         case MixBlendMode_Multiply:
             result.rgb = Multiply(Cb.rgb, Cs.rgb);
             break;
-        case MixBlendMode_Screen:
-            result.rgb = Screen(Cb.rgb, Cs.rgb);
-            break;
         case MixBlendMode_Overlay:
             // Overlay is inverse of Hardlight
             result.rgb = HardLight(Cs.rgb, Cb.rgb);

        case MixBlendMode_Luminosity:
            result.rgb = Luminosity(Cb.rgb, Cs.rgb);
            break;
        case MixBlendMode_PlusLighter:
            // This should be unreachable, since we implement
            // MixBlendMode::PlusLighter using glBlendFuncSeparate.
            break;
        default: break;


vec3 Difference(vec3 Cb, vec3 Cs) {
    return abs(Cb - Cs);
}

// These functions below are taken from the spec.
