Fix delta emitter sample weights in the direct integrator

src/integrators/direct.cpp

@@ -178,8 +178,8 @@ class DirectIntegrator : public SamplingIntegrator<Float, Spectrum> {
                 bsdf_val = si.to_world_mueller(bsdf_val, -wo, si.wi);
 
                 Float mis = dr::select(ds.delta, Float(1.f), mis_weight(
-                    ds.pdf * m_frac_lum, bsdf_pdf * m_frac_bsdf) * m_weight_lum);
-                result[active_e] += mis * bsdf_val * emitter_val;
+                    ds.pdf * m_frac_lum, bsdf_pdf * m_frac_bsdf));
+                result[active_e] += mis * bsdf_val * emitter_val * m_weight_lum;
             }
         }
 

src/integrators/tests/test_direct.py

@@ -0,0 +1,62 @@
+import pytest
+import drjit as dr
+import mitsuba as mi
+
+def test01_delta_emitter_samples_weighted(variants_all_rgb):
+    scene = mi.load_dict({
+        'type': 'scene',
+        'sensor': {
+            'type': 'perspective',
+            'to_world': mi.ScalarTransform4f().look_at(
+                origin=(0, 0, 5),
+                target=(0, 0, 0),
+                up=(0, 1, 0),
+            ),
+            'sampler': {
+                'type': 'independent',
+                'sample_count': 1
+            },
+            'film': {
+                'type': 'hdrfilm',
+                'width': 128, 'height': 128,
+                'crop_offset_x': 64,
+                'crop_offset_y': 64,
+                'crop_width': 1,
+                'crop_height': 1
+            },
+        },
+        'emitter' : {
+            'type': 'point',
+            'position': [0.0, 0.0, 2.0],
+            'intensity': {
+                'type': 'rgb',
+                'value': 1.0,
+            }
+        },
+        'sphere' : {
+            'type': 'sphere',
+            'center': [0.0, 0.0, 0.0],
+            'radius': 1,
+            'bsdf': {
+                'type': 'diffuse'
+            }
+        }
+    })
+
+    integrator = mi.load_dict({
+        'type': 'direct',
+        'emitter_samples': 1,
+        'bsdf_samples': 0,
+    })
+    single_sample_img = mi.render(scene, integrator=integrator)
+
+    integrator = mi.load_dict({
+        'type': 'direct',
+        'emitter_samples': 10,
+        'bsdf_samples': 0,
+    })
+    multi_sample_img = mi.render(scene, integrator=integrator)
+    # The number of emitter samples should not affect a scene with a single
+    # point light source.
+    diff = single_sample_img.array - multi_sample_img.array
+    assert dr.allclose(diff, 0)