CGAL users discussion list

[Quentin Merigot <>]

Hi,

I did not try actually running your code, but there is no reason why the convex hull of a set of points should be *strongly* convex, which means that all points are extreme points (think of the convex hull of the eight vertices of a cube, plus a point that lies on one of the facets of the cube: the ninth point is not an extreme point, but nonetheless belongs to the convex hull).

Best,
Quentin

2014-11-05 19:20 GMT+01:00 Илья Палачев <>:

Hi,

 

I'm using 4.3 version of CGAL. Do you advise to update it?

Ilya uses 'true' for the second parameter:

I did not change anything in Quentin's code except one that line.

 

BTW, I've found another issue related with this function.

After construction of the intersection, the assertion is_strongly_convex_3 fails.

 

Here is my code:

 

static VectorXd buildStartingVector(SupportFunctionDataPtr data)

{

// Some code

 

/* Construct intersection of support halfspaces represented by planes */

std::vector<Plane_3> planes = data->supportPlanes();

Polyhedron_3 intersection;

globalPCLDumper(PCL_DUMPER_LEVEL_DEBUG,

".support-planes-for-halfspaces_intersection.txt")

<< planes;

CGAL::internal::halfspaces_intersection(planes.begin(), planes.end(),

intersection, Kernel());

 

ASSERT(is_strongly_convex_3(intersection));

 

        // Some code

}

 

I have printed planes to file and attached them to this mail. Here is the way they were printed:

 

std::ostream &operator<<(std::ostream &stream, std::vector<Plane_3> planes)

{

for (auto plane = planes.begin(); plane != planes.end(); ++plane)

stream << plane->a() << " " << plane->b() << " " << plane->c()

<< " " << plane->d() << std::endl;

return stream;

}

 

It is interesting that even if I remove check "ASSERT(is_strongly_convex_3(intersection))" and if I get the hull of its points, the assertion also fails:

 

Polyhedron_3 hull;

std::vector<Point_3> points;

for (auto vertex = intersection.vertices_begin();

vertex != intersection.vertices_end(); ++vertex)

{

points.push_back(vertex->point());

}

CGAL::convex_hull_3(points.begin(), points.end(), hull);

ASSERT(is_strongly_convex_3(hull));

 

 

Do you think that upgrade to CGAL 4.5 can resolve this issue?

 

Best regards,

Ilya

 

03.11.2014, 19:14, "Laurent Rineau (CGAL/GeometryFactory)" <>:

Le Monday 03 November 2014 15:23:09 Quentin Merigot a écrit :

 Thank you for this bug report, it is now fixed in my repository. However, I
 must say that I am surprised that it did not work before this change: the
 numbers provided to the constructor of Polyhedron_incremental_builder_3 are
 in principle only used as estimations to reserve memory. What version of
 CGAL are you using ?

The constructor of CGAL::Polyhedron_incremental_builder has a second parameter
that is a Boolean:
http://doc.cgal.org/latest/Polyhedron/classCGAL_1_1Polyhedron__incremental__builder__3.html#acb8216ef40323e777f57cd4656d98c83

When it is set to 'true', then that class template does extra checks, and
display diagnostics.

Ilya uses 'true' for the second parameter:

       typename CGAL::Polyhedron_incremental_builder_3<HDS> B( hds, true);

       B.begin_surface(_primal.size_of_facets(),
                       _primal.size_of_vertices(),
                       _primal.size_of_vertices());

That explains the reported behavior.

--
Laurent Rineau, PhD
R&D Engineer at GeometryFactory           http://www.geometryfactory.com/
Release Manager of the CGAL Project       http://www.cgal.org/


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