float calculateLuminance(vec3 rgb) {
    // This is the luminance calculation part of the RGB to HSL formular.
    vec4 p = mix(
                 vec4(rgb.gb, 0.0, -1.0 / 3.0),
                 vec4(rgb.bg, -1.0, 2.0 / 3.0),
                 vec4(rgb.g < rgb.b)
                 );

    vec4 q = mix(
                 vec4(rgb.r, p.yzx),
                 vec4(p.xyw, rgb.r),
                 vec4(rgb.r < p.x)
                 );

    float croma = q.x - min(q.w, q.y);
    float luminance = q.x - croma * 0.5;
    return luminance;
}

kernel vec4 coreImageKernel(sampler image, float u_shadows, float u_highlights, vec4 c0, vec4 c1, vec4 c2, vec4 c3, vec4 bias) {
    vec4 color =  clamp(sample(image, samplerCoord(image)), 0.0, 1.0);

    // Apply shadows and highlights
    float luminance = calculateLuminance(color.rgb);

    float shadow = u_shadows >= 0.0
    ? clamp(
            pow(luminance, 1.0 / (u_shadows + 1.0))
            + pow(luminance, 2.0 / (u_shadows + 1.0)) * -0.76
            - luminance
            , 0.0, 1.0)
    : -clamp(
             pow(luminance, 1.0 / (-u_shadows + 1.0))
             + pow(luminance, 2.0 / (-u_shadows + 1.0)) * -0.76
             - luminance
             , 0.0, 1.0);

    float highlight = u_highlights < 0.0
    ? clamp(
            1.0
            - pow(1.0 - luminance, 1.0 / (1.0 - u_highlights))
            - pow(1.0 - luminance, 2.0 / (1.0 - u_highlights)) * -0.8
            - luminance
            , -1.0, 0.0)
    : -clamp(
             1.0
             - pow(1.0 - luminance, 1.0 / (1.0 + u_highlights))
             - pow(1.0 - luminance, 2.0 / (1.0 + u_highlights)) * -0.8
             - luminance
             , -1.0, 0.0);

    // Bright color need more contrast and dark color need more brightness.
    // This is to keep saturatation because the color information of a dark colors is lost.
    float shadowContrast   = shadow * luminance * luminance;
    float shadowBrightness = shadow - shadowContrast;
    color.rgb = clamp((luminance + highlight + shadowContrast) * ((color.rgb + shadowBrightness) / max(luminance, 0.0000001)), 0.0, 1.0);

    // Apply Color Matrix
    color.rgb /= max(color.a, 0.00001);
    color = clamp(vec4(dot(color,c0), dot(color,c1), dot(color,c2), dot(color,c3)) + bias, 0.0, 1.0);
    color.rgb *= color.a;
    return color;
}