Warp wrote:

> Scott Gammans <dee### [at] yahoocom> wrote:
> 
>>It never would have occurred to me to add the "hollow" keyword; I thought a
>>disc was supposed to be infinitely thin and that "hollow" wouldn't apply.
> 
> 
>   All objects in POV-Ray are "infinitely thin", yet most of them have an
> inside, even those which are not closed surfaces.
>   Right now I can only think of triangles, meshes and bicubic patches not
> having a defined inside.
They do have an orientation, depending on the order in
which the vertices are listed. If we number the vertices
of a triangle clockwise a,b, and c then (b-a)x(c-a)
always points outwards (with the 'x' denoting
the cross product. This crossproduct points in the
direction of the normal on the surface and the length is
the size of the triangle).In a closed triangular mesh all normals
should point outwards. That is also the reason why the
other side of the triangle (or mesh or patch) can be textured
differently by using the 'interior_texture' keyword.
If you think of closed surfaces this makes sense but it
looks a bit strange for essentially thin an flat objects
like pieces of paper or butterfly wings.
I must add here of course that in the real 3D world
there are no 2D objects. Perhaps using 'interior_texture'
is a good reminder that you take a shortcut ;).

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