CGAL Ipelets

*Sébastien Loriot and Sylvain Pion*

78.1 | Introduction | ||||

78.2 | CGAL Ipelets | ||||

78.3 | Example | ||||

78.4 | Installation of the Demo Ipelets | ||||

78.5 | Design and Implementation History |

The Ipe extensible drawing editor (http://tclab.kaist.ac.kr/ipe/) [Sch95, Che09]
is a tool used by computational geometry researchers to produce 2D figures for inclusion in articles or presentations.
The extensible adjective sheds a light on an important feature:
the possibility for users to write small extensions (called *ipelets*)
to interface any algorithm with Ipe. This feature is especially interesting
to produce illustrations in a paper describing a new algorithm.
This package provides one class
that eases the writing of ipelets based on Cgal, reducing the needed knowledge of the API of Ipe.
This class is designed to fulfill most of the needs to implement an ipelet for a 2D Cgal algorithm.
In addition, this package comes with more than ten complete examples
of ipelets: alpha-shape, arrangements, Voronoi diagrams, convex hulls, Hilbert curve, k-order Delaunay,
2D mesh, Minkowski sum, polygon partition, random generators, triangulations …

The class *CGAL::Ipelet_base* derives from the class *Ipelets* from Ipe
and has one template parameter indicating
which kernel must be used within the ipelet.
In practice, we recommend to use either *CGAL::Exact_predicate_exact_constructions_kernel*
or *CGAL::Exact_predicate_inexact_construction_kernel*.

Two main methods are provided by the *CGAL::Ipelet_base* class.
The first one, *read_active_objects* retrieves all
primitives selected in Ipe when calling an ipelet, and converts them into equivalent Cgal objects.
The second method, *draw_in_ipe* draws Cgal objects in the Ipe
window. These functions handle basic objects such as points, segments, polygons, circles and circular arcs.

The registration of an ipelet into Ipe can be done using the macro
*CGAL_IPELET*. *CGAL_IPELET(ipelet_class_name)*
must follow the definition of the ipelet class, in the same file source file.

The following example shows how the class *CGAL::Ipelet_base* can be used to easily
interface the Cgal 2D Delaunay triangulation with Ipe.

File:demo/CGAL_ipelets/simple_triangulation.cpp

#include <CGAL/Exact_predicates_inexact_constructions_kernel.h> #include <CGAL/Delaunay_triangulation_2.h> #include <CGAL/CGAL_Ipelet_base.h> namespace my_triangulation{ typedef CGAL::Exact_predicates_inexact_constructions_kernel Kernel; typedef CGAL::Delaunay_triangulation_2<Kernel> Delaunay; //Function names of the ipelet const std::string labels[] = { "Delaunay","Help" }; //Help message associated to the first function const std::string hmsg[] = { "Draw a Delaunay triangulation of a set of points" }; class Triangulation_ipelet : public CGAL::Ipelet_base<Kernel,2>{ public: //declare an ipelet called CGAL Delaunay, with 2 functions (including help message). Triangulation_ipelet() :CGAL::Ipelet_base<Kernel,2>("CGAL Delaunay",labels,hmsg){} void protected_run(int); }; //function called when using the ipelet. void Triangulation_ipelet::protected_run(int fn) { switch (fn){ case 1: show_help();//print an help message return; default: std::list<Point_2> pt_lst; //Recovering points using output iterator of type //Dispatch_or_drop_output_iterator read_active_objects( CGAL::dispatch_or_drop_output<Point_2>(std::back_inserter(pt_lst)) ); if (pt_lst.empty()) { print_error_message("No mark selected"); return; } Delaunay dt; dt.insert(pt_lst.begin(),pt_lst.end()); //draw the triangulation. draw_in_ipe(dt); }; } }//namespace my_triangulation //register the ipelet in Ipe CGAL_IPELET(my_triangulation::Triangulation_ipelet)

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