Distant plugin fix and docs and test updates

src/sensors/distant.cpp

@@ -32,9 +32,21 @@ Distant directional sensor (:monosp:`distant`)
      uniformly over the entire scene.
 
 This sensor plugin implements a distant directional sensor which records
-radiation leaving the scene in a given direction. If the ``target`` parameter
-is not set, rays cast by the sensor will be distributed uniformly on the cross
-section of the scene's bounding sphere.
+radiation leaving the scene in a given direction. By default, it records the 
+(spectral) radiant flux per unit solid angle leaving the scene in the specified
+direction (in unit power per unit solid angle). Rays cast by the sensor are 
+distributed uniformly on the cross section of the scene's bounding sphere.
+
+.. warning::
+
+    If this sensor is used with an environment map emitter, it will also record 
+    radiant flux coming from the part of emitter appearing through the scene's 
+    bounding sphere cross section. Care should be taken notably when using the
+    `constant` or `envmap` emitters.
+
+If the ``target`` parameter is set, the sensor looks at a single point and
+records a (spectral) radiant flux per unit surface area per unit solid angle 
+(in unit power per unit surface area per unit solid angle).
 
 */
 
@@ -116,7 +128,10 @@ MTS_VARIANT class DistantSensor final : public Sensor<Float, Spectrum> {
         }
 
         ray.update();
-        return std::make_pair(ray, wav_weight);
+        return std::make_pair(
+            ray, m_has_target
+                     ? wav_weight
+                     : wav_weight * (math::Pi<Float> * sqr(m_bsphere.radius)));
     }
 
     std::pair<RayDifferential3f, Spectrum> sample_ray_differential(
@@ -153,7 +168,10 @@ MTS_VARIANT class DistantSensor final : public Sensor<Float, Spectrum> {
         ray.has_differentials = false;
 
         ray.update();
-        return std::make_pair(ray, wav_weight);
+        return std::make_pair(
+            ray, m_has_target
+                     ? wav_weight
+                     : wav_weight * (math::Pi<Float> * sqr(m_bsphere.radius)));
     }
 
     /// This sensor does not occupy any particular region of space, return an

src/sensors/tests/test_distant.py

@@ -177,45 +177,56 @@ def test_intersection(variant_scalar_rgb, direction):
                        ek.dot(direction, [0, 0, -1]), atol=0.1)
 
 
-@pytest.mark.parametrize("radiance", [10**x for x in range(-3, 4)])
-def test_render(variant_scalar_rgb, radiance):
+def test_render(variant_scalar_rgb):
     # Test render results with a simple scene
     from mitsuba.core.xml import load_dict
-    from mitsuba.core import Bitmap, Struct
+    from mitsuba.core import Bitmap, Struct, ScalarTransform4f
 
-    def dict_scene(radiance=1.0, spp=1):
-        return {
-            "type": "scene",
-            "shape": {
+    for w_e, w_o in zip(([0, 0, -1], [0, 1, -1]), ([0, 0, -1], [0, 1, -1])):
+        l_e = 1.0  # Emitted radiance
+        w_e = list(ek.normalize(w_e))  # Emitter direction
+        w_o = list(ek.normalize(w_o))  # Sensor direction
+        cos_theta_e = abs(ek.dot(w_e, [0, 0, 1]))
+        cos_theta_o = abs(ek.dot(w_o, [0, 0, 1]))
+
+        scale = 0.5
+        rho = 1.0  # Surface reflectance
+        surface_area = 4. * scale ** 2
+
+    expected = l_e * cos_theta_e * surface_area * rho / np.pi * cos_theta_o
+
+    dict_scene = {
+        "type": "scene",
+        "shape": {
                 "type": "rectangle",
-                "bsdf": {"type": "conductor"},
-            },
-            "integrator": {"type": "path"},
-            "sensor": {
-                "type": "distant",
-                "film": {
+                "to_world": ScalarTransform4f.scale(scale),
+                "bsdf": {"type": "diffuse", "reflectance": rho},
+        },
+        "emitter": {
+            "type": "directional",
+            "irradiance": l_e,
+            "direction": w_e
+        },
+        "sensor": {
+            "type": "distant",
+            "direction": w_o,
+            "film": {
                     "type": "hdrfilm",
-                    "width": 1,
                     "height": 1,
-                    "pixel_format": "rgb",
+                    "width": 1,
+                    "pixel_format": "luminance",
                     "rfilter": {"type": "box"},
-                },
-                "sampler": {
-                    "type": "independent",
-                    "sample_count": spp
-                },
             },
-            "emitter": {
-                "type": "constant",
-                "radiance": {
-                    "type": "spectrum",
-                    "value": radiance
-                }
-            }
-        }
-
-    scene = load_dict(dict_scene(spp=1, radiance=radiance))
+            "sampler": {
+                "type": "independent",
+                "sample_count": 512
+            },
+        },
+        "integrator": {"type": "path"}
+    }
+
+    scene = load_dict(dict_scene)
     sensor = scene.sensors()[0]
     scene.integrator().render(scene, sensor)
-    img = sensor.film().bitmap()
-    assert np.allclose(np.array(img), radiance)
+    img = np.array(sensor.film().bitmap()).squeeze()
+    assert np.allclose(np.array(img), expected)