CGAL users discussion list

Quoting "Samer Afach" 
<>:

> But I've seen in the envelope program that it already supports triangles
> just like it supports planes with two different examples with similar
> outputs, isn't this the case? what's the difference between the
> implementation of triangles or planes equations? and I went through the
> functions in the reference manual in that class and couldn't find the
> function I need. Could you please tell me what's the name of that
> function if you know it?

CGAL::lower_envelope_3() ???

I believe that the lower envelope of planes will not get you the  
desired answer. Again, unless I misunderstand your problem.

> Thank you,
> Samer
>
>
>  wrote:
> > One way to do it is to triangulate the polyhedron facets (in case they
> > are not triangles to start with) and then compute the envelope of all
> > triangles. Consult the manual for further information. There are also
> > examples you can use as a starting point.
> >
> > Quoting "Samer Afach" 
> > <>:
> >
> > > Exactly! That's what I'm looking for actually! But could you please tell
> > > me how to implement it? does the envelope class have such a function?
> > >
> > > Thank you,
> > > Samer
> > >
> > >
> > >
> > >  wrote:
> > > > If you are looking for a function that accepts a polyhedron and a
> > > > point in the plane and returns the point on the lower envelope of the
> > > > polyhedron, the vertical projection of which coincides with the input
> > > > point, then you can implement it using the existing functionality.
> > > > Otherwise, I don't understand your question.
> > > >
> > > > Quoting "Samer Afach" 
> > > > <>:
> > > >
> > > > > Thank you for your reply.
> > > > >
> > > > > Actually I'm concerned now before this with the question whether the
> > > > > envelope algorithm offers the envelope as a function and not surfaces,
> > > > > because I couldn't find a single function in the corresponding class
> > > > > that does that. Does it do that? I mean I would like to give it a
> > > > > polyhedron, and on the other side using minimum and maximum get the
> > > > > envelopes as functions of x and y, is that possible?
> > > > >
> > > > > I'd appreciate any efforts so much,
> > > > > Thank you,
> > > > > Samer
> > > > >
> > > > >
> > > > >
> > > > >  wrote:
> > > > > > You need to use an exact-predicate and exact-construction kernel. If
> > > > > > you do that and the problem still occurs, please send the entire
> > > > > > (small) program that reproduces the problem.
> > > > > >
> > > > > > Quoting 
> > > > > > :
> > > > > >
> > > > > > > Hello,
> > > > > > >
> > > > > > > I eventually have found that the planes are being taken in the
> > > > > > > right way
> > > > > > > by dereferencing the iterator, but the program is failing in giving
> > > > > > > the
> > > > > > > right envelope, and giving an unexplainable exception, I started to
> > > > > > > believe that the algorithm has a problem... here's the part of the
> > > > > > > code,
> > > > > > > where P is a closed polyhedron object
> > > > > > >
> > > > > > >
> > > > > > >     for(plane_it = P.planes_begin(); plane_it !=
> > > > > > > P.planes_end();++plane_it)
> > > > > > >     {
> > > > > > >         planes.push_back(Surface_3(*plane_it));
> > > > > > >         cout<<*plane_it<<endl;
> > > > > > >     }
> > > > > > >
> > > > > > >     /*
> > > > > > >     //this is a part where the example inserted the points
> > > > > > >     planes.push_back (Surface_3(Plane_3(-1, 0, 1, 0)));
> > > > > > >     planes.push_back (Surface_3(Plane_3(0, 1 , 1, 0)));
> > > > > > >     planes.push_back (Surface_3(Plane_3(1, 0, 1,  0)));
> > > > > > >     */
> > > > > > >
> > > > > > >       // Compute and print the minimization diagram.
> > > > > > >     Envelope_diagram_2      min_diag;
> > > > > > >
