Those are essentially 2D planes with emissive surface and cutout opacity set to a really low value of 0.0000001. To work around that, people using DAZ Studio and Iray used a simple trick – they used something called a Ghost Light Kit. Iray was (and in my opinion still is despite the improvements that made use of light portals obsolete) not really great when it comes to interior lighting. To understand why it is a breaking change, you need to understand a bit of history. The other change is about luminosity calculation change and how it now affects emissive surfaces – it is much bigger change and it breaks a lot of existing 3D assets in many stores not just DAZ3D store, not to mention all the scene files already saved using those affected assets. One of them is thin film calculations and I won’t get into that, because DAZ team already has a partial workaround, though the previous color result cannot be restored completely in more complex scenarios.
Iray versions after version 334300.9558 have two significant changes which were made in an effort to make the raytracing more physically correct. I am posting this here because Iray and OptiX teams are in my understanding dependent and thus work closely together so I hope this will be read by NVIDIA developers and managers who decided to make those changes, rethink their decisions, and provide workarounds.
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