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我正在使用 CGAL 并遇到了一些奇怪的错误，我无法在小型测试程序中重现。这是给定的测试代码，但是当我在较大的程序（ROS节点）中有完全相同的代码时，它会出现错误：

#include <vector>
#include <boost/shared_ptr.hpp>
#include <CGAL/Exact_predicates_inexact_constructions_kernel.h>
#include <CGAL/Polygon_with_holes_2.h>
#include <CGAL/Boolean_set_operations_2.h>

#include "print.h"

typedef CGAL::Exact_predicates_inexact_constructions_kernel Kernel;
typedef Kernel::FT                          Ft;
typedef Kernel::Point_2                     Point;
typedef Kernel::Segment_2                   Segment;
typedef Kernel::Direction_2                 Direction;
typedef Kernel::Line_2                      Line;
typedef Kernel::Vector_2                    Vector;
typedef CGAL::Polygon_2<Kernel>             Polygon;
typedef CGAL::Polygon_with_holes_2<Kernel>  PolygonWithHoles;

main() {
    Polygon poly;

    float scale = 4.0/100;
    float max_y = 500*scale;
    poly.push_back(Point(76*scale, max_y-496*scale));
    poly.push_back(Point(660*scale, max_y-496*scale));
    poly.push_back(Point(660*scale, max_y-48*scale));
    poly.push_back(Point(71*scale, max_y-54*scale));

    // Holes must be clock wise!!
    Polygon holes[10];
    holes[0].push_back(Point(131*scale, max_y-86*scale));
    holes[0].push_back(Point(179*scale, max_y-85*scale));
    holes[0].push_back(Point(180*scale, max_y-238*scale));
    holes[0].push_back(Point(133*scale, max_y-239*scale));

    holes[1].push_back(Point(237*scale, max_y-84*scale));
    holes[1].push_back(Point(286*scale, max_y-84*scale));
    holes[1].push_back(Point(288*scale, max_y-237*scale));
    holes[1].push_back(Point(240*scale, max_y-238*scale));

    // Why does this hole make intersection() error?
    holes[2].push_back(Point(345*scale, max_y-84*scale));
    holes[2].push_back(Point(393*scale, max_y-83*scale));
    holes[2].push_back(Point(396*scale, max_y-236*scale));
    holes[2].push_back(Point(348*scale, max_y-236*scale));

    PolygonWithHoles polyHoles(poly);
    polyHoles.outer_boundary() = poly;
    for (int i=0; i<3; ++i)
        polyHoles.add_hole(holes[i]);

    std::cout << "\nPolygon:" << std::endl;
    print_polygon_with_holes(polyHoles);

    Polygon selection;
    float minx = -5.7669;
    float miny = -2.13124;
    float maxx = 0.396996;
    float maxy = 4.88933;

    selection.push_back(Point(minx, miny));
    selection.push_back(Point(maxx, miny));
    selection.push_back(Point(maxx, maxy));
    selection.push_back(Point(minx, maxy));

    std::cout << "\nSelection:" << std::endl;
    print_polygon(selection);

    std::vector<PolygonWithHoles> result;
    CGAL::intersection(polyHoles, selection, std::back_inserter(result));

    std::cout << "Intersection:" << std::endl;
    if (!result.empty())
        print_polygon_with_holes(result.front());
}

错误：

terminate called after throwing an instance of 'CGAL::Precondition_exception'
  what():  CGAL ERROR: precondition violation!
Expr: comp_f(object, parentP->object) != SMALLER
File: /usr/include/CGAL/Multiset.h
Line: 2128

我找到了两种解决方法：

  1. 稍微移动一点：将 83 替换为 84。
  2. 使用 Exact_predicates_exact_constructions_kernel

我的问题是：什么可能导致问题只存在于更大的程序中？

我想继续使用 unexact_constructions，我不明白为什么在这种情况下我必须使用exact_constructions（点不靠近彼此或任何东西），但因为我不知道交点是什么() 算法我可能错了。

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1 回答 1

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该intersect函数构造新点 - 新的交点。如果你没有使用精确的结构，那么这些新点就不能保证是正确的。如果您随后使用这些不精确的点进行进一步的计算，您将遇到问题。除非您对运行时间有重大限制，否则我相信您最好使用精确的结构。

示例：考虑单位圆x^2+y^2=1和直线y=x，令p为构造的交点。仅当您使用确切的结构时circle.has_on_boundary (p)才会返回。TRUE

于 2013-09-20T06:21:17.793 回答