Vertex positions can't be differentiated

[PidgeyBE]

Hey all

I'm trying to reproduce and play with the differentiable rendering example with the Cornell box described here: https://mitsuba2.readthedocs.io/en/latest/src/inverse_rendering/diff_render.html
All the steps described work, but if I e.g. want to try to differentiate/optimize OBJMesh_1.vertex_positions instead of red.reflectance.value, I get the following error when running the optimization:

---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-17-437578f9b545> in <module>
     10 
     11     # Back-propagate errors to input parameters
---> 12     ek.backward(ob_val)
     13 
     14     # Optimizer: take a gradient step
RuntimeError: set_gradient(): no gradients are associated with this variable (a prior call to requires_gradient() is required.) 

I tried all the Objects in the scene, but for none of them it is possible to optimize/differentiate with vertex_positions.
I find the error strange because the docs say the requires_gradient() call is automatically performed when initializing the Adam optimizer. Also if I do it manually before, it doesn't help...

So are there additional steps I should take to be able to optimize e.g. vertex_positions?

Thanks for helping! Pidgey

[PidgeyBE]

I found out that when I make e.g. one of the boxes an emitter, then I can '"optimize" its vertex coordinates...

[merlinND]

Hello @PidgeyBE,

Please see #26 and PR #44.