CGAL users discussion list

["Sebastien Loriot (GeometryFactory)" <>]

For information I've coded a way to clip a polyhedron by a plane
using some internal code. That is not the smartest way to do it
but it should work relatively well. A better version is on its
way but I need some time to finish it.


You need the include file internal/Polyhedron_plane_clipping_3.h
available here:
https://github.com/sloriot/cgal/raw/Operations_on_polyhedra-plane_clipping_with_corefinement-sloriot/Operations_on_polyhedra/include/CGAL/internal/Polyhedron_plane_clipping_3.h

And you can test it with the following program:
https://github.com/sloriot/cgal/raw/Operations_on_polyhedra-plane_clipping_with_corefinement-sloriot/Operations_on_polyhedra/examples/Operations_on_polyhedra/polyhedron_clipping.cpp

You can run the program without arguments if you have the file
elephant.off in data/ where the program is ran.

Sebastien.


On 10/21/2015 12:34 PM, gunshi gupta wrote:
> Thanks Vinicius, this looks helpful.
> I shall do so and revert back soon.
> Gunshi.
>
> On Tue, Oct 20, 2015 at 4:34 PM, Vinicius Azevedo
> <
>  
> <mailto:>>
>  wrote:
>
>     Hi Gunshi,
>
>     You may want to look my old thread about polygon slicing:
>     
> http://cgal-discuss.949826.n4.nabble.com/Slicing-splitting-cutting-polyhedral-meshes-td4660731.html
>     The snippet that I provide at the end can work for some cases, but
>     its not the optimal approach. I can help if you have more questions.
>
>     Vinicius C. Azevedo.
>
>     On Tue, Oct 20, 2015 at 7:51 AM, gunshi gupta
>     
> <
>  
> <mailto:>>
>  wrote:
>
>         I wanted to add:
>         Or should I go about it using boolean operations on nef
>         polyhedra, using the halfspaces I want as NOT of(halfspaces of
>         the plane I have)?
>         Thanks.
>
>         On 20 Oct 2015 14:51, "gunshi gupta" 
> <
>         
> <mailto:>>
>  wrote:
>
>             Thank you for your response Sebastien.
>
>             I now understand how the function works mostly, but i still
>             have a couple of doubts.
>
>             1)Initially in my program i have a cube with 6 plane
>             equations for their faces. I can, after some calculations
>             compute which half spaces of these planes i want. I then
>             intend to introduce n cutting planes, one by one, and
>             partition this cube into two polyhedrons, then partition the
>             two polyhedrons created into more polyhedrons by the next
>             cutting plane, and so on.
>
>             I basically have a cube and n cutting planes, and i want to
>             generate all the polyhedrons(guaranteed convex) that are
>             generated between these 6+n planes when we see them in 3d space.
>             The functions computes an intersection of half spaces, so
>             that means an intersection of the negative sides of these
>             hyperplanes right?
>             But what if i know which half space i want to be used in the
>             intersection? Is there any way to modify the plane_3
>             variable to make that happen,or some other way,  since i
>             guess its quite unlikely that i will get the polyhedrons i
>             want with only the negative half spaces being taken into
>             consideration, since i know my polyhedron will lie in the
>             first octant.
>
>             If it is possible, then i can just specify half spaces every
>             time, for the polyhedron, and turn wise use both half spaces
>             of the cutting plane to create the two polyhedrons i want
>             each time.
>
>             2) The last optional argument of the halfspace intersection
>             method is to be supplied as a point that we are certain lies
>             inside the polyhedron. This is supposed to greatly speed up
>             computation.
>             In my case i can only provide a point that lies ON the
>             polyhedron (i have this point from some other part of the
>             program),
>             I know that internally the point-in-polygon test takes place
>             calls :
>
>             if( !plane.has_on_negative_side(p) )
>
>             return false;
>
>             so i guess it'll return false for my case, and i should not
>             supply it, right?
>
>
>             Thanks again!
>
>
>             On Mon, Oct 19, 2015 at 12:22 PM, Sebastien Loriot
>             (GeometryFactory) 
> <
>             
> <mailto:>>
>  wrote:
>
>                 On 10/17/2015 05:58 PM, gunshi gupta wrote:
>
>                     Hi,
>                     I have a function written in MATLAB which is
>                     clipping a convex
>                     polyhedron, given an initial convex polyhedron and a
>                     plane, and it
>                     returns the new polyhedron that was formed as a
>                     result of the clipping
>                     (defined in terms of its faces and vertices).
>                        This function has been implemented by using the
>                     clipPolygon3D function
>                     that MATLAB provides, internally, by clipping each
>                     polygonal face of the
>                     polyhedron and then doing the necessary computation
>                     to combine the results.
>                     link:
>                     
> http://in.mathworks.com/matlabcentral/fileexchange/24484-geom3d/content/geom3d/geom3d/clipPolygon3dHP.m
>
>                     Now i want to do the same in c++, and preferably
>                     using CGAL.
>                     So
>                     1) i have searched and was not able to find a
>                     function for polygon
>                     clipping in CGAL, but i wanted to confirm if that is
>                     right since CGAL
>                     covers quite a wide range of algorithms.
>                     2)i have also come across the function "void
>                     CGAL::halfspace_intersection_3" from section 3D
>                     convex hulls in CGAL
>                     which seems to compute the intersection of a plane
>                     and a convex hull.
>                     link:
>                     
> http://doc.cgal.org/latest/Convex_hull_3/group__PkgConvexHull3Functions.html#ga6118d2c46934c541669ab871f8e1fd24
>
>                     It seems that if i am able to define a polyhedron as
>                     a convex hull, then
>                     from what i understand from the documentation (i
>                     understand very
>                     little), i should be able to do what i described
>                     above(polyhedron
>                     clipping) without having to implement polyhedron
>                     clipping through
>                     polygon clipping. Can someone help me with
>                     this/verify if this is possible?
>                     Thanks a lot!
>
>                 If the polyhedron is defined as a set of planes, you
>                 indeed can use
>                 this function.
>
>                 If you use a CGAL polyhedron with plane associated to
>                 faces, this is
>                 direct.
>
>                 See this example [1] (only the last line) to see how to
>                 associate
>                 a plane to each face.
>
>                 [1]
>                 
> http://doc.cgal.org/latest/Convex_hull_3/Convex_hull_3_2quickhull_3_8cpp-example.html
>
>                 Sebastien.
>
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