From: <HIDDEN>
Newsgroups: comp.soft-sys.matlab
Subject: Re: Normal direction of faceNormals
Date: Thu, 22 Apr 2010 22:29:07 +0000 (UTC)
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Cheers, Roger. Shame there's no 2D Mobius strip!!

I thought that the direction might propagate through from the ordering of the vertices in the TRI arrays which are output by the delaunay triangulation. For example, you could say the order of the three vertices is given in the clockwise direction when facing outward - therefore defining the normal direction within the TRI matrix. 

However, this isn't the case.  I tried the example for faceNormals given in the MATLAB documentation (convex hull of random points on a sphere).
Result: It gives a consistently outward normal direction. 
I repeated the experiment, having swapped the second and third columns of the second half of the input TRI matrix rows. I then recomputed the faceNormals. Half of the resultant normals should point inward.
2nd Result: Normal directions are still consistently outward pointing.
Conclusion: faceNormals must check and enforce consistent normal directions.

How it does this, and whether it's reliable for open or nonconvex hulls, I don't know. Any mathworkers in the building?? :)