The second triangle (the one using the x+y+z corner) is at a different angle
towards the lightsource and thus gets lit differently. You should try
brilliance 0 to see if that changes anything. All this is just a guess based
on the code, not an actual render.
Regards,
Tim
--
"Tim Nikias v2.0"
Homepage: <http://www.nolights.de>
