Added an overlay effect, implementing the atop effect.

[movit] / overlay_effect.frag

diff --git a/overlay_effect.frag b/overlay_effect.frag
new file mode 100644 (file)
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);
+       }
+}