bsdf_ashikhmin_velvet.h

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/*
 * Adapted from Open Shading Language with this license:
 *
 * Copyright (c) 2009-2010 Sony Pictures Imageworks Inc., et al.
 * All Rights Reserved.
 *
 * Modifications Copyright 2011, Blender Foundation.
 * 
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 * modification, are permitted provided that the following conditions are
 * met:
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 *   notice, this list of conditions and the following disclaimer.
 * * Redistributions in binary form must reproduce the above copyright
 *   notice, this list of conditions and the following disclaimer in the
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 *   contributors may be used to endorse or promote products derived from
 *   this software without specific prior written permission.
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*/

#ifndef __BSDF_ASHIKHMIN_VELVET_H__
#define __BSDF_ASHIKHMIN_VELVET_H__

CCL_NAMESPACE_BEGIN

typedef struct BsdfAshikhminVelvetClosure {
    //float3 m_N;
    float m_invsigma2;
} BsdfAshikhminVelvetClosure;

__device void bsdf_ashikhmin_velvet_setup(ShaderData *sd, ShaderClosure *sc, float sigma)
{
    sigma = fmaxf(sigma, 0.01f);

    float m_invsigma2 = 1.0f/(sigma * sigma);

    sc->type = CLOSURE_BSDF_ASHIKHMIN_VELVET_ID;
    sd->flag |= SD_BSDF|SD_BSDF_HAS_EVAL;
    sc->data0 = m_invsigma2;
}

__device void bsdf_ashikhmin_velvet_blur(ShaderClosure *sc, float roughness)
{
}

__device float3 bsdf_ashikhmin_velvet_eval_reflect(const ShaderData *sd, const ShaderClosure *sc, const float3 I, const float3 omega_in, float *pdf)
{
    float m_invsigma2 = sc->data0;
    float3 m_N = sd->N;

    float cosNO = dot(m_N, I);
    float cosNI = dot(m_N, omega_in);
    if(cosNO > 0 && cosNI > 0) {
        float3 H = normalize(omega_in + I);

        float cosNH = dot(m_N, H);
        float cosHO = fabsf(dot(I, H));

        if(!(fabsf(cosNH) < 1.0f-1e-5f && cosHO > 1e-5f))
            return make_float3(0, 0, 0);

        float cosNHdivHO = cosNH / cosHO;
        cosNHdivHO = fmaxf(cosNHdivHO, 1e-5f);

        float fac1 = 2 * fabsf(cosNHdivHO * cosNO);
        float fac2 = 2 * fabsf(cosNHdivHO * cosNI);

        float sinNH2 = 1 - cosNH * cosNH;
        float sinNH4 = sinNH2 * sinNH2;
        float cotangent2 = (cosNH * cosNH) / sinNH2;

        float D = expf(-cotangent2 * m_invsigma2) * m_invsigma2 * M_1_PI_F / sinNH4;
        float G = min(1.0f, min(fac1, fac2)); // TODO: derive G from D analytically

        float out = 0.25f * (D * G) / cosNO;

        *pdf = 0.5f * M_1_PI_F;
        return make_float3(out, out, out);
    }

    return make_float3(0, 0, 0);
}

__device float3 bsdf_ashikhmin_velvet_eval_transmit(const ShaderData *sd, const ShaderClosure *sc, const float3 I, const float3 omega_in, float *pdf)
{
    return make_float3(0.0f, 0.0f, 0.0f);
}

__device float bsdf_ashikhmin_velvet_albedo(const ShaderData *sd, const ShaderClosure *sc, const float3 I)
{
    return 1.0f;
}

__device int bsdf_ashikhmin_velvet_sample(const ShaderData *sd, const ShaderClosure *sc, float randu, float randv, float3 *eval, float3 *omega_in, float3 *domega_in_dx, float3 *domega_in_dy, float *pdf)
{
    float m_invsigma2 = sc->data0;
    float3 m_N = sd->N;

    // we are viewing the surface from above - send a ray out with uniform
    // distribution over the hemisphere
    sample_uniform_hemisphere(m_N, randu, randv, omega_in, pdf);

    if(dot(sd->Ng, *omega_in) > 0) {
        float3 H = normalize(*omega_in + sd->I);

        float cosNI = dot(m_N, *omega_in);
        float cosNO = dot(m_N, sd->I);
        float cosNH = dot(m_N, H);
        float cosHO = fabsf(dot(sd->I, H));

        if(fabsf(cosNO) > 1e-5f && fabsf(cosNH) < 1.0f-1e-5f && cosHO > 1e-5f) {
            float cosNHdivHO = cosNH / cosHO;
            cosNHdivHO = fmaxf(cosNHdivHO, 1e-5f);

            float fac1 = 2 * fabsf(cosNHdivHO * cosNO);
            float fac2 = 2 * fabsf(cosNHdivHO * cosNI);

            float sinNH2 = 1 - cosNH * cosNH;
            float sinNH4 = sinNH2 * sinNH2;
            float cotangent2 =  (cosNH * cosNH) / sinNH2;

            float D = expf(-cotangent2 * m_invsigma2) * m_invsigma2 * M_1_PI_F / sinNH4;
            float G = min(1.0f, min(fac1, fac2)); // TODO: derive G from D analytically

            float power = 0.25f * (D * G) / cosNO;

            *eval = make_float3(power, power, power);

#ifdef __RAY_DIFFERENTIALS__
            // TODO: find a better approximation for the retroreflective bounce
            *domega_in_dx = (2 * dot(m_N, sd->dI.dx)) * m_N - sd->dI.dx;
            *domega_in_dy = (2 * dot(m_N, sd->dI.dy)) * m_N - sd->dI.dy;
            *domega_in_dx *= 125.0f;
            *domega_in_dy *= 125.0f;
#endif
        }
        else
            *pdf = 0.0f;
    }
    else
        *pdf = 0.0f;

    return LABEL_REFLECT|LABEL_DIFFUSE;
}

CCL_NAMESPACE_END

#endif /* __BSDF_ASHIKHMIN_VELVET_H__ */