I don't have the POV 3.5 source available, but in POV 3.1:
lighting.c, Determine_Apparent_Colour()
...
VDot(Normal_Direction, Raw_Normal, Ray->Direction);
if (Normal_Direction > 0.0)
{
VScaleEq(Raw_Normal, -1.0);
}
--
--
Christopher James Huff <chr### [at] maccom>