Initial refraction approximation in GLSL (#918)

This changelist introduces a rough approximation of refraction in GLSL, allowing transmissive materials such as glass to respond visually to changes in specular_IOR, specular_roughness, and transmission_color. Because this initial approximation is limited to a single render pass, only the environment radiance map is currently visible in refractions, and opaque objects behind transmissive surfaces are not yet visible.
AcademySoftwareFoundation · Apr 25, 2022 · f5fc395 · f5fc395
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diff --git a/javascript/MaterialXView/source/viewer.js b/javascript/MaterialXView/source/viewer.js
@@ -382,7 +382,8 @@ export class Material
             u_envRadiance: { value: radianceTexture },
             u_envRadianceMips: { value: Math.trunc(Math.log2(Math.max(radianceTexture.image.width, radianceTexture.image.height))) + 1 },
             u_envRadianceSamples: { value: 16 },
-            u_envIrradiance: { value: irradianceTexture }
+            u_envIrradiance: { value: irradianceTexture },
+            u_refractionEnv: { value: true }
         });
 
         // Create Three JS Material

diff --git a/libraries/pbrlib/genglsl/lib/mx_environment_fis.glsl b/libraries/pbrlib/genglsl/lib/mx_environment_fis.glsl
@@ -34,7 +34,7 @@ vec3 mx_environment_radiance(vec3 N, vec3 V, vec3 X, vec2 alpha, int distributio
 
         // Compute the half vector and incoming light direction.
         vec3 H = mx_ggx_importance_sample_NDF(Xi, alpha);
-        vec3 L = -reflect(V, H);
+        vec3 L = fd.refraction ? mx_refraction_solid_sphere(-V, H, fd.ior.x) : -reflect(V, H);
 
         // Compute dot products for this sample.
         float NdotH = clamp(H.z, M_FLOAT_EPS, 1.0);
@@ -54,13 +54,16 @@ vec3 mx_environment_radiance(vec3 N, vec3 V, vec3 X, vec2 alpha, int distributio
         // Compute the geometric term.
         float G = mx_ggx_smith_G2(NdotL, NdotV, avgAlpha);
 
+        // Compute the combined FG term, which is inverted for refraction.
+        vec3 FG = fd.refraction ? vec3(1.0) - (F * G) : F * G;
+
         // Add the radiance contribution of this sample.
         // From https://cdn2.unrealengine.com/Resources/files/2013SiggraphPresentationsNotes-26915738.pdf
         //   incidentLight = sampleColor * NdotL
         //   microfacetSpecular = D * F * G / (4 * NdotL * NdotV)
         //   pdf = D * NdotH / (4 * VdotH)
         //   radiance = incidentLight * microfacetSpecular / pdf
-        radiance += sampleColor * F * G * VdotH / (NdotV * NdotH);
+        radiance += sampleColor * FG * VdotH / (NdotV * NdotH);
     }
 
     // Normalize and return the final radiance.

diff --git a/libraries/pbrlib/genglsl/lib/mx_microfacet_specular.glsl b/libraries/pbrlib/genglsl/lib/mx_microfacet_specular.glsl
@@ -26,6 +26,9 @@ struct FresnelData
     // Thin film
     float tf_thickness;
     float tf_ior;
+
+    // Refraction
+    bool refraction;
 };
 
 // https://media.disneyanimation.com/uploads/production/publication_asset/48/asset/s2012_pbs_disney_brdf_notes_v3.pdf
@@ -217,6 +220,13 @@ float mx_ior_to_f0(float ior)
     return mx_square((ior - 1.0) / (ior + 1.0));
 }
 
+// Convert normal-incidence reflectivity to real-valued index of refraction.
+float mx_f0_to_ior(float F0)
+{
+    float sqrtF0 = sqrt(F0);
+    return (1.0 + sqrtF0) / (1.0 - sqrtF0);
+}
+
 // https://seblagarde.wordpress.com/2013/04/29/memo-on-fresnel-equations/
 float mx_fresnel_dielectric(float cosTheta, float ior)
 {
@@ -387,7 +397,7 @@ vec3 mx_fresnel_airy(float cosTheta, vec3 ior, vec3 extinction, float tf_thickne
 
