ b/mix_effect.frag
@@ -1,5 +1,33 @@
 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;
 }