// 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);

  float blend = smoothstep(0.0, 1.0, color.r - color.g);
  color.rgb = mix(vec3(color.r, luma, luma), vec3(luma, color.g, luma), blend);
}