CGAL users discussion list

["Sebastien Loriot (GeometryFactory)" <>]

Sorry for not having answered before.
If you submit a pull request over https://github.com/CGAL/cgal
I can have a look at it to integrate it into CGAL.

Thanks,

Sebastien.

On 04/30/2015 09:45 PM, Vinicius Azevedo wrote:
> Hey guys,
>
> I've fixed the halfedgeDS_cut_component function to project the newly
> created points on the cutting plane. To my surprise, that worked:
> https://dl.dropboxusercontent.com/u/5919734/bunnyW1.png
> https://dl.dropboxusercontent.com/u/5919734/bunnyw2.png
>
> To do that, I've changed all Vertex() constructors from
> halfedgeDS_cut_component
> to Vertex(pred.getPlane().projection(h->vertex()->point())). I've also
> made some few adjustments on the polyhedron_cut_plane_3. If anyone is
> interested I can post the full modified code.
>
>
> Vinicius C. Azevedo.
>
> On Thu, Apr 30, 2015 at 2:02 PM, Vinicius Azevedo
> <
>  
> <mailto:>>
>  wrote:
>
>     Update:
>
>     The polyhedron_cut_plane_3 function
>     calls I_polyhedron_cut_component( poly, h, pred) routine. This
>     returns a set of invalid (uninitialized?) edges/faces that are
>     connected to the newly created cut border.
>
>     Vinicius C. Azevedo.
>
>     On Wed, Apr 29, 2015 at 2:48 PM, Vinicius Azevedo
>     
> <
>  
> <mailto:>>
>     wrote:
>
>         OK, so I think that I found a potential problem. Inside the
>         polyhedron_cut_plane_3 function we have:
>
>         Halfedge_handle start = h;
>         do {
>             h->vertex()->point() = construct_point( plane,
>         h->next()->opposite()->facet()->plane(),
>         h->opposite()->facet()->plane());
>             h = h->next();
>         } while ( h != start);
>
>         This function basically computes the intersection point from 3
>         planes: the cutting plane and the two planes that pass through
>         the current edge. The function construct_point is:
>
>           operator()( const Plane_3& p, const Plane_3& q, const Plane_3&
>         r) const {
>             Object obj = intersection( p, q);
>             Line_3 line;
>             if ( assign( line, obj)) {
>                  obj = intersection( r, line);
>                  Point_3 pt;
>                  if ( assign( pt, obj)) {
>                      return pt;
>                  }
>             }
>             std::cerr << "ERROR: coplanar planes used for computing "
>         "intersecting point." << std::endl;
>             std::cerr << "       Return ORIGIN. Don't trust result." <<
>         std::endl;
>             return ORIGIN;
>         }
>
>         This function tries to find the intersection point by
>         successively computing intersections between the planes. But
>         this algorithm *will fail if r and q planes are colinear. *That
>         means that if an edge has two colinear planes (e.g. a simple
>         plane), that algorithm will not work. So perhaps a /function
>         that better computes the intersection between a plane and an
>         edge has to be used to solved this problem/.
>
>         Any ideas? Is what I said correct?
>
>         Vinicius.
>
>         Vinicius C. Azevedo.
>
>         On Wed, Apr 29, 2015 at 11:15 AM, Vinicius Azevedo
>         
> <
>         
> <mailto:>>
>  wrote:
>
>             Sure, here's the code snippet: http://pastebin.com/C892FnCu
>
>             The interesting thing though is that the halfedge handle
>             that I'm passing is not one that directly crosses the
>             polyline intersection.If I try to pass an edge that is being
>             actually intersected by the polyline I get an assertion error.
>
>             Vinicius C. Azevedo.
>
>             On Wed, Apr 29, 2015 at 11:01 AM, Sebastien Loriot
>             (GeometryFactory) 
> <
>             
> <mailto:>>
>  wrote:
>
>                 On 04/29/2015 04:56 PM, Vinicius Azevedo wrote:
>
>                     I was confused by the cut/slice examples on the
>                     Polyhedron demo. I
>                     thought you was mentioning that, my bad. That's what
>                     I was looking for,
>                     thanks! I have some questions though on how to use
>                     this function:
>
>                        * What should I pass to the function as starting
>                     halfedge? Is it an
>                          edge returned by any_intersection?
>
>                 I think so.
>
>                        * Does this function add vertices from the
>                     polyline that is generated
>                          from the intersection of the polyhedron with
>                     the plane? That's not
>                          what I'm getting on my results
>
>                     (https://dl.dropboxusercontent.com/u/5919734/bunnyCut1.png
>                     &&
>                     
> https://dl.dropboxusercontent.com/u/5919734/bunnyCut2.png).
>                     It only
>                          cuts the mesh by following pre-existent
>                     vertices. Also I'm having
>                          the error "coplanar planes used for computing
>                     the intersection
