[Bf-blender-cvs] [f4b61c696b7] principled-v2: Simplify v1 Fresnel computation

[Lukas Stockner]

Commit: f4b61c696b740cf173d1ba7decc3f776955cf02b
Author: Lukas Stockner
Date:   Sun Oct 30 00:14:03 2022 +0200
Branches: principled-v2
https://developer.blender.org/rBf4b61c696b740cf173d1ba7decc3f776955cf02b

Simplify v1 Fresnel computation

===================================================================

M	intern/cycles/app/cycles_precompute.cpp
M	intern/cycles/kernel/closure/bsdf_microfacet_util.h
M	intern/cycles/kernel/closure/bsdf_util.h

===================================================================

diff --git a/intern/cycles/app/cycles_precompute.cpp b/intern/cycles/app/cycles_precompute.cpp
index 6f9d9601b1e..b55246e9e91 100644
--- a/intern/cycles/app/cycles_precompute.cpp
+++ b/intern/cycles/app/cycles_precompute.cpp
@@ -213,7 +213,7 @@ static float precompute_ggx_dielectric_E(float rough, float mu, float eta, float
      * is an issue since there are changes in that range at higher roughnesses.
      * Therefore, the remapping is blended with the identity function for a compromise.
      */
-    float F0 = fresnel_dielectric_cos(1.0f, eta);
+    float F0 = fresnel_dielectric_F0(eta);
     auto get_remap = [eta, F0](float x) {
       return mix(x, inverse_lerp(1.0f, F0, fresnel_dielectric_cos(x, eta)), 0.5f);
     };
diff --git a/intern/cycles/kernel/closure/bsdf_microfacet_util.h b/intern/cycles/kernel/closure/bsdf_microfacet_util.h
index b531c192e03..8a0902f054b 100644
--- a/intern/cycles/kernel/closure/bsdf_microfacet_util.h
+++ b/intern/cycles/kernel/closure/bsdf_microfacet_util.h
@@ -106,7 +106,7 @@ ccl_device_forceinline float microfacet_ggx_dielectric_E(KernelGlobals kg,
                            kernel_data.tables.ggx_dielectric_E_offset;
 
   float macro_fresnel = fresnel_dielectric_cos(mu, ior);
-  float F0 = fresnel_dielectric_cos(1.0f, ior);
+  float F0 = fresnel_dielectric_F0(ior);
   float x = mix(mu, inverse_lerp(1.0f, F0, macro_fresnel), 0.5f);
   float y = 1 - rough;
   float z = sqrtf(0.5f * ((inv_table ? 1.0f / ior : ior) - 1.0f));
diff --git a/intern/cycles/kernel/closure/bsdf_util.h b/intern/cycles/kernel/closure/bsdf_util.h
index 13de4ac15b0..47a37a43c87 100644
--- a/intern/cycles/kernel/closure/bsdf_util.h
+++ b/intern/cycles/kernel/closure/bsdf_util.h
@@ -71,6 +71,13 @@ ccl_device float fresnel_dielectric_cos(float cosi, float eta)
   return 1.0f;  // TIR(no refracted component)
 }
 
+/* Returns F0 (perpendicular) reflectivity for given eta,
+ * equivalent to fresnel_dielectric_cos(1.0f, eta). */
+ccl_device float fresnel_dielectric_F0(float eta)
+{
+  return sqr((eta - 1.0f) / (eta + 1.0f));
+}
+
 ccl_device Spectrum fresnel_conductor(float cosi, const Spectrum eta, const Spectrum k)
 {
   Spectrum cosi2 = make_spectrum(sqr(cosi));
@@ -124,11 +131,9 @@ ccl_device_forceinline Spectrum interpolate_fresnel_color(Spectrum L,
 {
   /* Compute the real Fresnel term and remap it from real_F0...1 to F0...1.
    * We could also just use actual Schlick fresnel (mix(F0, 1, (1-cosI)^5)) here. */
-  float real_F0 = fresnel_dielectric_cos(1.0f, ior);
-  float F0_norm = 1.0f / (1.0f - real_F0);
-  float FH = (fresnel_dielectric_cos(dot(L, H), ior) - real_F0) * F0_norm;
-
-  return mix(F0, one_spectrum(), FH);
+  float real_F0 = fresnel_dielectric_F0(ior);
+  float mixFactor = inverse_lerp(real_F0, 1.0f, fresnel_dielectric_cos(dot(L, H), ior));
+  return mix(F0, one_spectrum(), mixFactor);
 }
 
 ccl_device float3 ensure_valid_reflection(float3 Ng, float3 I, float3 N)