diff -r 000000000000 -r 1918ee327afb src/opengl/util/composition_mode_screen.glsl
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/opengl/util/composition_mode_screen.glsl	Mon Jan 11 14:00:40 2010 +0000
@@ -0,0 +1,6 @@
+// Dca' = Sca + Dca - Sca.Dca
+// Da'  = Sa + Da - Sa.Da 
+vec4 composite(vec4 src, vec4 dst)
+{
+    return src + dst - src * dst;
+}