// Values obtained from: http://en.wikipedia.org/wiki/Grayscale#Luma_coding_in_video_systems
vec3 LumaFactors = vec3(0.299, 0.587, 0.114);

void windowShader(inout vec4 color) {
  // average brightness
  float luma = dot(vec3(color.r, color.g, color.b), LumaFactors);

  if (color.g < color.b) {
    color = vec4(luma, luma, color.b, color.a);
  } else {
    color = vec4(luma, color.g, luma, color.a);
  }
  // color = vec4(luma, color.g, color.b, color.a);
}