Remove medium samples from RaySamples used in wavefront integrator.

The previous implementation would inadvertently reuse the same sample value
for medium distance sampling when rays passed through interfaces between
different scattering media. In turn, this led to significant errors in
rendered images.

See issue #221...

src/pbrt/wavefront/media.cpp

@@ -55,8 +55,8 @@ void WavefrontPathIntegrator::SampleMediumInteraction(int wavefrontDepth) {
             bool scattered = false;
 
             RaySamples raySamples = pixelSampleState.samples[w.pixelIndex];
-            Float uDist = raySamples.media.uDist;
-            Float uMode = raySamples.media.uMode;
+            Float uDist = rng.Uniform<Float>();
+            Float uMode = rng.Uniform<Float>();
 
             SampledSpectrum T_maj = SampleT_maj(
                 ray, tMax, uDist, rng, lambda,

src/pbrt/wavefront/samples.cpp

@@ -39,8 +39,6 @@ void WavefrontPathIntegrator::GenerateRaySamples(int wavefrontDepth, int sampleI
             int dimension = 6 + 7 * w.depth;
             if (haveSubsurface)
                 dimension += 3 * w.depth;
-            if (haveMedia)
-                dimension += 2 * w.depth;
 
             // Initialize _Sampler_ for pixel, sample index, and dimension
             ConcreteSampler pixelSampler = *sampler.Cast<ConcreteSampler>();
@@ -62,12 +60,6 @@ void WavefrontPathIntegrator::GenerateRaySamples(int wavefrontDepth, int sampleI
                 rs.subsurface.u = pixelSampler.Get2D();
             }
 
-            rs.haveMedia = haveMedia;
-            if (haveMedia) {
-                rs.media.uDist = pixelSampler.Get1D();
-                rs.media.uMode = pixelSampler.Get1D();
-            }
-
             // Store _RaySamples_ in pixel sample state
             pixelSampleState.samples[w.pixelIndex] = rs;
         });

src/pbrt/wavefront/workitems.h

@@ -35,10 +35,6 @@ struct RaySamples {
         Float uc;
         Point2f u;
     } subsurface;
-    bool haveMedia;
-    struct {
-        Float uDist, uMode;
-    } media;
 };
 
 template <>
@@ -62,7 +58,6 @@ struct SOA<RaySamples> {
         rs.direct.uc = dir.v[2];
 
         rs.haveSubsurface = int(dir.v[3]) & 1;
-        rs.haveMedia = int(dir.v[3]) & 2;
 
         Float4 ind = Load4(indirect + i);
         rs.indirect.uc = ind.v[0];
@@ -75,11 +70,6 @@ struct SOA<RaySamples> {
             rs.subsurface.u = Point2f(ss.v[1], ss.v[2]);
         }
 
-        if (rs.haveMedia) {
-            rs.media.uDist = mediaDist[i];
-            rs.media.uMode = mediaMode[i];
-        }
-
         return rs;
     }
 
@@ -89,7 +79,7 @@ struct SOA<RaySamples> {
 
         PBRT_CPU_GPU
         void operator=(RaySamples rs) {
-            int flags = (rs.haveSubsurface ? 1 : 0) | (rs.haveMedia ? 2 : 0);
+            int flags = rs.haveSubsurface ? 1 : 0;
             soa->direct[index] =
                 Float4{rs.direct.u[0], rs.direct.u[1], rs.direct.uc, Float(flags)};
             soa->indirect[index] = Float4{rs.indirect.uc, rs.indirect.rr,
@@ -97,10 +87,6 @@ struct SOA<RaySamples> {
             if (rs.haveSubsurface)
                 soa->subsurface[index] =
                     Float4{rs.subsurface.uc, rs.subsurface.u.x, rs.subsurface.u.y, 0.f};
-            if (rs.haveMedia) {
-                soa->mediaDist[index] = rs.media.uDist;
-                soa->mediaMode[index] = rs.media.uMode;
-            }
         }
 
         SOA *soa;