Fixed issue with prbvolpath gradient propagation

Author: Microno95

src/emitters/volumelight.cpp

@@ -74,7 +74,6 @@ class VolumeLight final : public Emitter<Float, Spectrum> {
         m_flags = +EmitterFlags::Medium;
 
         dr::set_attr(this, "flags", m_flags);
-        dr::set_attr(this, "radiance", m_radiance);
     }
 
     void traverse(TraversalCallback *callback) override {

src/python/python/ad/integrators/prbvolpath.py

@@ -268,11 +268,14 @@ def sample(self,
                     null_scatter = (sampler.next_1d(active_medium) < (emission_prob + null_prob))
                     act_null_scatter = null_scatter & active_medium
                     act_medium_scatter = ~null_scatter & active_medium
-                    L[act_null_scatter] += throughput * radiance * dr.detach(weight * emission_weight)
+                    contrib = throughput * radiance * dr.detach(emission_weight * weight)
+                    L[act_null_scatter] += dr.detach(contrib if is_primal else -contrib)
                     weight[act_null_scatter] *= mei.sigma_n * dr.detach(null_weight)
                 else:
                     scatter_weight = mi.Float(1.0)
                     act_medium_scatter = active_medium
+                    contrib = dr.zeros(mi.UnpolarizedSpectrum)
+
 
                 depth[act_medium_scatter] += 1
                 last_scatter_event[act_medium_scatter] = dr.detach(mei)
@@ -291,6 +294,8 @@ def sample(self,
                 if not is_primal and dr.grad_enabled(weight):
                     Lo = dr.detach(dr.select(active_medium | escaped_medium, L / dr.maximum(1e-8, weight), 0.0))
                     dr.backward(δL * weight * Lo)
+                if not is_primal and dr.grad_enabled(contrib):
+                    dr.backward(δL * contrib)
 
                 phase_ctx = mi.PhaseFunctionContext(sampler)
                 phase = mei.medium.phase_function()