Some fixes for energy conservation when subsurface is used with transmission. Still some issues though

Author: knightcrawler25

src/shaders/common/disney.glsl

@@ -31,7 +31,6 @@
  * [6] http://shihchinw.github.io/2015/07/implementing-disney-principled-brdf-in-arnold.html
  * [7] https://github.com/mmp/pbrt-v4/blob/0ec29d1ec8754bddd9d667f0e80c4ff025c900ce/src/pbrt/bxdfs.cpp#L76-L286
  * [8] https://www.cs.cornell.edu/~srm/publications/EGSR07-btdf.pdf
- * [9] https://graphics.pixar.com/library/RadiosityCaching/paper.pdf
  */
 
 //-----------------------------------------------------------------------
@@ -120,7 +119,7 @@ vec3 EvalSubsurface(State state, vec3 V, vec3 N, vec3 L, inout float pdf)
     float FL = SchlickFresnel(abs(dot(N, L)));
     float FV = SchlickFresnel(dot(N, V));
     float Fd = (1.0f - 0.5f * FL) * (1.0f - 0.5f * FV);
-    return sqrt(state.mat.albedo) * state.mat.subsurface * (1.0 / PI) * Fd * (1.0 - state.mat.metallic);
+    return sqrt(state.mat.albedo) * state.mat.subsurface * (1.0 / PI) * Fd * (1.0 - state.mat.metallic) * (1.0 - state.mat.specTrans);
 }
 
 //-----------------------------------------------------------------------
@@ -172,8 +171,7 @@ vec3 DisneySample(inout State state, vec3 V, vec3 N, inout vec3 L, inout float p
     {
         if (rand() < diffuseRatio)
         {
-            // This way of performing subsurface scattering was taken from Tinsel [4] and is similar [9].
-            // Simpler than random walk but not sure if accurate.
+            // Diffuse transmission. A way to approximate subsurface scattering
             if (rand() < state.mat.subsurface)
             {
                 L = UniformSampleHemisphere(r1, r2);
@@ -271,7 +269,7 @@ vec3 DisneyEval(State state, vec3 V, vec3 N, vec3 L, inout float pdf)
         // Subsurface
         if (dot(N, L) < 0.0)
         {
-            // TODO: Double check this. Causing occassional NaNs and fails furnace test when used with transmission
+            // TODO: Double check this. Fails furnace test when used with rough transmission
             if (state.mat.subsurface > 0.0)
             {
                 brdf = EvalSubsurface(state, V, N, L, m_pdf);

src/shaders/common/globals.glsl

@@ -127,10 +127,13 @@ struct LightSampleRec
 
 uniform Camera camera;
 
-//-----------
 float rand()
-//-----------
 {
     seed -= randomVector.xy;
     return fract(sin(dot(seed, vec2(12.9898, 78.233))) * 43758.5453);
+}
+
+vec3 FaceForward(vec3 a, vec3 b)
+{
+    return dot(a, b) < 0.0 ? -b : b;
 }

src/shaders/common/pathtrace.glsl

@@ -139,16 +139,10 @@ vec3 DirectLight(in Ray r, in State state)
 //-----------------------------------------------------------------------
 {
     vec3 Li = vec3(0.0);
-    vec3 surfacePos;
+    vec3 surfacePos = state.fhp;
 
     BsdfSampleRec bsdfSampleRec;
 
-    // If sampling subsurface then push surfacePos towards the outer surface to be able to hit a light source
-    if(state.isSubsurface)
-        surfacePos = state.fhp + state.normal * EPS;
-    else
-        surfacePos = state.fhp + state.ffnormal * EPS;
-
     // Environment Light
 #ifdef ENVMAP
 #ifndef CONSTANT_BG
@@ -160,7 +154,7 @@ vec3 DirectLight(in Ray r, in State state)
 
         if (state.isSubsurface || dot(lightDir, state.ffnormal) > 0.0)
         {
-            Ray shadowRay = Ray(surfacePos, lightDir);
+            Ray shadowRay = Ray(surfacePos + FaceForward(state.normal, lightDir) * EPS, lightDir);
             bool inShadow = AnyHit(shadowRay, INFINITY - EPS);
 
             if (!inShadow)
@@ -209,7 +203,7 @@ vec3 DirectLight(in Ray r, in State state)
         if (!state.isSubsurface && (dot(lightDir, state.ffnormal) <= 0.0 || dot(lightDir, lightSampleRec.normal) >= 0.0))
             return Li;
 
-        Ray shadowRay = Ray(surfacePos, lightDir);
+        Ray shadowRay = Ray(surfacePos + FaceForward(state.normal, lightDir) * EPS, lightDir);
         bool inShadow = AnyHit(shadowRay, lightDist - EPS);
 
         if (!inShadow)
@@ -293,7 +287,7 @@ vec3 PathTrace(Ray r)
 
         // Set absorption only if the ray is currently inside the object.
         if (dot(state.ffnormal, bsdfSampleRec.L) < 0.0)
-            absorption = -log(state.mat.extinction) / vec3(0.1);
+            absorption = -log(state.mat.extinction) / vec3(0.2); // TODO: Add atDistance
 
         if (bsdfSampleRec.pdf > 0.0)
             throughput *= bsdfSampleRec.f * abs(dot(state.ffnormal, bsdfSampleRec.L)) / bsdfSampleRec.pdf;