Get material properties of interesected surface inside Mitsuba Integrator sample loop

[USharma002]

Hi, I'm trying to set up a path guiding integrator framework in python for testing and am currently trying to integrate the Neural Radiance Cache within the Integrator. For the inputs of radiance cache I have planned to take 16 dimensional input.

Total input dimensions = 3 (pos) + 3 (normal) + 3 (diffuse color) + 3 (specular color) + 3 (dir) + 1 (roughness) = 16

but I am unable to get these properties of the intersected surface inside the loop. For bsdf I can think of a workaround but the specular color/reflectance and roughness I am unable to do but they are defined in the scene dictionary.

Is there some way to access these properties inside the integrator (or probably any custom property I add to surfaces as well) ?

Here is the implementation of the integrator : https://github.com/USharma002/py-neural-path-guiding/blob/main/path_guiding_integrator.py

        while loop(active):
            
            # Compute a surface interaction with given ray
            si = scene.ray_intersect(ray,
                                     ray_flags=mi.RayFlags.All,
                                     coherent=dr.eq(depth, 0))

            # Get the BSDF, potentially computes texture-space differentials
            bsdf = si.bsdf(ray)
           
            # Need specular reflectance, roughness here !

[merlinND]

Hello @USharma002,

Have you tried calling eval_attribute() on the BSDF? (see also eval_attribute_1, eval_attribute_3)

mitsuba3/include/mitsuba/render/bsdf.h

Lines 483 to 501 in 8120648

	/**
	* \brief Evaluate a specific BSDF attribute at the given surface interaction.
	*
	* BSDF attributes are user-provided fields that provide extra
	* information at an intersection. An example of this would be a per-vertex
	* or per-face color on a triangle mesh.
	*
	* \param name
	* Name of the attribute to evaluate
	*
	* \param si
	* Surface interaction associated with the query
	*
	* \return
	* An unpolarized spectral power distribution or reflectance value
	*/
	virtual UnpolarizedSpectrum eval_attribute(const std::string &name,
	const SurfaceInteraction3f &si,
	Mask active = true) const;