 FresnelData mx_init_fresnel_data(int model)
 {
-    return FresnelData(model, vec3(0.0), vec3(0.0), vec3(0.0), vec3(0.0), 0.0, 0.0, 0.0);
+    return FresnelData(model, vec3(0.0), vec3(0.0), vec3(0.0), vec3(0.0), 0.0, 0.0, 0.0, false);
 }
 
 FresnelData mx_init_fresnel_dielectric(float ior)
@@ -462,6 +472,14 @@ vec3 mx_compute_fresnel(float cosTheta, FresnelData fd)
     }
 }
 
+// Compute the refraction of a ray through a solid sphere.
+vec3 mx_refraction_solid_sphere(vec3 R, vec3 N, float ior)
+{
+    R = refract(R, N, 1.0 / ior);
+    vec3 N1 = normalize(R * dot(R, N) - N * 0.5);
+    return refract(R, N1, ior);
+}
+
 vec2 mx_latlong_projection(vec3 dir)
 {
     float latitude = -asin(dir.y) * M_PI_INV + 0.5;

diff --git a/libraries/pbrlib/genglsl/mx_dielectric_bsdf.glsl b/libraries/pbrlib/genglsl/mx_dielectric_bsdf.glsl
@@ -27,7 +27,7 @@ void mx_dielectric_bsdf_reflection(vec3 L, vec3 V, vec3 P, float occlusion, floa
     }
     else
     {
-         fd = mx_init_fresnel_dielectric(ior);
+        fd = mx_init_fresnel_dielectric(ior);
     }
     vec3  F = mx_compute_fresnel(VdotH, fd);
     float D = mx_ggx_NDF(Ht, safeAlpha);
@@ -44,14 +44,7 @@ void mx_dielectric_bsdf_reflection(vec3 L, vec3 V, vec3 P, float occlusion, floa
 
 void mx_dielectric_bsdf_transmission(vec3 V, float weight, vec3 tint, float ior, vec2 roughness, vec3 N, vec3 X, int distribution, int scatter_mode, inout BSDF bsdf)
 {
-    if (scatter_mode == 1)
-    {
-        bsdf.response = tint * weight;
-        bsdf.throughput = bsdf.response;
-        return;
-    }
-
-    if (weight < M_FLOAT_EPS)
+    if (weight < M_FLOAT_EPS || scatter_mode == 0)
     {
         return;
     }
@@ -61,20 +54,29 @@ void mx_dielectric_bsdf_transmission(vec3 V, float weight, vec3 tint, float ior,
 
     FresnelData fd;
     if (bsdf.thickness > 0.0)
+    { 
         fd = mx_init_fresnel_dielectric_airy(ior, bsdf.thickness, bsdf.ior);
+    }
     else
+    {
         fd = mx_init_fresnel_dielectric(ior);
-
+    }
     vec3 F = mx_compute_fresnel(NdotV, fd);
 
     vec2 safeAlpha = clamp(roughness, M_FLOAT_EPS, 1.0);
     float avgAlpha = mx_average_alpha(safeAlpha);
+
     float F0 = mx_ior_to_f0(ior);
     vec3 comp = mx_ggx_energy_compensation(NdotV, avgAlpha, F);
     vec3 dirAlbedo = mx_ggx_dir_albedo(NdotV, avgAlpha, F0, 1.0) * comp;
     bsdf.throughput = 1.0 - dirAlbedo * weight;
 
-    bsdf.response = (scatter_mode == 2) ? tint * weight * bsdf.throughput : vec3(0.0);
+    // For now, we approximate the appearance of dielectric transmission as
+    // glossy environment map refraction, ignoring any scene geometry that
+    // might be visible through the surface.
+    fd.refraction = true;
+    vec3 Li = $refractionEnv ? mx_environment_radiance(N, V, X, safeAlpha, distribution, fd) : $refractionColor;
+    bsdf.response = Li * tint * weight;
 }
 
 void mx_dielectric_bsdf_indirect(vec3 V, float weight, vec3 tint, float ior, vec2 roughness, vec3 N, vec3 X, int distribution, int scatter_mode, inout BSDF bsdf)
@@ -90,14 +92,18 @@ void mx_dielectric_bsdf_indirect(vec3 V, float weight, vec3 tint, float ior, vec
 
