Re: [rp-ml] STL files

From: C. Brock Rooney <>
Date: Thu Dec 07 2006 - 15:08:18 EET

1) The normal is Calculated using the right hand rule. This normal must point
Out of the volume. The normal info in the STL file is redundant.
2) There will be no gaps if (3) is followed. This is a fact, not a constraint.
3) Edge matching is the issue. Each Edge of each triangle must have a
corresponding Edge of another triangle, whose vertices match exactly, ordered
in the opposite direction. That is, all edges come in pairs.

The STL spec makes this clear. It even has examples.

At 08:52 AM 12/7/06 +0000, Wesley Brooks wrote
> Anyway, going back to the three rules;
> 1. All triangles much conform to the right hand rule
> 2. All exterior surfaces must be devoid of gaps
> 3. Each triangle must share two of its vertices with each adjacent triangle
> Should point three be changed to; 'Each triangle must share at most two of
> its vertices with each of the adjacent triangles'? Consider the simplest 3D
> shape that can be represented by triangles, a triangular base pyramid. In
> this case the rule holds. Go to a square based pyramid and you get triangles
> which share only one point, the sharp end of the pyramid.

C. Brock Rooney (President) Brock Rooney & Associates Inc.
915 Westwood, Birmingham, MI 48009 USA
(248) 645-0236 Fax (248) 645-9020 Email:
Received on Thu Dec 07 14:13:29 2006

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