// 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) {
  // Desaturate to luma only
  float luma = dot(vec3(color.r, color.g, color.b), LumaFactors);

  color.rgb = vec3(luma, luma, luma);
}