In my opinion, IDL irradiance caching has issues and SM can't fix this or they would have by now. I am very frustrated with this. I reported it years ago, and they did improve it a lot, but it's still there. I have test cases showing that any surfaces meeting at right angles produce some artifacts from IDL. If a significant portion of the light in those areas comes from IDL (in this case all of it does), then the artifacts are very obvious, and you can't get rid of them.

It is why my Soft Studio Lighting sample scene does not only use glowing props for light, but also some traditional lights.

I posted this image to SM (along with the scene) in a bug report in 2009.

It's no longer as bad as this, but you can't make it go away.

In this scene by xpdev, it is lit correctly. All of the couch light is bounced or from the env sphere which is mostly occluded by the enclosure. There will be artifacts, unless you compromise your lighting and add a point light inside the enclosure, or other similar hack.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)