[🐛 bug report] weight differentiation for blended BSDFs dysfunctional

[lo-zed]

Summary

Computing an inverse rendering on blended BSDFs weight parameter yields false results.

System configuration

• Platform: Ubuntu 20.04.2 LTS (GNU/Linux 5.4.0-65-generic x86_64)
• Python version: Python 3.8.5
• Mitsuba2 version: commit fe2a1bd

Description

When trying to recover the bsdf type on a simple scene composed of two rectangles with different bsdf, the objective value increases instead of decreasing. The algorithms converges towards an image that seems inverted.
I tried clipping the weights between [0, 1] but it didn't help.

Related

I noticed @LeonidMill asked a question about this in issue #26. I read the follow-up recommendations through PRs #44 & #157 but was told by @Speierers this was unrelated.

Steps to reproduce

run attached file
rectangle_blended_weight_differentiation.zip