Why is specular_reflectance defined as a Texture in the BSDF class?

[Virtual-Optix]

I was going through the Custom Python plugin tutorial to define your own BSDF and comparing it with the c++ code. Most lines are very similar, however, there are a couple of lines which I don't fully understand and are omitted from the python tutorial.

In the in dielectric BSDF source code the specular reflectance is initiated as:

if (props.has_property("specular_reflectance"))
            m_specular_reflectance   = props.texture<Texture>("specular_reflectance", 1.f);

The same is also encountered in roughdielectric when initiating alpha_u and alpha_v.

Why are in these cases the specular reflectance and alpha initiated as a texture? And how would this initiation be done with Python?

[njroussel]

Hi @Virtual-Optix,

These fields are initiated as Textures such that they can be spatially varying. For example, in the roughdielectric we could have a specific area of the shape which is rougher. The props.texture method has some extra logic so that it can create a uniform texture from the default value if the property is not already a Texture.
See here for some examples of plugin descriptions in Python. If you want to replicate the props.texture behaviour, you can just instantiate the Texture yourself using mi.load_dict/load_string.

[Virtual-Optix]

Hey @njroussel,

Thanks for the answer, it explains a lot! Looking at the textures listed in the plugin description it is not quite clear to me which one would be used in the case of initiating the specular_reflectance. To me it seems like a constant valued textured is initiated but as far as I can see this is not one of the possible option. What kind of Texture is initiated in:

m_specular_reflectance   = props.texture<Texture>("specular_reflectance", 1.f);

[njroussel]

You're looking for this plugin: uniform. Spectra plugins share the same interface as textures (very top of the spectra plugin page), you can even see this in their implementation, as they inherit from Texture.