CGAL users discussion list

[Andreas Fabri <>]

Hi Benjamin,

it should be spread out.
Note also that this problem might even occur with one connected
component.

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If the channel that connects the cavity with the rest
is extremely narrow it might happen that the algorithm
never detects the channel and the cavity.

andreas

On 18/07/2014 13:07, Benjamin Kehlet wrote:
> Ah, I see. Thanks!
>
> I will try to come with an implementation of
> Construct_initial_points() that makes sure all components are
> represented.
> Is the reason for shooting random rays that the points should be
> spread out over the surface or could I just pick some points (eg. via
> the vertex iterator)? (I generally find (bug) reproducability a nice
> feature :) )
>
> Benjamin
>
> 2014-07-18 12:00 GMT+02:00 Andreas Fabri 
> <>:
> > Hi Benjamin,
> >
> > the polyhedral domain class has a function
> > Polyhedral_mesh_domain::Construct_initial_points
> >
> > that provides an intial set of points on the surface.
> > It currently does this by shooting rays in random
> > directions from the center of the bounding box.
> >
> > When the surface has several connected components
> > and for a component no intial point is given
> > it may happen that the mesh generator later
> > never finds it (by also shooting rays, not random
> > though).
> >
> > What you have to do is to initialize
> > the Complex_3_in_triangulation_3 with better
> > points by doing what
> > Polyhedral_mesh_domain::Construct_initial_points
> > Mesh_3::init_c3_t3  do.
> >
> > If I find the time next week, I will try to provide
> > an example.
> >
> > best,
> >
> > andreas
> >
> >
> > On 17/07/2014 01:40, Benjamin Kehlet wrote:
> > > Hello CGAL people!
> > >
> > > I got a report of some unexpected results in mshr (a mesh generation
> > > component in the FEniCS project which uses CGAL heavily). I have
> > > simplified it a bit and reproduced the issue as a standalone CGAL
> > > program.
> > >
> > > When meshing a box with a crude triangulation of a sphere subtracted,
> > > the sphere is sometimes "missing" in the resulting mesh.
> > >
> > > When meshing the polyhedron in smaller.off (which is basically an axis
> > > aligned box with corners at (-1, -1, -.4) and (1,1,1) with a sphere
> > > with center at origin and radius .3 subtracted) it works fine. See the
> > > surface plot of the resulting mesh in smaller.png.
> > >
> > > However, with bigger.off (where the first corner is at (-1,-1,-.7) and
> > > the rest is equal) the inner sphere can not be seen in the plot. See
> > > bigger.png.
> > >
> > > I confirmed that it is not a plotting issue by calculating the volume
> > > of the mesh. For bigger.off the result is 6.8 which is equal to the
> > > volume of the box (2*2*1.7). For smaller.off it calculates the volume
> > > to 5.5223. I assume is the correct solution since it is smaller than
> > > 5.6 which is the volume of the box (2*2*1.4=5.6). The program source
> > > code is attached. The issue seems to be insensitive to the meshing
> > > parameters.
> > >
> > > Best regards
> > >
> > > Benjamin Kehlet
> >
> > --
> > Andreas Fabri, PhD
> > Chief Officer, GeometryFactory
> > Editor, The CGAL Project
> >
> > phone: +33.492.954.912    skype: andreas.fabri
> >
> > --
> > You are currently subscribed to cgal-discuss.
> > To unsubscribe or access the archives, go to
> > https://sympa.inria.fr/sympa/info/cgal-discuss

--
Andreas Fabri, PhD
Chief Officer, GeometryFactory
Editor, The CGAL Project

phone: +33.492.954.912    skype: andreas.fabri