CGAL users discussion list

[Roger House <>]

To avoid obscure errors when calling CGAL functions, I am trying to 
validate
objects of type Polygon_2 and Polygon_with_holes_2 before doing such things
as boolean operations.  To my chagrin, is_valid_2_object blew up when I
called it on this polygon:

   (2 0) (4 0) (4 4) (2 0) (0 0) (0 -4)

It seems to me that the worst a validation function should do is return
false.  Am I calling the wrong function?

Below is the output produced by the attached program.

Roger House
Software Developer

Written to stderr by error handler

CGAL problem:  precondition
 Expression:  compare_xy(cv1.right(), p) == LARGER && 
compare_xy(cv2.right(), p) == LARGER
 File:  
c:\users\roghouse\332\build\projects\cgal_eesof\include\cgal\arr_segment_traits_2.h
 Line:  609

Written to stdout

Begin test of is_valid_2_object

Two triangles touching
 [ 6 vertices: (2 0) (4 0) (4 4) (2 0) (0 0) (0 -4) ]
 P is not simple
 P is not convex
 P has area 8

Just before calling is_valid_2_object
EXCEPTION THROWN!
CGAL ERROR: precondition violation!
Expr: compare_xy(cv1.right(), p) == LARGER && compare_xy(cv2.right(), p) 
== LARGER
File: 
c:\users\roghouse\332\build\projects\cgal_eesof\include\cgal\arr_segment_traits_2.h
Line: 609
End test of is_valid_2_object

Press any nonwhitespace key and Enter to terminate

// isvalid.cpp  --  Demonstrate problem with is_valid_2_object
//

#include "CGAL\Cartesian.h"
#include "CGAL\Polygon_2.h"
#include <CGAL\Boolean_set_operations_2.h>
#include <CGAL\assertions.h>
#include <CGAL\Polygon_set_2.h>
#include <list>
#include <cassert>
#include <iostream>
#include <sstream>

using namespace std;

typedef CGAL::Cartesian<double>       K;
typedef K::Point_2                    Point;
typedef list<Point>                   Point_list;
typedef CGAL::Polygon_2<K>            Polygon_2;
typedef CGAL::Polygon_set_2<K>        Polygon_set_2;
typedef CGAL::Polygon_with_holes_2<K> Polygon_with_holes_2;
typedef list<Polygon_with_holes_2>    Pwh_list_2;

// CGAL error handler

void handle_CGAL_errors(
    const char *type,
    const char *expression,
    const char *file,
    int line,
    const char *explanation)
{
    cerr << "CGAL problem:  " << type << endl;
    cerr << "  Expression:  " << expression << endl;
    cerr << "  File:  " << file << endl;
    cerr << "  Line:  " << line << endl;
    if (explanation != NULL && *explanation != '\0')
        cerr << "  Explanation:  " << explanation << endl;
}


void print_polygon(const Polygon_2 &poly)
{
    Polygon_2::Vertex_const_iterator vit;

    cout << "[ " << poly.size() << " vertices:";
    for (vit = poly.vertices_begin(); vit != poly.vertices_end(); ++vit)
        cout << " (" << *vit << ')';
    cout << " ]" << endl;
}


void print_polygon_info(
    const Polygon_2 &poly, 
    const string &name, 
    const string &desc)
{
    bool isSimple = poly.is_simple();
    bool isConvex = poly.is_convex();
    bool isCCW    = false;
    if (isSimple)
        isCCW = poly.is_counterclockwise_oriented();

    cout << desc << endl;
    cout << "  ";
    print_polygon(poly);
    cout << "  " << name << " is " << (isSimple ? "" : "not ") 
                                                    << "simple" << endl;
    cout << "  " << name << " is " << (isConvex ? "" : "not ") 
                                                    << "convex" << endl;
    if (isSimple)
    cout << "  " << name << " is " << (isCCW    ? "counterclockwise" 
                                                : "clockwise") << endl;
    cout << "  " << name << " has area " << poly.area() << endl;
    cout << endl;
}


int main()
{
    CGAL::Failure_function prev_error_handler;
    CGAL::Failure_function prev_warning_handler;
    prev_error_handler   = CGAL::set_error_handler  (handle_CGAL_errors);
    prev_warning_handler = CGAL::set_warning_handler(handle_CGAL_errors);

    cout << "Begin test of is_valid_2_object" << endl;
    cout << endl;

    try
    { 
        Point points[] = { Point(2,0), Point(4,0), Point(4,4), Point(2,0), 
            Point(0,0), Point(0,-4) };
        Polygon_2 P(points, points+6);
        print_polygon_info(P, "P", "Two triangles touching");

        cout << "Just before calling is_valid_2_object" << endl;
        bool isValid = Polygon_set_2::Traits_2().is_valid_2_object()(P);

        cout << "  P is is " << (isValid ? "" : "not ") << "valid" << endl;
    }
    catch (exception& e)
    {
        cout << "EXCEPTION THROWN!" << endl;
        cout << e.what() << endl;
    }

    cout << "End test of is_valid_2_object" << endl;
    cout << endl; 

    cout << "Press any nonwhitespace key and Enter to terminate"  << endl;
    string c;
    cin >> c;

}   //  end main


// end isvalid.cpp