Ah, I guess my summary was a bit exaggerated, sorry. And technically it's about raytracing rather than IDL.

Here's the actual quote from the installation readme:

Quote -
 

• There are some situations where polygonal smoothing and raytracing should not be used at the same time since shadows at the edge of object can appear with black artifacts. Disabling smoothing or disabling raytracing solves this issue. If you are using reflections, try disabling polygonal smoothing or adjusting the shadow bias to minimize or eliminate artifacts.

 

• Raytraced shadows or reflections cannot always accurately take into account micro-polygon smoothed or micro-displaced polygons.

 

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Hardware: AMD Ryzen 9 3900X/MSI MAG570 Tomahawk X570/Zotac Geforce GTX 1650 Super 4GB/32GB OLOy RAM

Software: Windows 10 Professional/Poser Pro 11/Photoshop/Postworkshop 3