@@ -383,6 +419,7 @@ ccl_device Spectrum bsdf_microfacet_estimate_albedo(KernelGlobals kg,
   if (!is_zero(reflectance) && bsdf->fresnel_type == MicrofacetFresnel::GENERALIZED_SCHLICK) {
     ccl_private FresnelGeneralizedSchlick *fresnel = (ccl_private FresnelGeneralizedSchlick *)
                                                          bsdf->fresnel;
+    /* TODO: Iridescence */

@@ -238,0 +272,4 @@
+  }
+
+  float R123 = R12 * R23;
+  float r123 = sqrt(R123);

@@ -238,0 +288,4 @@
+
+/* Computes reflectivity (R) and phase shift (phi) for perpendicular (.x) and parallel (.y)
+ * polarized light. */
+ccl_device_inline void fresnel_dielectric_polarized(const float cosThetaI,

@@ -238,0 +331,4 @@
+  /* Compute angle inside the thin film (after refraction at the top interface). */
+  float cosTheta2_sq = 1.0f - sqr(eta1 / eta2) * (1.0f - sqr(cosTheta));
+  if (cosTheta2_sq < 0.0f) {
+    /* TIR at the top interface. */

@@ -261,0 +261,4 @@
+
+  /* Panel for Thin Film settings. */
+  PanelDeclarationBuilder &film = b.add_panel("Thin Film").default_closed(true);
+  film.add_input<decl::Float>("Thin Film Thickness").default_value(0.0).min(0.0f).max(100000.0f);

@@ -261,0 +263,4 @@
+  PanelDeclarationBuilder &film = b.add_panel("Thin Film").default_closed(true);
+  film.add_input<decl::Float>("Thin Film Thickness").default_value(0.0).min(0.0f).max(100000.0f);
+#define SOCK_THIN_FILM_THICKNESS_ID 28
+  film.add_input<decl::Float>("Thin Film IOR").default_value(1.3f).min(1.0f).max(1000.0f);

@@ -390,3 +425,1 @@
-      float z = sqrtf(fabsf((bsdf->ior - 1.0f) / (bsdf->ior + 1.0f)));
-      s = lookup_table_read_3D(
-          kg, rough, cos_NI, z, kernel_data.tables.ggx_gen_schlick_ior_s, 16, 16, 16);
+    /* Precomputing LUTs for thin-film iridescence isn't viable, so fall back to the specular

@@ -39,0 +56,4 @@
+  }
+
+  /* Return squared amplitude to get the fraction of reflected energy. */
+  return make_float2(sqr(r_s), sqr(r_p));

@@ -39,0 +64,4 @@
+                                                float eta,
+                                                ccl_private float *r_cos_theta_t)
+{
+  return average(fresnel_dielectric_polarized(cos_theta_i, eta, r_cos_theta_t, NULL));

@@ -500,2 +542,3 @@
 template<MicrofacetType m_type>
-ccl_device Spectrum bsdf_microfacet_eval(ccl_private const ShaderClosure *sc,
+ccl_device Spectrum bsdf_microfacet_eval(KernelGlobals kg,
+                                         const ShaderClosure *sc,