//--------------------------------------------------------------------------------------
// AMD Tonemapper
//--------------------------------------------------------------------------------------
// General tonemapping operator, build 'b' term.
float ColToneB(float hdrMax, float contrast, float shoulder, float midIn, float midOut) 
{
    return
        -((-pow(midIn, contrast) + (midOut*(pow(hdrMax, contrast*shoulder)*pow(midIn, contrast) -
            pow(hdrMax, contrast)*pow(midIn, contrast*shoulder)*midOut)) /
            (pow(hdrMax, contrast*shoulder)*midOut - pow(midIn, contrast*shoulder)*midOut)) /
            (pow(midIn, contrast*shoulder)*midOut));
}

// General tonemapping operator, build 'c' term.
float ColToneC(float hdrMax, float contrast, float shoulder, float midIn, float midOut) 
{
    return (pow(hdrMax, contrast*shoulder)*pow(midIn, contrast) - pow(hdrMax, contrast)*pow(midIn, contrast*shoulder)*midOut) /
           (pow(hdrMax, contrast*shoulder)*midOut - pow(midIn, contrast*shoulder)*midOut);
}

// General tonemapping operator, p := {contrast,shoulder,b,c}.
float ColTone(float x, vec4 p) 
{ 
    float z = pow(x, p.r); 
    return z / (pow(z, p.g)*p.b + p.a); 
}

vec3 ToneMapping(vec3 color)
{
    const float hdrMax = 16.0; // How much HDR range before clipping. HDR modes likely need this pushed up to say 25.0.
    const float contrast = 2.0; // Use as a baseline to tune the amount of contrast the tonemapper has.
    const float shoulder = 1.0; // Likely don�t need to mess with this factor, unless matching existing tonemapper is not working well..
    const float midIn = 0.18; // most games will have a {0.0 to 1.0} range for LDR so midIn should be 0.18.
    const float midOut = 0.18; // Use for LDR. For HDR10 10:10:10:2 use maybe 0.18/25.0 to start. For scRGB, I forget what a good starting point is, need to re-calculate.

    float b = ColToneB(hdrMax, contrast, shoulder, midIn, midOut);
    float c = ColToneC(hdrMax, contrast, shoulder, midIn, midOut);

    #define EPS 1e-6f
    float peak = max(color.r, max(color.g, color.b));
    peak = max(EPS, peak);

    vec3 ratio = color / peak;
    peak = ColTone(peak, vec4(contrast, shoulder, b, c) );
    // then process ratio

    // probably want send these pre-computed (so send over saturation/crossSaturation as a constant)
    float crosstalk = 4.0; // controls amount of channel crosstalk
    float saturation = contrast; // full tonal range saturation control
    float crossSaturation = contrast*16.0; // crosstalk saturation

    float white = 1.0;

    // wrap crosstalk in transform
    ratio = pow(abs(ratio), vec3(saturation / crossSaturation));
    ratio = mix(ratio, vec3(white), vec3(pow(peak, crosstalk)));
    ratio = pow(abs(ratio), vec3(crossSaturation));

    // then apply ratio to peak
    color = peak * ratio;
    return color;
}