diff -r 000000000000 -r 1918ee327afb src/opengl/util/painter.glsl
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/opengl/util/painter.glsl	Mon Jan 11 14:00:40 2010 +0000
@@ -0,0 +1,21 @@
+uniform sampler2D dst_texture;
+uniform sampler2D mask_texture;
+uniform vec2 inv_mask_size;
+uniform vec2 inv_dst_size;
+uniform vec2 mask_offset;
+uniform vec4 mask_channel;
+
+float mask()
+{
+    return dot(mask_channel, texture2D(mask_texture, (gl_FragCoord.xy + mask_offset) * inv_mask_size));
+}
+
+void main()
+{
+    vec4 dst = texture2D(dst_texture, gl_FragCoord.xy * inv_dst_size);
+
+    // combine clip and coverage channels
+    float mask_alpha = mask();
+
+    gl_FragColor = mix(dst, composite(brush(), dst), mask_alpha);
+}