Chris Huff wrote:
> 
> In article <3974264c@news.povray.org>, Warp <war### [at] tagpovrayorg>
> wrote:
> 
> > John VanSickle <van### [at] erolscom> wrote:
> > : For flat triangles the normal is derived from a cross-product of
> > : two of the edge vectors (I presume) and therefore depends on the
> > : order the vertices are defined.
> >
> >   I think it doesn't matter which side the normal is pointing to (as
> > in any other surface in povray). This specially doesn't matter with
> > triangles since they do not have interior.
> 
> John VanSickle is right, the normal is derived using the cross-product
> of the edge vectors.(or something like that) The "up" side of the
> triangle is determined by the order of the vertices.
> For lighting calculations, the normal is always flipped so it points
> to the side the intersecting ray came from, so you don't notice a
> difference.

This is what I tested; the normal does indeed need to be flipped to
achieve this effect.  I'm fairly sure this was done in a recent
version of POV-Ray; I distinctly remember having to reverse the order
of some defined points in order to get the normals pointed the right
way.

Regards,
John
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ICQ: 46085459

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