[SHADER] hue-lightness ordered dithering - http://alex-charlton.com/posts/Dithering_on_the_GPU/

dither-hue-lightness.glsl

const float lightnessSteps = 4.0;

float lightnessStep(float l) {
    /* Quantize the lightness to one of `lightnessSteps` values */
    return floor((0.5 + l * lightnessSteps)) / lightnessSteps;
}

vec3 dither(vec3 color) {
    vec3 hsl = rgbToHsl(color);

    vec3 cs[2] = closestColors(hsl.x);
    vec3 c1 = cs[0];
    vec3 c2 = cs[1];
    float d = indexValue();
    float hueDiff = hueDistance(hsl.x, c1.x) / hueDistance(c2.x, c1.x);

    float l1 = lightnessStep(max((hsl.z - 0.125), 0.0));
    float l2 = lightnessStep(min((hsl.z + 0.124), 1.0));
    float lightnessDiff = (hsl.z - l1) / (l2 - l1);

    vec3 resultColor = (hueDiff < d) ? c1 : c2;
    resultColor.z = (lightnessDiff < d) ? l1 : l2;
    return hslToRgb(resultColor);
}