Parameter update in the C++ interface

[lcit]

I would like to update the parameters of the scene at runtime through the C++ interface but I cannot figure out how to do it. I understand that the an object that inherit from TraversalCallback is required to get the parameters recursively via traverse(). This class exists in the Python interface but I cannot find an equivalent in C++. Is it the case?
If it is the case I assume I have to create one by myself, then modify the buffers as needed, and notify the objects that parameters have changed?

Any hint is welcome and thank you for the great work.

[njroussel]

Hi @lcit

If it is the case I assume I have to create one by myself, then modify the buffers as needed, and notify the objects that parameters have changed?

Exactly. We don't have a cpp equivalent to the python traverse(), so all of that logic would need to be re-implemented.

[lcit]

Hi @njroussel,

I have implemented an attribute search mechanism by matching key name and type. That allows me to modify an attribute and to update it . Printing the scene to console shows that the parameter has changed, however, the image does not change. To update the attribute I simply do obj->parameters_changed({name}) solely on the object being modified. An example could be to_world of object Instance. Is this sufficient to update the scene? More generally, is there anything that pops on your mind that has to be updated that I am missing?

[njroussel]

Hi @lcit

That's unfortunately not enough. The scene structure is a tree where the Scene object is the root node and its children are BSDFs, shapes, etc. An update to one node, should trigger a call to parameters_changed() to its parent, and that until the root node is reached.
In your specific case, the Instance will update its own to_world but the Scene object must be aware of it, or else it will not re-build the acceleration structure. Effectively, because it has not received a call to parameters_changed, it will re-use the previous acceleration structure, i.e the one that was created before you updated the parameter.

In Python, this logic is in this snippet: https://github.com/mitsuba-renderer/mitsuba3/blob/master/src/python/python/util.py#L223-L235