Cleanup: remove redundant fresnel_dielectric()

We already have two other functions doing very much the same thing.
`bsdf_microfacet_sample()` seems to be the only place where this
function was used; there we always sample visible normals, so the extra
`inside` check is not needed.

intern/cycles/kernel/closure/bsdf_microfacet.h

@@ -590,6 +590,7 @@ ccl_device int bsdf_microfacet_sample(ccl_private const ShaderClosure *sc,
   }
 
   const float m_eta = bsdf->ior;
+  const float m_inv_eta = 1.0f / bsdf->ior;
   const bool m_refraction = CLOSURE_IS_REFRACTION(bsdf->type);
   const bool m_glass = CLOSURE_IS_GLASS(bsdf->type);
   const float alpha_x = bsdf->alpha_x;
@@ -624,14 +625,14 @@ ccl_device int bsdf_microfacet_sample(ccl_private const ShaderClosure *sc,
 
     H = X * local_H.x + Y * local_H.y + N * local_H.z;
   }
+  const float cos_HI = dot(H, wi);
 
-  bool valid;
+  /* The angle between the half vector and the refracted ray. Not used when sampling reflection. */
+  float cos_HO;
   bool do_refract;
   float lobe_pdf;
   if (m_refraction || m_glass) {
-    bool inside;
-    float fresnel = fresnel_dielectric(m_eta, H, wi, wo, &inside);
-    valid = !inside;
+    float fresnel = fresnel_dielectric(cos_HI, m_eta, &cos_HO);
 
     /* For glass closures, we decide between reflection and refraction here. */
     if (m_glass) {
@@ -640,32 +641,30 @@ ccl_device int bsdf_microfacet_sample(ccl_private const ShaderClosure *sc,
     }
     else {
       /* For pure refractive closures, refraction is the only option. */
+      if (fresnel == 1.0f) {
+        return LABEL_NONE;
+      }
       do_refract = true;
       lobe_pdf = 1.0f;
-      valid = valid && (fresnel != 1.0f);
     }
   }
   else {
     /* Pure reflective closure, reflection is the only option. */
-    valid = true;
     lobe_pdf = 1.0f;
     do_refract = false;
   }
-  const float cos_HI = dot(H, wi);
 
   if (do_refract) {
-    /* wo was already set to the refracted direction by fresnel_dielectric. */
-    // valid = valid && (dot(Ng, *wo) < 0);
+    *wo = refract_angle(wi, H, cos_HO, m_inv_eta);
     /* If the IOR is close enough to 1.0, just treat the interaction as specular. */
     m_singular = m_singular || (fabsf(m_eta - 1.0f) < 1e-4f);
   }
   else {
     /* Eq. 39 - compute actual reflected direction */
     *wo = 2 * cos_HI * H - wi;
-    valid = valid && (dot(Ng, *wo) > 0);
   }
 
-  if (!valid) {
+  if ((dot(Ng, *wo) < 0) != do_refract) {
     return LABEL_NONE;
   }
 
@@ -706,9 +705,8 @@ ccl_device int bsdf_microfacet_sample(ccl_private const ShaderClosure *sc,
       lambdaI = bsdf_aniso_lambda<m_type>(alpha_x, alpha_y, local_I);
     }
 
-    const float cos_HO = dot(H, *wo);
     const float common = D / cos_NI *
-                         (do_refract ? fabsf(cos_HI * cos_HO) / sqr(cos_HO + cos_HI / m_eta) :
+                         (do_refract ? fabsf(cos_HI * cos_HO) / sqr(cos_HO + cos_HI * m_inv_eta) :
                                        0.25f);
 
     *pdf = common * lobe_pdf / (1.0f + lambdaI);
@@ -718,7 +716,7 @@ ccl_device int bsdf_microfacet_sample(ccl_private const ShaderClosure *sc,
   }
 
   *sampled_roughness = make_float2(alpha_x, alpha_y);
-  *eta = do_refract ? 1.0f / m_eta : m_eta;
+  *eta = do_refract ? m_inv_eta : m_eta;
 
   return (do_refract ? LABEL_TRANSMIT : LABEL_REFLECT) |
          (m_singular ? LABEL_SINGULAR : LABEL_GLOSSY);

intern/cycles/kernel/closure/bsdf_util.h

@@ -17,15 +17,15 @@ ccl_device float fresnel_dielectric(float cos_theta_i, float eta, ccl_private fl
 
   /* Using Snell's law, calculate the squared cosine of the angle between the surface normal and
    * the transmitted ray. */
-  const float cos_theta_t_sq = 1.0f - (1.0f - sqr(cos_theta_i)) / sqr(eta);
-  if (cos_theta_t_sq <= 0) {
+  const float eta_cos_theta_t_sq = sqr(eta) - (1.0f - sqr(cos_theta_i));
+  if (eta_cos_theta_t_sq <= 0) {
     /* Total internal reflection. */
     return 1.0f;
   }
 
   cos_theta_i = fabsf(cos_theta_i);
   /* Relative to the surface normal. */
-  const float cos_theta_t = -safe_sqrtf(cos_theta_t_sq);
+  const float cos_theta_t = -safe_sqrtf(eta_cos_theta_t_sq) / eta;
 
   if (r_cos_theta_t) {
     *r_cos_theta_t = cos_theta_t;
@@ -48,45 +48,6 @@ ccl_device_inline float3 refract_angle(const float3 incident,
   return (inv_eta * dot(normal, incident) + cos_theta_t) * normal - inv_eta * incident;
 }
 
-ccl_device float fresnel_dielectric(
-    float eta, const float3 N, const float3 I, ccl_private float3 *T, ccl_private bool *is_inside)
-{
-  float cos = dot(N, I), neta;
-  float3 Nn;
-
-  // check which side of the surface we are on
-  if (cos > 0) {
-    // we are on the outside of the surface, going in
-    neta = 1 / eta;
-    Nn = N;
-    *is_inside = false;
-  }
-  else {
-    // we are inside the surface
-    cos = -cos;
-    neta = eta;
-    Nn = -N;
-    *is_inside = true;
-  }
-
-  float arg = 1 - (neta * neta * (1 - (cos * cos)));
-  if (arg < 0) {
-    *T = make_float3(0.0f, 0.0f, 0.0f);
-    return 1;  // total internal reflection
-  }
-  else {
-    float dnp = max(sqrtf(arg), 1e-7f);
-    float nK = (neta * cos) - dnp;
-    *T = -(neta * I) + (nK * Nn);
-    // compute Fresnel terms
-    float cosTheta1 = cos;  // N.R
-    float cosTheta2 = -dot(Nn, *T);
-    float pPara = (cosTheta1 - eta * cosTheta2) / (cosTheta1 + eta * cosTheta2);
-    float pPerp = (eta * cosTheta1 - cosTheta2) / (eta * cosTheta1 + cosTheta2);
-    return 0.5f * (pPara * pPara + pPerp * pPerp);
-  }
-}
-
 ccl_device float fresnel_dielectric_cos(float cosi, float eta)
 {
   // compute fresnel reflectance without explicitly computing