The lighting of a surface must be correct with regard to the light
source regardless of which way the surface normal points at. The normal
can in some cases point to the other side of the surface (without it
being wrong in any way). The simplest example is putting the camera
and a light source inside a sphere.

-- 
plane{-x+y,-1pigment{bozo color_map{[0rgb x][1rgb x+y]}turbulence 1}}
sphere{0,2pigment{rgbt 1}interior{media{emission 1density{spherical
density_map{[0rgb 0][.5rgb<1,.5>][1rgb 1]}turbulence.9}}}scale
<1,1,3>hollow}text{ttf"timrom""Warp".1,0translate<-1,-.1,2>}//  - Warp -

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