Fix regression: crash when using light tree and motion blur (appleseedhq#2659)

Author: oktomus

src/appleseed/renderer/kernel/lighting/backwardlightsampler.cpp

@@ -288,9 +288,11 @@ void BackwardLightSampler::sample_light_tree(
     else
     {
         assert(light_type == EmittingShapeType);
-        sample_emitting_shapes(
+        sample_emitting_shape(
             time,
-            s,
+            Vector2f(s[1], s[2]),
+            light_index,
+            light_prob,
             light_sample);
     }
 

src/appleseed/renderer/kernel/lighting/lightsamplerbase.cpp

@@ -633,19 +633,39 @@ void LightSamplerBase::store_object_area_in_shadergroups(
     }
 }
 
+void LightSamplerBase::sample_emitting_shape(
+    const ShadingRay::Time&             time,
+    const Vector2f&                     s,
+    const size_t                        shape_index,
+    const float                         shape_prob,
+    LightSample&                        light_sample) const
+{
+    // Fetch the emitting shape.
+    const EmittingShape& emitting_shape = m_emitting_shapes[shape_index];
+
+    // Uniformly sample the surface of the shape.
+    light_sample.m_light = nullptr;
+    emitting_shape.sample_uniform(s, shape_prob, light_sample);
+
+    assert(light_sample.m_shape);
+    assert(light_sample.m_probability > 0.0f);
+}
+
 void LightSamplerBase::sample_emitting_shapes(
     const ShadingRay::Time&             time,
     const Vector3f&                     s,
     LightSample&                        light_sample) const
 {
     assert(m_emitting_shapes_cdf.valid());
 
+    // Fetch the emitting shape.
     const EmitterCDF::ItemWeightPair result = m_emitting_shapes_cdf.sample(s[0]);
     const size_t emitter_index = result.first;
     const float emitter_prob = result.second;
+    const EmittingShape& emitting_shape = m_emitting_shapes[emitter_index];
 
+    // Uniformly sample the surface of the shape.
     light_sample.m_light = nullptr;
-    const EmittingShape& emitting_shape = m_emitting_shapes[emitter_index];
     emitting_shape.sample_uniform(Vector2f(s[1], s[2]), emitter_prob, light_sample);
 
     assert(light_sample.m_shape);

src/appleseed/renderer/kernel/lighting/lightsamplerbase.h

@@ -140,6 +140,14 @@ class LightSamplerBase
         const float                         object_area,
         const MaterialArray&                materials);
 
+    // Sample a given emitting shape.
+    void sample_emitting_shape(
+        const ShadingRay::Time&             time,
+        const foundation::Vector2f&         s,
+        const size_t                        shape_index,
+        const float                         shape_prob,
+        LightSample&                        light_sample) const;
+
     // Sample the set of emitting shapes.
     void sample_emitting_shapes(
         const ShadingRay::Time&             time,