Okay... so upping the "Intensity" setting in the Dimension 3d Render FF dialog to 100 was just resulting in a massively over-exposed looking image. I have just set that back to 1.4... so either I was mixing up this setting with something else or it just doesn't need to be so high for my scene.

Irradiance cache at 100 was just taking silly amounts of time to render. I've gone back to 20 there. From looking at the render (as far as it got before I lost patience, about 5 hours in, and aborted it) I just couldn't see any clear benefit.

Obviously my scene isn't particularly enclosed other than by the envsphere, which has an exterior skyscape panorama on it.... I guess I should try making up some sort of room prop with just a window or two and / or skylight andsee how that differs?

I realise I need to try a lighting set up that creates more "chiaroscuro", in order to really test the use of IDL only lighting... after all, it's primarily this feature that I have still been using Poser lights for, on top of using IDL for ambient light.

What else am I missing on both these points regarding forcing the ommission of the IDL precalc??

Anyway, I brought the "shading rate" down to 0.2 and upped the "Samples" in the IDL section of the D3d dialog to 500 and that looks to be helping with those artifacts, far as I can see from the IDL pre-pass. I'll play around with that some more and post the results later today some point

Does mesh topology perhaps contribute to these shading artifacts, such as I got under the cushion of that chair prop?

Also doing a birds-eye view render of the light set up I've used to make this clearer....

Cheers