kernel FadeToHistory
<   nameSpace : "Kevin's Tutorial Filters";
    vendor : "Kevin's Filters, Inc";
    version : 1;
    description : "Fade from color to B&W to sepia";
>
{
    parameter float crossfade<minValue:0.0; maxValue:2.0; defaultValue:0.0;>;
    
    const float3 lumMult = float3(0.3, 0.59, 0.11);
    const float3x3 sepiaMatrix = float3x3(0.400, 0.769, 0.189,
                                          0.349, 0.686, 0.168,
                                          0.272, 0.534, 0.131);
    
    void
    evaluatePixel(in image4 src, out pixel4 dst)
    {
        dst = sampleNearest(src,outCoord());
        float3 luminance = float3( dot(dst.rgb, lumMult) );
        float3 sepia = dst.rgb * sepiaMatrix;
        
        float3 startMix = dst.rgb;
        startMix = (crossfade <= 1.0) ? startMix : luminance;
        float3 endMix = (crossfade <= 1.0) ? luminance : sepia;
        float mixMinusOne = crossfade - 1.0;
        float mixValue = (crossfade <= 1.0) ? crossfade : mixMinusOne;
        
        dst.rgb = mix(startMix, endMix, mixValue);
    }
}