CGAL users discussion list

["Sebastien Loriot (GeometryFactory)" <>]

On 11/12/2014 11:03 PM, Qingnan Zhou wrote:
> Hi Sebastien,
>
> OK, we have tested your code.  It only fails with gcc 4.8.2 which
> happens to be the compiler we were using on both linux machines.
>
> gcc version 4.4.7 (GCC) : OK
> gcc version 4.7.2 (GCC) : OK
> gcc version 4.8.2 (GCC): Failed
> gcc version 4.9.2 (GCC): OK
>
> It might have been a bug with gcc 4.8.2.  :(

Just in case:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58800

Sebastien.

> best,
> James
>
> On Wed, Nov 12, 2014 at 3:31 PM, Sebastien Loriot (GeometryFactory)
> <
>  
> <mailto:>>
>  wrote:
>
>     I compiled the program attached on your input converted to off using
>     meshlab using
>     g++-4.8 (Debian 4.8.3-2) 4.8.3
>     g++-4.7 (Debian 4.7.3-14) 4.7.3
>     g++-4.6 (Debian 4.6.4-7) 4.6.4
>     g++-4.4 (Debian 4.4.7-8) 4.4.7
>     with CGAL-4.5 and all was fine.
>
>     Could you give it a try and let us know?
>
>     Thanks,
>
>     Sebastien.
>
>
>     On 11/12/2014 06:04 PM, Julian Panetta wrote:
>
>         Dear Sebastien and Laurent,
>
>         The main point of the example was to show that querying the AABB
>         for a
>         triangle by its centroid is finding the incorrect triangle. The
>         distance
>         printout shows that the error made is many orders of magnitude
>         greater
>         than machine precision (the example mesh bounding box is
>         26.6x19.2x16.1). You can see a full printout of the large errors
>         here:
>
>         http://julianpanetta.com/bad.__txt
>         <http://julianpanetta.com/bad.txt>
>
>         Notice that even when the AABB tree gets the correct face,
>         sometimes the
>         distance is not close to zero:
>         11208
>         distance  = 0.0363889
>         face idx  = 11208
>         (the first number means the centroid of 11208 was queried, and
>         face idx
>         = 11208 means the AABB returned face 11208).
>
>         It's difficult to believe this is a simple floating point error; the
>         example mesh doesn't have small or degenerate triangles, and the
>         errors
>         are visually obvious:
>         http://julianpanetta.com/__incorrect_result.png
>         <http://julianpanetta.com/incorrect_result.png>
>         (a query and the incorrectly retrieved triangle are outlined).
>
>         Furthermore, the results are compiler dependent: when gcc 4.8.2
>         is used
>         to compile CGAL and the example, both kernels Cartesian<double> and
>         Exact_predicates_inexact___constructions_kernel, give the
>         incorrect AABB
>         results. When gcc 4.9.2 is used to compile CGAL and the example,
>         both
>         kernels give the correct AABB results. It also works to compile CGAL
>         with gcc 4.8.2 but the example with gcc 4.9.2.
>
>         -Julian
>
>         On Wed, Nov 12, 2014 at 11:03 AM, Laurent Rineau
>         (CGAL/GeometryFactory)
>         
> <
>  
> <mailto:>
>         <
>         
> <mailto:>>>
>  wrote:
>
>              Le Wednesday 12 November 2014 10:38:03 Qingnan Zhou a écrit :
>              > Hi Sebastien,
>              >
>              > Changing the kernel to
>         CGAL::Exact_predicates___inexact_constructions_kernel
>              > does not help in this case.  The closest point and
>         primitive returned are
>              > still incorrect for many faces.  However, switching to
>         gcc-4.9.2 on linux
>              > seems to make the problem go away.  Maybe it is related
>         to how CGAL is
>              > built?
>              >
>              > Is anyone able to reproduce this problem?  I.e. compute
>         the distance
>              > between face center to the mesh for every face and check
>         the distance is
>              > 0.  Try it on this mesh: "suzanne.obj
>               >
>         <https://www.dropbox.com/s/__j1xj8nfy68xpdqc/suzanne_tri.__obj?dl=0
>         <https://www.dropbox.com/s/j1xj8nfy68xpdqc/suzanne_tri.obj?dl=0>>"
>
>              The computation of a distance is a construction. With the
>         kernel
>              CGAL::Exact_predicates___inexact_constructions_kernel, then the
>              constructions
>              are not exact, and thus the computation of a distance is
>         not exact
>              either.
>              That is expected.
>
>              Please explain what sort of problem you want to solve, that is
>              impacted by
>              this "bug", and we might be able to propose other solutions.
>
>              --
>              Laurent Rineau, PhD
>              R&D Engineer at GeometryFactory
>         http://www.geometryfactory.__com/ <http://www.geometryfactory.com/>
>              Release Manager of the CGAL Project http://www.cgal.org/
>
>
>
>
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