> > > > > > >     CGAL::lower_envelope_3 (planes.begin(), planes.end(), min_diag);
> > > > > > >
> > > > > > >     std::cout << std::endl << "The minimization diagram:" <<
> > > > > > > std::endl;
> > > > > > >     print_diagram (min_diag);
> > > > > > >
> > > > > > >     // Compute and print the maximization diagram.
> > > > > > >     Envelope_diagram_2      max_diag;
> > > > > > >
> > > > > > >     CGAL::upper_envelope_3 (planes.begin(), planes.end(), max_diag);
> > > > > > >
> > > > > > >     std::cout << std::endl << "The maximization diagram:" <<
> > > > > > > std::endl;
> > > > > > >     print_diagram (max_diag);
> > > > > > >
> > > > > > > The polyhedron I'm using has like 30 facets or more, and is
> > > > > > > closed and
> > > > > > > follow the Euler characteristic, and I could view it using
> > > > > > > Geomview. So
> > > > > > > I'm very confused and don't know where the error could be. The
> > > > > > > exception
> > > > > > > differs everytime because I'm every time generating a new polyhedron
> > > > > > > that
> > > > > > > fulfills the conditions mention. here's the error I'm getting in
> > > > > > > Terminal:
> > > > > > >
> > > > > > > terminate called after throwing an instance of
> > > > > > > 'CGAL::Precondition_exception'
> > > > > > >   what():  CGAL ERROR: precondition violation!
> > > > > > > Expr: cv.has_left()
> > > > > > > File: /usr/local/include/CGAL/Arr_linear_traits_2.h
> > > > > > > Line: 682
> > > > > > > Aborted
> > > > > > >
> > > > > > > and here's another one too:
> > > > > > > terminate called after throwing an instance of
> > > > > > > 'CGAL::Precondition_exception'
> > > > > > >   what():  CGAL ERROR: precondition violation!
> > > > > > > Expr: f == f2
> > > > > > > File:
> > > > > > > /usr/local/include/CGAL/Arrangement_2/Arrangement_on_surface_2_impl.h
> > > > > > >
> > > > > > > Line: 2639
> > > > > > > Explanation: The two halfedges must share the same incident face.
> > > > > > > Aborted
> > > > > > >
> > > > > > > This last error I can't understand, what does the envelope thing
> > > > > > > have to
> > > > > > > do with the halfedges? I'm giving the program plane equations and
> > > > > > > it has
> > > > > > > nothing to do with my polyhedron.
> > > > > > >
> > > > > > > Any effort is highly appreciated,
> > > > > > >
> > > > > > > Thank you,
> > > > > > > Samer Afach
> > > > > > >
> > > > > > >
> > > > > > >
> > > > > > >
> > > > > > > > Hello programmers,
> > > > > > > >
> > > > > > > > I have a question since I'm not a professional C++ programmer. I'm
> > > > > > > > trying to apply the 3D envelope example 32.3.3 in
> > > > > > > >
> > > > > > > > http://www.cgal.org/Manual/last/doc_html/cgal_manual/Envelope_3/Chapter_main.html
> > > > > > > >
> > > > > > > >
> > > > > > > >
> > > > > > > >
> > > > > > > > but I'm taking the planes from a polyhedron, and I have them in the
> > > > > > > > type
> > > > > > > > Plane_3 through the iterator Polyhedron::begin() to end(). The
> > > > > > > > example
> > > > > > > > takes them in the form Surface_3(Plane_3()), how can I do this
> > > > > > > > transformation? I tried to use std::transform for this but I
> > > > > > > > failed.
> > > > > > > > The example provides it using the 4 coefficients of a plane, I can
> > > > > > > > see
> > > > > > > > these coefficients of my Plane_3 object on a cout stream, but I
> > > > > > > > can't
> > > > > > > > extract them alone one by one.
> > > > > > > >
> > > > > > > > Could you please either tell me the correct transformation
> > > > > > > > method for
> > > > > > > > this or tell me how to extract the coefficients from my Plane_3
> > > > > > > > object?
> > > > > > > >
> > > > > > > > Thank you,
> > > > > > > > Samer Afach
> > > > > > > >
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