     FresnelData fd;
     if (bsdf.thickness > 0.0)
+    { 
         fd = mx_init_fresnel_dielectric_airy(ior, bsdf.thickness, bsdf.ior);
+    }
     else
+    {
         fd = mx_init_fresnel_dielectric(ior);
-
+    }
     vec3 F = mx_compute_fresnel(NdotV, fd);
 
     vec2 safeAlpha = clamp(roughness, M_FLOAT_EPS, 1.0);
     float avgAlpha = mx_average_alpha(safeAlpha);
+
     float F0 = mx_ior_to_f0(ior);
     vec3 comp = mx_ggx_energy_compensation(NdotV, avgAlpha, F);
     vec3 dirAlbedo = mx_ggx_dir_albedo(NdotV, avgAlpha, F0, 1.0) * comp;

diff --git a/libraries/pbrlib/genglsl/mx_generalized_schlick_bsdf.glsl b/libraries/pbrlib/genglsl/mx_generalized_schlick_bsdf.glsl
@@ -36,14 +36,7 @@ void mx_generalized_schlick_bsdf_reflection(vec3 L, vec3 V, vec3 P, float occlus
 
 void mx_generalized_schlick_bsdf_transmission(vec3 V, float weight, vec3 color0, vec3 color90, float exponent, vec2 roughness, vec3 N, vec3 X, int distribution, int scatter_mode, inout BSDF bsdf)
 {
-    if (scatter_mode == 1)
-    {
-        bsdf.response = color0 * weight;
-        bsdf.throughput = bsdf.response;
-        return;
-    }
-
-    if (weight < M_FLOAT_EPS)
+    if (weight < M_FLOAT_EPS || scatter_mode == 0)
     {
         return;
     }
@@ -56,12 +49,20 @@ void mx_generalized_schlick_bsdf_transmission(vec3 V, float weight, vec3 color0,
 
     vec2 safeAlpha = clamp(roughness, M_FLOAT_EPS, 1.0);
     float avgAlpha = mx_average_alpha(safeAlpha);
+
     vec3 comp = mx_ggx_energy_compensation(NdotV, avgAlpha, F);
     vec3 dirAlbedo = mx_ggx_dir_albedo(NdotV, avgAlpha, color0, color90) * comp;
     float avgDirAlbedo = dot(dirAlbedo, vec3(1.0 / 3.0));
     bsdf.throughput = vec3(1.0 - avgDirAlbedo * weight);
 
-    bsdf.response = (scatter_mode == 2) ? color0 * weight * bsdf.throughput : vec3(0.0);
+    // For now, we approximate the appearance of Schlick transmission as
+    // glossy environment map refraction, ignoring any scene geometry that
+    // might be visible through the surface.
+    float avgF0 = dot(color0, vec3(1.0 / 3.0));
+    fd.refraction = true;
+    fd.ior.x = mx_f0_to_ior(avgF0);
+    vec3 Li = $refractionEnv ? mx_environment_radiance(N, V, X, safeAlpha, distribution, fd) : $refractionColor;
+    bsdf.response = Li * color0 * weight;
 }
 
 void mx_generalized_schlick_bsdf_indirect(vec3 V, float weight, vec3 color0, vec3 color90, float exponent, vec2 roughness, vec3 N, vec3 X, int distribution, int scatter_mode, inout BSDF bsdf)

