X-Git-Url: https://git.sesse.net/?p=movit;a=blobdiff_plain;f=overlay_effect.frag;fp=overlay_effect.frag;h=38970eab7e85469943353b85e9690d87ba59e5a8;hp=0000000000000000000000000000000000000000;hb=5e05605a85a15944b95d7000745446756cdcbfa8;hpb=49ef2523611e215b327cd26f7bf698d1516edd46

diff --git a/overlay_effect.frag b/overlay_effect.frag
new file mode 100644
index 0000000..38970ea
--- /dev/null
+++ b/overlay_effect.frag
@@ -0,0 +1,26 @@
+// If we didn't have to worry about alpha in the bottom layer,
+// this would be a simple mix(). However, since people might
+// compose multiple layers together and we don't really have
+// any control over the order, it's better to do it right.
+//
+// These formulas come from Wikipedia:
+//
+//   http://en.wikipedia.org/wiki/Alpha_compositing
+
+vec4 FUNCNAME(vec2 tc) {
+	vec4 bottom = INPUT1(tc);
+	vec4 top = INPUT2(tc);
+	float new_alpha = mix(bottom.a, 1.0, top.a);
+	if (new_alpha < 1e-6) {
+		// new_alpha = 0 only if top.a = bottom.a = 0, at least as long as
+		// both alphas are within range. (If they're not, the result is not
+		// meaningful anyway.) And if new_alpha = 0, we don't really have
+		// any meaningful output anyway, so just set it to zero instead of
+		// getting division-by-zero below.
+		return vec4(0.0);
+	} else {
+		vec3 premultiplied_color = mix(bottom.rgb * bottom.aaa, top.rgb, top.a);
+		vec3 color = premultiplied_color / new_alpha;
+		return vec4(color.r, color.g, color.b, new_alpha);
+	}
+}