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.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 * * Redistributions of source code must retain the above copyright
 *   notice, this list of conditions and the following disclaimer.
 * * Redistributions in binary form must reproduce the above copyright
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 *   contributors may be used to endorse or promote products derived from
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#ifndef __BSDF_ASHIKHMIN_VELVET_H__
#define __BSDF_ASHIKHMIN_VELVET_H__
 
CCL_NAMESPACE_BEGIN
 
typedef ccl_addr_space struct VelvetBsdf {
  SHADER_CLOSURE_BASE;
 
  float sigma;
  float invsigma2;
} VelvetBsdf;
 
static_assert(sizeof(ShaderClosure) >= sizeof(VelvetBsdf), "VelvetBsdf is too large!");
 
ccl_device int bsdf_ashikhmin_velvet_setup(VelvetBsdf *bsdf)
{
  float sigma = fmaxf(bsdf->sigma, 0.01f);
  bsdf->invsigma2 = 1.0f / (sigma * sigma);
 
  bsdf->type = CLOSURE_BSDF_ASHIKHMIN_VELVET_ID;
 
  return SD_BSDF | SD_BSDF_HAS_EVAL;
}
 
ccl_device bool bsdf_ashikhmin_velvet_merge(const ShaderClosure *a, const ShaderClosure *b)
{
  const VelvetBsdf *bsdf_a = (const VelvetBsdf *)a;
  const VelvetBsdf *bsdf_b = (const VelvetBsdf *)b;
 
  return (isequal_float3(bsdf_a->N, bsdf_b->N)) && (bsdf_a->sigma == bsdf_b->sigma);
}
 
ccl_device float3 bsdf_ashikhmin_velvet_eval_reflect(const ShaderClosure *sc,
                                                     const float3 I,
                                                     const float3 omega_in,
                                                     float *pdf)
{
  const VelvetBsdf *bsdf = (const VelvetBsdf *)sc;
  float m_invsigma2 = bsdf->invsigma2;
  float3 N = bsdf->N;
 
  float cosNO = dot(N, I);
  float cosNI = dot(N, omega_in);
  if (cosNO > 0 && cosNI > 0) {
    float3 H = normalize(omega_in + I);
 
    float cosNH = dot(N, H);
    float cosHO = fabsf(dot(I, H));
 
    if (!(fabsf(cosNH) < 1.0f - 1e-5f && cosHO > 1e-5f))
      return make_float3(0.0f, 0.0f, 0.0f);
 
    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(0.0f, 0.0f, 0.0f);
}
 
ccl_device float3 bsdf_ashikhmin_velvet_eval_transmit(const ShaderClosure *sc,
                                                      const float3 I,
                                                      const float3 omega_in,
                                                      float *pdf)
{
  return make_float3(0.0f, 0.0f, 0.0f);
}
 
ccl_device int bsdf_ashikhmin_velvet_sample(const ShaderClosure *sc,
                                            float3 Ng,
                                            float3 I,
                                            float3 dIdx,
                                            float3 dIdy,
                                            float randu,
                                            float randv,
                                            float3 *eval,
                                            float3 *omega_in,
                                            float3 *domega_in_dx,
                                            float3 *domega_in_dy,
                                            float *pdf)
{
  const VelvetBsdf *bsdf = (const VelvetBsdf *)sc;
  float m_invsigma2 = bsdf->invsigma2;
  float3 N = bsdf->N;
 
  // we are viewing the surface from above - send a ray out with uniform
  // distribution over the hemisphere
  sample_uniform_hemisphere(N, randu, randv, omega_in, pdf);
 
  if (dot(Ng, *omega_in) > 0) {
    float3 H = normalize(*omega_in + I);
 
    float cosNI = dot(N, *omega_in);
    float cosNO = dot(N, I);
    float cosNH = dot(N, H);
    float cosHO = fabsf(dot(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(N, dIdx)) * N - dIdx;
      *domega_in_dy = (2 * dot(N, dIdy)) * N - dIdy;
#endif
    }
    else
      *pdf = 0.0f;
  }
  else
    *pdf = 0.0f;
 
  return LABEL_REFLECT | LABEL_DIFFUSE;
}
 
CCL_NAMESPACE_END
 
#endif /* __BSDF_ASHIKHMIN_VELVET_H__ */