Expose PhaseFunction of Medium with mi.traverse()

include/mitsuba/render/medium.h

@@ -85,6 +85,8 @@ class MI_EXPORT_LIB Medium : public Object {
         return m_has_spectral_extinction;
     }
 
+    void traverse(TraversalCallback *callback) override;
+
     /// Return a string identifier
     std::string id() const override { return m_id; }
 

src/media/heterogeneous.cpp

@@ -51,6 +51,7 @@ Heterogeneous medium (:monosp:`heterogeneous`)
    - A nested phase function that describes the directional scattering properties of
      the medium. When none is specified, the renderer will automatically use an instance of
      isotropic.
+   - |exposed|, |differentiable|
 
 
 This plugin provides a flexible heterogeneous medium implementation, which acquires its data

src/media/homogeneous.cpp

@@ -51,6 +51,7 @@ Homogeneous medium (:monosp:`homogeneous`)
    - A nested phase function that describes the directional scattering properties of
      the medium. When none is specified, the renderer will automatically use an instance of
      isotropic.
+   - |exposed|, |differentiable|
 
 This class implements a homogeneous participating medium with support for arbitrary
 phase functions. This medium can be used to model effects such as fog or subsurface scattering.

src/render/medium.cpp

@@ -33,6 +33,10 @@ MI_VARIANT Medium<Float, Spectrum>::Medium(const Properties &props) : m_id(props
 
 MI_VARIANT Medium<Float, Spectrum>::~Medium() {}
 
+MI_VARIANT void Medium<Float, Spectrum>::traverse(TraversalCallback *callback) {
+    callback->put_object("phase_function", m_phase_function.get(), +ParamFlags::Differentiable);
+}
+
 MI_VARIANT
 typename Medium<Float, Spectrum>::MediumInteraction3f
 Medium<Float, Spectrum>::sample_interaction(const Ray3f &ray, Float sample,