Global Tonemapping Operator - Lottes (2016)

global_tonemapping_operator_Lottes2016.rpp

// https://github.com/KhronosGroup/glTF-Compressonator/blob/master/Compressonator/Applications/_Plugins/C3DModel_viewers/glTF_DX12_EX/DX12Util/shaders/Tonemapping.hlsl

kernel GlobalTonemappingOperator_Lottes2016 : ImageComputationKernel<ePixelWise>
{
    Image<eRead, eAccessPoint, eEdgeClamped> src;
    Image<eWrite> dst; 

    param:
        float hdrMax;
        float contrast;
        float shoulder;
        float midIn;
        float midOut;

    void define() {
        defineParam(hdrMax, "hdrMax", 16.0f);
        defineParam(contrast, "contrast", 2.0f);
        defineParam(shoulder, "shoulder", 1.0f);
        defineParam(midIn, "midIn", 0.18f);
        defineParam(midOut, "midOut", 0.18f);
    }

    void init() {
    }

    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, float4 p) 
    { 
        float z = pow(x, p.x); 
        return z / (pow(z, p.y)*p.z + p.w); 
    }

    float3 lerp(float3 a, float3 b, float c) {
        return (1 - c) * a + c * b;
    }

    float3 GlobalTonemappingOperator(float3 color)
    {
        float b = ColToneB(hdrMax, contrast, shoulder, midIn, midOut);
        float c = ColToneC(hdrMax, contrast, shoulder, midIn, midOut);

        float peak = max(color.x, max(color.y, color.z));
        float3 ratio = color / peak;
        peak = ColTone(peak, float4(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
        float saturationCrossSaturation = saturation / crossSaturation;
        ratio = pow(fabs(ratio), float3(saturationCrossSaturation, saturationCrossSaturation, saturationCrossSaturation));
        ratio = lerp(ratio, float3(white, white, white), pow(peak, crosstalk));
        ratio = pow(fabs(ratio), float3(crossSaturation, crossSaturation, crossSaturation));

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

    void process() {
        // Read the input image
        SampleType(src) input = src();

        // Isolate the RGB components
        float3 srcPixel(input.x, input.y, input.z);

        float3 tonemappedPixel = GlobalTonemappingOperator(srcPixel);

        // Write the result to the output image
        dst() = float4(tonemappedPixel.x, tonemappedPixel.y, tonemappedPixel.z, input.w);
    }
};