diff --git a/resources/Materials/Examples/StandardSurface/standard_surface_glass_tinted.mtlx b/resources/Materials/Examples/StandardSurface/standard_surface_glass_tinted.mtlx
@@ -0,0 +1,15 @@
+<?xml version="1.0"?>
+<materialx version="1.38" colorspace="lin_rec709">
+  <standard_surface name="SR_glass_tinted" type="surfaceshader">
+    <input name="base" type="float" value="0" />
+    <input name="specular" type="float" value="1" />
+    <input name="specular_color" type="color3" value="1, 1, 1" />
+    <input name="specular_roughness" type="float" value="0.15" />
+    <input name="specular_IOR" type="float" value="1.54" />
+    <input name="transmission" type="float" value="1" />
+    <input name="transmission_color" type="color3" value="0.2, 0.1, 1" />
+  </standard_surface>
+  <surfacematerial name="GlassTinted" type="material">
+    <input name="surfaceshader" type="surfaceshader" nodename="SR_glass_tinted" />
+  </surfacematerial>
+</materialx>
diff --git a/resources/Materials/TestSuite/pbrlib/bsdf/dielectric.mtlx b/resources/Materials/TestSuite/pbrlib/bsdf/dielectric.mtlx
@@ -1,122 +1,43 @@
 <?xml version="1.0"?>
 <materialx version="1.38">
-  <!-- Test dielectric_bsdf in various scatter modes -->
-  <nodegraph name="dielectric_bsdf_R">
-    <dielectric_bsdf name="bsdf1" type="BSDF">
+  <nodegraph name="dielectric_bsdf">
+    <dielectric_bsdf name="dielectric_R" type="BSDF">
       <input name="weight" type="float" value="1.0" />
       <input name="tint" type="color3" value="0.7, 0.7, 0.7" />
-      <input name="ior" type="float" value="1.7" />
       <input name="scatter_mode" type="string" value="R" />
     </dielectric_bsdf>
-    <surface name="surface1" type="surfaceshader">
-      <input name="bsdf" type="BSDF" nodename="bsdf1" />
+    <surface name="surface_R" type="surfaceshader">
+      <input name="bsdf" type="BSDF" nodename="dielectric_R" />
     </surface>
-    <output name="out" type="surfaceshader" nodename="surface1" />
-  </nodegraph>
-  <nodegraph name="dielectric_bsdf_T">
-    <dielectric_bsdf name="bsdf1" type="BSDF">
+    <output name="R_out" type="surfaceshader" nodename="surface_R" />
+
+    <dielectric_bsdf name="dielectric_T" type="BSDF">
       <input name="weight" type="float" value="1.0" />
       <input name="tint" type="color3" value="0.7, 0.7, 0.7" />
-      <input name="ior" type="float" value="1.7" />
       <input name="scatter_mode" type="string" value="T" />
     </dielectric_bsdf>
-    <surface name="surface1" type="surfaceshader">
-      <input name="bsdf" type="BSDF" nodename="bsdf1" />
+    <surface name="surface_T" type="surfaceshader">
+      <input name="bsdf" type="BSDF" nodename="dielectric_T" />
     </surface>
-    <output name="out" type="surfaceshader" nodename="surface1" />
-  </nodegraph>
-  <nodegraph name="dielectric_bsdf_RT">
-    <dielectric_bsdf name="bsdf1" type="BSDF">
+    <output name="T_out" type="surfaceshader" nodename="surface_T" />
+
+    <dielectric_bsdf name="dielectric_RT" type="BSDF">
       <input name="weight" type="float" value="1.0" />
       <input name="tint" type="color3" value="0.7, 0.7, 0.7" />
-      <input name="ior" type="float" value="1.7" />
       <input name="scatter_mode" type="string" value="RT" />
     </dielectric_bsdf>
-    <surface name="surface1" type="surfaceshader">
-      <input name="bsdf" type="BSDF" nodename="bsdf1" />
-    </surface>
-    <output name="out" type="surfaceshader" nodename="surface1" />
-  </nodegraph>
-  <nodegraph name="dielectric_bsdf_layeredRT">
-    <dielectric_bsdf name="bsdf1" type="BSDF">
-      <input name="weight" type="float" value="1.0" />
-      <input name="tint" type="color3" value="0.7, 0.7, 0.7" />
-      <input name="ior" type="float" value="1.7" />
-      <input name="scatter_mode" type="string" value="R" />
-    </dielectric_bsdf>
-    <dielectric_bsdf name="bsdf2" type="BSDF">
-      <input name="weight" type="float" value="1.0" />
-      <input name="tint" type="color3" value="0.7, 0.7, 0.7" />
-      <input name="ior" type="float" value="1.7" />
-      <input name="scatter_mode" type="string" value="T" />
-    </dielectric_bsdf>
-    <layer name="layer1" type="BSDF">
-      <input name="top" type="BSDF" nodename="bsdf1" />
-      <input name="base" type="BSDF" nodename="bsdf2" />
-    </layer>
-    <surface name="surface1" type="surfaceshader">
-      <input name="bsdf" type="BSDF" nodename="layer1" />
+    <surface name="surface_RT" type="surfaceshader">
+      <input name="bsdf" type="BSDF" nodename="dielectric_RT" />
     </surface>
-    <output name="out" type="surfaceshader" nodename="surface1" />
-  </nodegraph>
+    <output name="RT_out" type="surfaceshader" nodename="surface_RT" />
 
