Are you just not supposed to use spotlights with Poser's GI?  In Cinema I always use area lights/area shadows, and I'm beginning to suspect that the artifacts I'm coming are at least partly caused by all the rays being emitted from a single point (probably wastefully).  I fell back to my old method of a small array of spotlights and it seems to be quite a bit better.  Everyone else seems to be using Infinite lights (which is nuts imo, except for simulating sunlight) or point lights.  Actually this looks pretty good indeed, to me.  It's a hell of a lot better than I ever got with a single light.  Four lights here, a little triangle of three in the upper right foreground and one weak fill on the lower left.

Lowering Diffuse_Value did not help me with the corner-y artifacts I've been getting, although it does seem to help for large smooth surfaces, and I agree that it should probably be kept lower than 1 for all surfaces, I'll do that for everything GI-related in the future.

I still get some garbage around the border of the image but that's not quite so bad, I can just render a few pixels larger and crop the outside to get rid of it.  There is still a little trash around the thigh and boot.  I think if I add another light I could get rid of it.  Render time wasn't too bad either.  Antialasing needs a little work - fairly sure I left post filter at default, I'll change that.

Ignore that shit in the bottom right, that is coming from the "environment cube" I replaced the environment sphere with, to simplify things, but I didn't remember to drop it below the backdrop.

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