[Radiance-general] Strange halo artifact in rendering

[Mark Stock]

Folks,

I am stumped. There is a halo around a black box on a grey background
in a rendering, and I can't figure out how to get rid of it (because I
can't imagine that it's real).

I am preparing renderings for an art submission, and I am simulating a
little model mountain inside a small flat black frame on a light grey
wall, in a room with an array of ceiling lights and one spot. Here is
what happens (rpict and mountain-removed rvu):

http://markjstock.org/transfer/whyTheHaloMaster.png
http://markjstock.org/transfer/whyTheHalo.png

The black material is plastic with 0.0 specularity.
The halo is independent of the ambient parameters.
The halo is independent of -lw refinement.
The halo is still there if the black box touches the wall or is offset a little.
The halo doesn't go away when I render at high resolution.

Can anyone tell me what is going on here? Unzip and run "make" on:
http://markjstock.org/transfer/whatGives.zip

Mark