-  <!-- Test generalized_schlick_bsdf in various scatter modes -->
-  <nodegraph name="generalized_schlick_bsdf_R">
-    <generalized_schlick_bsdf name="bsdf1" type="BSDF">
-      <input name="weight" type="float" value="1.0" />
-      <input name="color0" type="color3" value="0.7, 0.7, 0.7" />
-      <input name="color90" type="color3" value="1.0, 1.0, 1.0" />
-      <input name="scatter_mode" type="string" value="R" />
-    </generalized_schlick_bsdf>
-    <surface name="surface1" type="surfaceshader">
-      <input name="bsdf" type="BSDF" nodename="bsdf1" />
-    </surface>
-    <output name="out" type="surfaceshader" nodename="surface1" />
-  </nodegraph>
-  <nodegraph name="generalized_schlick_bsdf_T">
-    <generalized_schlick_bsdf name="bsdf1" type="BSDF">
-      <input name="weight" type="float" value="1.0" />
-      <input name="color0" type="color3" value="0.7, 0.7, 0.7" />
-      <input name="color90" type="color3" value="1.0, 1.0, 1.0" />
-      <input name="scatter_mode" type="string" value="T" />
-    </generalized_schlick_bsdf>
-    <surface name="surface1" type="surfaceshader">
-      <input name="bsdf" type="BSDF" nodename="bsdf1" />
-    </surface>
-    <output name="out" type="surfaceshader" nodename="surface1" />
-  </nodegraph>
-  <nodegraph name="generalized_schlick_bsdf_RT">
-    <generalized_schlick_bsdf name="bsdf1" type="BSDF">
-      <input name="weight" type="float" value="1.0" />
-      <input name="color0" type="color3" value="0.7, 0.7, 0.7" />
-      <input name="color90" type="color3" value="1.0, 1.0, 1.0" />
-      <input name="scatter_mode" type="string" value="RT" />
-    </generalized_schlick_bsdf>
-    <surface name="surface1" type="surfaceshader">
-      <input name="bsdf" type="BSDF" nodename="bsdf1" />
-    </surface>
-    <output name="out" type="surfaceshader" nodename="surface1" />
-  </nodegraph>
-  <nodegraph name="generalized_schlick_bsdf_layeredRT">
-    <generalized_schlick_bsdf name="bsdf1" type="BSDF">
-      <input name="weight" type="float" value="1.0" />
-      <input name="color0" type="color3" value="0.7, 0.7, 0.7" />
-      <input name="color90" type="color3" value="1.0, 1.0, 1.0" />
-      <input name="scatter_mode" type="string" value="R" />
-    </generalized_schlick_bsdf>
-    <generalized_schlick_bsdf name="bsdf2" type="BSDF">
-      <input name="weight" type="float" value="1.0" />
-      <input name="color0" type="color3" value="1.0, 1.0, 1.0" />
-      <input name="color90" type="color3" value="1.0, 1.0, 1.0" />
-      <input name="scatter_mode" type="string" value="T" />
-    </generalized_schlick_bsdf>
-    <layer name="layer1" type="BSDF">
-      <input name="top" type="BSDF" nodename="bsdf1" />
-      <input name="base" type="BSDF" nodename="bsdf2" />
+    <layer name="layer_RT" type="BSDF">
+      <input name="top" type="BSDF" nodename="dielectric_R" />
+      <input name="base" type="BSDF" nodename="dielectric_T" />
     </layer>
-    <surface name="surface1" type="surfaceshader">
-      <input name="bsdf" type="BSDF" nodename="layer1" />
+    <surface name="surface_layer_RT" type="surfaceshader">
+      <input name="bsdf" type="BSDF" nodename="layer_RT" />
     </surface>
-    <output name="out" type="surfaceshader" nodename="surface1" />
+    <output name="layer_RT_out" type="surfaceshader" nodename="surface_layer_RT" />
   </nodegraph>
 </materialx>