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[Max Harris <>]

OK, your suggestion worked for that case, but I want to write a function that finds the correct Halfedge_handle for a given cutting plane. What follows is my attempt, which doesn't work. What's wrong with it?

Halfedge_handle FindUsableHandle(Polyhedron P, Plane pl)

{

    Halfedge_iterator hi;

    for (hi = P.halfedges_begin(); hi != P.halfedges_end(); hi++) 

    {

        CGAL::Oriented_side test = pl.oriented_side(hi->vertex()->point());

        if (test == CGAL::ON_POSITIVE_SIDE)

        { 

            cout << "point on positive side" << endl; 

        }

        else if (test == CGAL::ON_NEGATIVE_SIDE)

        { 

            cout << "point on negative side" << endl;

            return hi;

        }

        else

        { 

            cout << "point over line" << endl; 

        }

    }

    

    return hi;

}

        Polyhedron result;

        Halfedge_handle he = make_cube_3(result);

        

        CGAL::polyhedron_cut_plane_3(result, he->opposite()->next(), pl3); // works

        CGAL::polyhedron_cut_plane_3(result, FindUsableHandle(P, pl3), pl3); // does not work. see output below 

output: 

Facet number: 0

Plane: -0 -1 -0 -0

Normal: -0 -1 -0

Scaled normal: -0 0.1 -0

Transformed plane: -0 -1 -0 0.1

point on positive side

point on positive side

point on positive side

point on positive side

point on positive side

point on positive side

point on negative side

terminate called after throwing an instance of 'CGAL::Postcondition_exception'

  what():  CGAL ERROR: postcondition violation!

Expr: D.is_valid(false,3)

File: /usr/local/include/CGAL/halfedgeDS_cut_component.h

Line: 126

(gdb) 

On May 27, 2011, at 2:29 AM, Sebastien Loriot (GeometryFactory) wrote:

Try with

CGAL::polyhedron_cut_plane_3(result, he->opposite(), pl3);
instead of
CGAL::polyhedron_cut_plane_3(result, he, pl3);

he->vertex() need to be on the negative side of the plane.

S.

Max Harris wrote:

polyhedron_cut_plane_3() breaks when I try running the code below. What am I doing wrong?

Here's the output:

*Plane: -0 -1 -0 -0*

**

*Normal: -0 -1 -0*

**

*Scaled normal: -0 0.1 -0*

**

*Transformed plane: -0 -1 -0 0.1*

*terminate called after throwing an instance of 'CGAL::Precondition_exception'*

*  what():  CGAL ERROR: precondition violation!*

*Expr: pred( h->vertex())*

*File: /usr/local/include/CGAL/polyhedron_cut_plane_3.h*

*Line: 120*

Root::Root()

{

   Polyhedron P;

   make_cube_3(P);

   Facet_iterator f = P.facets_begin();        Plane pl2 = f->plane(); // plane equation of the facet

   cout << "Plane: " << pl2 << endl;

       CGAL::Vector_3<Kernel> normal = pl2.orthogonal_vector();

   cout << "\nNormal: " << normal << endl;

       Vector sn = normal.transform(Transformation(CGAL::SCALING, -0.1));

   cout << "\nScaled normal: " << sn << endl;

       Plane pl3 = pl2.transform(Transformation(CGAL::TRANSLATION, sn));

   cout << "\nTransformed plane: " << pl3 << endl;

       Polyhedron result;

   Halfedge_handle he = make_cube_3(result);

       CGAL::polyhedron_cut_plane_3(result, he, pl3);

}

template <class Poly>

typename Poly::Halfedge_handle Root::make_cube_3(Poly& P)

{

   // appends a cube of size [0,1]^3 to the polyhedron P.

   CGAL_precondition( P.is_valid());

   typedef typename Poly::Point_3         Point;

   typedef typename Poly::Plane_3         Plane;

   typedef typename Poly::Halfedge_handle Halfedge_handle;

   Halfedge_handle h = P.make_tetrahedron(Point( 1, 0, 0),

  Point( 0, 0, 1),

  Point( 0, 0, 0),

  Point( 0, 1, 0));

   Halfedge_handle g = h->next()->opposite()->next();

   P.split_edge( h->next());

   P.split_edge( g->next());

   P.split_edge( g);

   h->next()->vertex()->point()     = Point( 1, 0, 1);

   g->next()->vertex()->point()     = Point( 0, 1, 1);

   g->opposite()->vertex()->point() = Point( 1, 1, 0);

   Halfedge_handle f = P.split_facet( g->next(), g->next()->next()->next());

   Halfedge_handle e = P.split_edge( f);

   e->vertex()->point() = Point( 1, 1, 1);

   P.split_facet( e, f->next()->next());

   CGAL_postcondition( P.is_valid());

   g = h;

   g->facet()->plane() = Plane( g->vertex()->point(),

g->next()->vertex()->point(),

g->next()->next()->vertex()->point());

   g = h->opposite();

   g->facet()->plane() = Plane( g->vertex()->point(),

g->next()->vertex()->point(),

g->next()->next()->vertex()->point());

   g = h->next()->opposite();

   g->facet()->plane() = Plane( g->vertex()->point(),

g->next()->vertex()->point(),

g->next()->next()->vertex()->point());

   g = h->next()->next()->opposite();

   g->facet()->plane() = Plane( g->vertex()->point(),

g->next()->vertex()->point(),

g->next()->next()->vertex()->point());

   g = h->next()->next()->next()->opposite();

   g->facet()->plane() = Plane( g->vertex()->point(),

g->next()->vertex()->point(),

g->next()->next()->vertex()->point());

   g = g->next()->next()->opposite();

   g->facet()->plane() = Plane( g->vertex()->point(),

g->next()->vertex()->point(),

g->next()->next()->vertex()->point());

   return h;

}

On May 25, 2011, at 12:47 AM, Marc Glisse wrote:

On Wed, 25 May 2011, harris.max wrote:

I'm trying to get CGAL to generate a plane that's offset by some specifiable

distance from an existing plane, but everything I've tried so far isn't

working. What's the best way to do this?

What did you try that isn't working?

There doesn't seem to be any function to do that directly, but you could for instance use Construct_orthogonal_vector_3 to get a normal vector, scale it to whatever distance you want, then translate the plane?

--

Marc Glisse

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