Seele/res/shaders/lib/BRDF.slang

import Common;

interface IBRDF
{
    float3 evaluate(float3 view, float3 light, float3 normal, float3 tangent, float3 biTangent, float3 lightColor);
};

struct BlinnPhong : IBRDF
{
    float3 baseColor;
    float metallic = 0;
    float3 normal = float3(0, 1, 0);
    float specular = 0.5;
    float roughness = 0.5;
    float sheen = 1.f;

    float3 evaluate(float3 view, float3 light, float3 surfaceNormal, float3 tangent, float3 biTangent, float3 lightColor)
    {
        float nDotL = saturate(dot(normal, light));
        float3 h = normalize(light + view);
        float nDotH = saturate(dot(normal, h));
		
        return baseColor * nDotL + lightColor * specular * pow(nDotH, sheen);
    }
};

struct DisneyBRDF : IBRDF
{
    float3 baseColor;
    float metallic = 0;
    float3 normal = float3(0, 1, 0);
    float subsurface = 0;
    float specular = 0.5;
    float roughness = 0.5;
    float specularTint = 0;
    float anisotropic = 0;
    float sheen = 0;
    float sheenTint = 0.5f;
    float clearCoat = 0;
    float clearCoatGloss = 1;

    float sqr(float x)
    {
        return x * x;
    }

    float SchlickFresnel(float u)
    {
        float m = clamp(1 - u, 0, 1);
        float m2 = m * m;
        return m2 * m2 * m; // pow(m,5)
    }
   
    float GTR1(float NdotH, float a)
    {
        if (a >= 1)
            return 1 / PI;
        float a2 = a * a;
        float t = 1 + (a2 - 1) * NdotH * NdotH;
        return (a2 - 1) / (PI * log(a2) * t);
    }
   
    float GTR2(float NdotH, float a)
    {
        float a2 = a * a;
        float t = 1 + (a2 - 1) * NdotH * NdotH;
        return a2 / (PI * t * t);
    }
   
    float GTR2_aniso(float NdotH, float HdotX, float HdotY, float ax, float ay)
    {
        return 1 / (PI * ax * ay * sqr(sqr(HdotX / ax) + sqr(HdotY / ay) + NdotH * NdotH));
    }

    float smithG_GGX(float NdotV, float alphaG)
    {
        float a = alphaG * alphaG;
        float b = NdotV * NdotV;
        return 1 / (NdotV + sqrt(a + b - a * b));
    }

    float smithG_GGX_aniso(float NdotV, float VdotX, float VdotY, float ax, float ay)
    {
        return 1 / (NdotV + sqrt(sqr(VdotX * ax) + sqr(VdotY * ay) + sqr(NdotV)));
    }

    float3 mon2lin(float3 x)
    {
        return float3(pow(x[0], 2.2), pow(x[1], 2.2), pow(x[2], 2.2));
    }


    float3 evaluate(float3 V, float3 L, float3 surfaceNormal, float3 X, float3 Y, float3 lightColor)
    {
		float3 N = normal;
        float NdotL = dot(N, L);
        float NdotV = dot(N, V);
        if (NdotL < 0 || NdotV < 0)
            return float3(0);
        float3 H = normalize(L + V);
       
        float NdotH = dot(N, H);
        float LdotH = dot(L, H);
        float3 Cdlin = mon2lin(baseColor);
        float Cdlum = .3 * Cdlin[0] + .6 * Cdlin[1] + .1 * Cdlin[2]; // luminance approx.
        float3 Ctint = Cdlum > 0 ? Cdlin / Cdlum : float3(1); // normalize lum. to isolate hue+sat
        float3 Cspec0 = lerp(specular * .08 * lerp(float3(1), Ctint, specularTint), Cdlin, metallic);
        float3 Csheen = lerp(float3(1), Ctint, sheenTint);
        
        // Diffuse fresnel - go from 1 at normal incidence to .5 at grazing
        // and mix in diffuse retro-reflection based on roughness
        float FL = SchlickFresnel(NdotL), FV = SchlickFresnel(NdotV);
        float Fd90 = 0.5 + 2 * LdotH * LdotH * roughness;
        float Fd = lerp(1.0, Fd90, FL) * lerp(1.0, Fd90, FV);
        
        // Based on Hanrahan-Krueger brdf approximation of isotropic bssrdf
        // 1.25 scale is used to (roughly) preserve albedo
        // Fss90 used to "flatten" retroreflection based on roughness
        float Fss90 = LdotH * LdotH * roughness;
        float Fss = lerp(1.0, Fss90, FL) * lerp(1.0, Fss90, FV);
        float ss = 1.25 * (Fss * (1 / (NdotL + NdotV) - .5) + .5);
        
        // specular
        float aspect = sqrt(1 - anisotropic * .9);
        float ax = max(.001, sqr(roughness) / aspect);
        float ay = max(.001, sqr(roughness) * aspect);
        float Ds = GTR2_aniso(NdotH, dot(H, X), dot(H, Y), ax, ay);
        float FH = SchlickFresnel(LdotH);
        float3 Fs = lerp(Cspec0, float3(1), FH);
        float Gs;
        Gs = smithG_GGX_aniso(NdotL, dot(L, X), dot(L, Y), ax, ay);
        Gs *= smithG_GGX_aniso(NdotV, dot(V, X), dot(V, Y), ax, ay);
               
        // sheen
        float3 Fsheen = FH * sheen * Csheen;
               
        // clearcoat (ior = 1.5 -> F0 = 0.04)
        float Dr = GTR1(NdotH, lerp(.1, .001, clearCoatGloss));
        float Fr = lerp(.04, 1.0, FH);
        float Gr = smithG_GGX(NdotL, .25) * smithG_GGX(NdotV, .25);
       
        return ((1 / PI) * lerp(Fd, ss, subsurface) * Cdlin + Fsheen)
           * (1 - metallic)
           + Gs * Fs * Ds + .25 * clearCoat * Gr * Fr * Dr;
    }
};