myCookTorrance.h

#include "math.h"

vector Schlick(float CosT; vector FZero)
{
    return (FZero + (1-FZero) * pow(1-CosT, 5));
}


void myCookTorrance(vector nN, FZero, specular; float roughness; string categories, lightmask)
{
    vector nI = normalize(-I);  //from eye to surface
    float r2 = roughness * roughness;

    illuminance(P, nN, M_PI, bouncemask("reflect"), "categories", categories, "lightmask", lightmask)
    {
        //prepare variables
        vector nL = normalize(L); //direction to light
        vector H = normalize(nI + nL); // half vector

        float NdotH = dot(nN, H);
        float NdotL = dot(nN, nL);
        float IdotH = dot(nI, H);
        float NdotI = dot(nN, nI);


        //fresnel
        vector kr = Schlick(NdotI, FZero);


        //Geometry Function
        float G = min(1, (2*NdotH*NdotI/IdotH), (2*NdotH*NdotL/IdotH));


        //beckman
        float roughness_a = 1.0f / ( 4.0f * r2 * pow( NdotH, 4 ) );
        float roughness_b = NdotH * NdotH - 1.0f;
        float roughness_c = r2 * NdotH * NdotH;


        //Distribution Function
        float D = roughness_a * exp( roughness_b / roughness_c );
        

        //final result
        vector _specular = shadow(Cl) * (kr * D * G) / (M_PI * NdotI);
        storelightexport(getlightname(), "specular", _specular);
        specular += _specular;
    }
}