RuntimeError: Python -> C++ conversions

[bullette007]

Hi everyone,

I'm currently working on a project where I'd like to implement a custom BSDF plugin, so I started with the tutorial (https://mitsuba.readthedocs.io/en/stable/src/others/custom_plugin.html).

Unfortunately, even the unmodified sample (only modified by setting the variant to scalar_rgb) does fail while calling mi.render with the following output:
RuntimeError: When called outside a bound function, py::cast() cannot do Python -> C++ conversions which require the creation of temporary values

This seems to have broken during the transition from version 3.0.2 (here it still works) to 3.1.0.

Any thoughts / help on this?

Many thanks in advance!

[njroussel]

Hi @bullette007

In general custom Python plugins are not meant to be used with scalar-* variants. As mentioned in the tutorial, there is a significant performance overhead with Python and the JIT enabled variants mitigate that overhead.

I'm unable to fully reproduce the behaviour you describe. My best guess is that the eval_pdf method is returning return 0.0, 0.0 when it should maybe explicitly return return Color3f(0.0), 0.0. More generally, I'm actually surprised you got this too work at all, even in 3.02. Are you running this code on a single-threaded machine?