add bent normals-based IBL anisotropy rendering.

Author: bhouston

libraries/pbrlib/genglsl/lib/mx_environment_fis.glsl

@@ -5,10 +5,13 @@ vec3 mx_environment_radiance(vec3 N, vec3 V, vec3 X, vec2 alpha, int distributio
     // Generate tangent frame.
     X = normalize(X - dot(X, N) * N);
     vec3 Y = cross(N, X);
-    mat3 tangentToWorld = mat3(X, Y, N);
+    vec3 bentNormal = mx_bent_normal_anisotropy(N, V, X, Y, alpha);
+    X = normalize(X - dot(X, bentNormal) * bentNormal);
+    Y = cross(bentNormal, X);
+    mat3 tangentToWorld = mat3(X, Y, bentNormal);
 
     // Transform the view vector to tangent space.
-    V = vec3(dot(V, X), dot(V, Y), dot(V, N));
+    V = vec3(dot(V, X), dot(V, Y), dot(V, bentNormal));
 
     // Compute derived properties.
     float NdotV = clamp(V.z, M_FLOAT_EPS, 1.0);

libraries/pbrlib/genglsl/lib/mx_environment_prefilter.glsl

@@ -10,7 +10,10 @@ float mx_latlong_alpha_to_lod(float alpha)
 vec3 mx_environment_radiance(vec3 N, vec3 V, vec3 X, vec2 alpha, int distribution, FresnelData fd)
 {
     N = mx_forward_facing_normal(N, V);
-    vec3 L = fd.refraction ? mx_refraction_solid_sphere(-V, N, fd.ior.x) : -reflect(V, N);
+    X = normalize(X - dot(X, N) * N);
+    vec3 Y = cross(N, X);
+    vec3 bentNormal = mx_bent_normal_anisotropy(N, V, X, Y, alpha);
+    vec3 L = fd.refraction ? mx_refraction_solid_sphere(-V, N, fd.ior.x) : -reflect(V, bentNormal);
 
     float NdotV = clamp(dot(N, V), M_FLOAT_EPS, 1.0);
 

libraries/pbrlib/genglsl/lib/mx_microfacet_specular.glsl

@@ -192,6 +192,31 @@ float mx_average_alpha(vec2 alpha)
     return sqrt(alpha.x * alpha.y);
 }
 
+// Approximate anisotropic IBL with a bent normal from alpha.x / alpha.y.
+// https://google.github.io/filament/Filament.md.html Section Anisotropy
+vec3 mx_bent_normal_anisotropy(vec3 N, vec3 V, vec3 X, vec3 Y, vec2 alpha)
+{
+    float maxAlpha = max(max(alpha.x, alpha.y), M_FLOAT_EPS);
+    float minOverMax = min(alpha.x, alpha.y) / maxAlpha;
+    float anisotropy = 1.0 - clamp(minOverMax, 0.0, 1.0);
+    float anisotropySign = (alpha.x >= alpha.y) ? 1.0 : -1.0;
+    float signedAnisotropy = anisotropy * anisotropySign;
+
+    vec3 anisotropicDirection = (signedAnisotropy >= 0.0) ? Y : X;
+    vec3 anisotropicTangent = cross(anisotropicDirection, V);
+    float tangentLenSq = dot(anisotropicTangent, anisotropicTangent);
+    vec3 anisotropicNormal = (tangentLenSq > M_FLOAT_EPS) ?
+        normalize(cross(anisotropicTangent, anisotropicDirection)) : N;
+
+    float roughness = clamp(sqrt(mx_average_alpha(alpha)), 0.0, 1.0);
+    float bend = abs(signedAnisotropy) * (1.0 - roughness);
+    float normalWeight = 1.0 - bend;
+    normalWeight *= normalWeight;
+    normalWeight *= normalWeight;
+
+    return normalize(mix(anisotropicNormal, N, normalWeight));
+}
+
 // Convert a real-valued index of refraction to normal-incidence reflectivity.
 float mx_ior_to_f0(float ior)
 {