texcoords[0].zw).rgb,                      1.0);
  vec4 pixel3 = vec4(texture2D(s_texture, v_texcoords[1].xy).rgb,                      1.0);
  vec4 pixel4 = vec4(texture2D(s_texture, v_texcoords[1].zw).rgb,                      1.0);
  vec4 pixel12 = (pixel1 + pixel2) / 2.0;
  vec4 pixel34 = (pixel3 + pixel4) / 2.0;
  gl_FragData[0] = vec4(dot(pixel1, rgb_to_plane0),
                        dot(pixel2, rgb_to_plane0),
                        dot(pixel3, rgb_to_plane0),
                        dot(pixel4, rgb_to_plane0));
  gl_FragData[1] = vec4(dot(pixel12, rgb_to_plane1),
                        dot(pixel34, rgb_to_plane1),
                        dot(pixel12, rgb_to_plane2),
                        dot(pixel34, rgb_to_plane2));