>                          plane, Return ORIGIN, Don't trust the result".
>
>                 I never used the function. Could you share your code so
>                 that I can
>                 give it a try?
>
>                 Sebastien.
>
>                     Thanks for your help so far!
>
>                     Vinicius C. Azevedo.
>
>                     On Tue, Apr 28, 2015 at 10:24 AM, Sebastien Loriot
>                     (GeometryFactory)
>                     
> <
>  
> <mailto:>
>                     
> <mailto:
>                     
> <mailto:>>>
>  wrote:
>
>                          On 04/28/2015 02:15 PM, Vinicius Azevedo wrote:
>
>                              Hi Sebastian,
>
>                              Thanks for your answer. I didn't expressed
>                     myself so well. What
>                              I want
>                              is to actually cut the mesh into two
>                     different independent parts
>                              with a
>                              given cut plane. In this video
>                     https://www.youtube.com/watch?v=IOnk7PBv1AU and paper
>                     http://www.math.ucla.edu/~jteran/papers/WJST14.pdf,
>                     they present
>                              some
>                              techniques to do that robustly. I saw the
>                     examples that you
>                              mentioned,
>                              and it seems that they only compute the
>                     intersections of planes and
>                              Polyhedrons, but don't actually split it
>                     into different parts.
>
>
>                          I believe this is what polyhedron_cut_plane_3.h
>                     is doing. The comment
>                          for this function is:
>                          // Cuts the polyhedron `poly' at plane `plane'
>                     starting at halfedge `h'.
>                          // Traces the intersection curve of `plane'
>                     with `poly' starting at `h',
>                          // cuts `poly' along that intersection curve
>                     and deletes the (connected)
>                          // component on the negative side of the plane.
>                     The hole created along
>                          // the intersection curve is filled with a new
>                     face containing the plane
>                          // and the points in the vertices are computed.
>
>                          Sebastien.
>
>
>                              Vinicius C. Azevedo.
>
>                              On Tue, Apr 28, 2015 at 1:48 AM, Sebastien
>                     Loriot (GeometryFactory)
>                              
> <
>                     
> <mailto:>
>                     
> <mailto:
>                     
> <mailto:>>
>                              
> <mailto:
>                     
> <mailto:>
>                     
> <mailto:
>                     
> <mailto:>>>>
>  wrote:
>
>                                   On 04/27/2015 04:31 PM, Vinicius
>                     Azevedo wrote:
>
>                                       Hi all,
>
>                                       I would like to know if anyone has
>                     a code snippet for
>                                       slicing/splitting/cutting
>                     polyhedral meshes in two
>                              different
>                                       parts with
>                                       arbitrary planes. I've seen some
>                     old threads about it
>
>
>                     
> (http://cgal-discuss.949826.n4.nabble.com/Slices-in-a-mesh-td951695.html),
>                                       but didn't find any code examples
>                     so far.
>
>                                       Thanks!
>                                       Vinicius C. Azevedo.
>
>
>
>                                   The thread you mention is to compute
>                     the intersection of a
>                              polyhedron
>                                   with a plane which is one or several
>                     polygons. There is a
>                              non documented
>                                   functor that is doing this in
>                     CGAL/Polyhedron_slicer_3.h
>                                   A better and documented version will
>                     be included in CGAL 4.8
>
>                                   In your case, it seems you want the
>                     intersection of a
>                              polyhedron with
>                                   a halfspace.
>                                   There is another documented function
>                              'CGAL::polyhedron_cut_plane_3'
>                                   I never used that is defined in
>                     CGAL/polyhedron_cut_plane_3.h
>                                   that should be doing what you want.
>
>                                   Sebastien.
>
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