gray, try it with IDL IC = 100. in technical terms, this means that GIMaxError = 0 (no interpolation) in FFRender.  GIVariables are used in FFRender to calculate IDL bounces.  use d3d's Render Firefly script in scripts/partners menu.  there are two IC variables.  first: ordinary raytrace IC, which can be set as in above render settings screen.  second: IDL IC.  when IDL is enabled, raytrace IC value is ignored by FFRender.

blotchy artifacts may also be seen where transparent/refractive posersurface contacts another posersurface, e.g. windowpane touching frame.  bill discussed contact artifacts, along with corner blotches.  possible solution: micro-bevelled corner, in case anybody wants to modify one of these poser models.  nobody has tried it yet AFAIK, as they may feel it's too much work and they might lose the UVMapping.