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[Stephen Wong <>]

Hi Gilbert,

I don't believe that my input file is non-manifold.  The facets that you were 
describing as "shared facets" between the inner and outer triangular prism - 
those facets are non-convex and have the inner prism removed (you can see 
that in the attached OFF file, which I describe later in this post).  There 
are two facets in the input file which are not convex.

I have tried converting the OFF file into a Nef_polyhedron using the function 
OFF_to_nef_3() as you have described.  When executing the 
complex_construction.cpp with that file, I get the following output:

Nef vertices: 12
Nef edges: 21
Nef facets: 11
Nef volumes: 2
number of discarded facets: 0

That seems to indicate that the input file is not degenerate.

I converted the resultant Nef_polyhedron_3 that is generated to a 
polyhedron_3 using convert_to_Polyhedron_3(), and then wrote the polyhedron 
to OFF.  I have attached this file here - you can see that the solid is 
identical to the input, except with the facets triangulated.

So, I still believe that there is something awry during the conversion 
between Polyhedron_3 and Nef_polyhedron_3 for this input OFF file.

Stephen

-----Original Message-----
From: Gilbert Bernstein 
[mailto:]
Sent: January-12-08 6:47 PM
To: 

Subject: Re: [cgal-discuss] Crash on converting Polyhedron_3 to 
Nef_polyhedron_3 on an OFF input.

Stephen,

The mesh you're trying to read in is non-manifold.  This is probably
why it is not working.  Inside of the triangular prism is a second
triangular prism sharing part of a face.  The surface represented
itself is non-manifold, and your data file contains T-Junctions, so
the connectivity of the mesh isn't made explicit.  One or both of
these problems might be the cause of a bad conversion.

If you check the examples folder, there is a program called
"complex_construction.cpp"  It demonstrates the use of the following
function:

#include<CGAL/OFF_to_nef_3.h>

Nef_3 N;
std::size_t discarded = CGAL::OFF_to_nef_3 (std::cin, N, true);

discarded contains the number of facets of the model which were
discarded by the conversion process.  You might want to try running
the model through complex_construction and see what happens.

-- Gilbert


On Jan 12, 2008, at 7:47 PM, Stephen Wong wrote:

Hi there,

On this seemingly innocent input solid in the form of a OFF file,
CGAL's Nef_polyhedron_3 constructor using Polyhedron_3 crashes.  I
believe that the solid represented by the OFF file is valid based on
the specification (you can see the model in geomview or any other OFF
viewer).  I caused this crash by modifying the Nef_3.sln's
interface_polyhedron:

Polyhedron P ;
std::ifstream fin( "Triangle - Copy.off" ) ;
fin >> P ;
if ( P.is_closed() && P.is_valid() )
{
   Nef_polyhedron_3 N1( poly ) ;
}

Thanks,

Stephen
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Attachment: Triangle - copyOut.off
Description: Triangle - copyOut.off