 # Geometry :

Subject: [geometry] Order of the result from geometry::difference algorithm
From: You Li (liyou5678_at_[hidden])
Date: 2019-02-28 00:40:45

Hi all,

I wonder if the result from geometry::difference have a specific order?

A test shows that the result comes along the linestring from start point to
end point as below.
( Two difference lines returned. The order of result is alwasy AC the
first, DB the second. )

My work depends on the order of result. I am afraid of:
1. Under other circumstances, that order varies.
2. That order of result will change in the future version of boost library.
( I use 1.58.0 for now).

documentation.
Can you help me?


[image: image.png]
 https://pastebin.com/xNj1W41i

#include <iostream>

#include <boost/geometry.hpp>
#include <boost/geometry/geometries/point_xy.hpp>

#include <boost/foreach.hpp>

int main()
{
typedef boost::geometry::model::d2::point_xy<double> POINT;
typedef boost::geometry::model::linestring<POINT> LINE;
typedef boost::geometry::model::polygon<POINT> POLYGON;

LINE line;
POLYGON polygon;

// Y
// |
// -------------
// | | |
// | | |
// ------A>>>>>C>>>>>>>>>>>D>>>>>B-----> X
// start | | | end
// point | | | point
// ------|------
// |
//

// Create line from A to B.
boost::geometry::correct(line);

boost::geometry::read_wkt("POLYGON((1 1, -1 1, -1 -1, 1 -1))", polygon);
boost::geometry::correct(polygon);

std::vector<LINE> output;
boost::geometry::difference(line, polygon, output);

// Result will be 2 lines.
// A to C : LINESTRING(-2 0,-1 0)
// D to B : LINESTRING(1 0,2 0)
BOOST_FOREACH(LINE const& l, output)
{
std::cout << boost::geometry::wkt(l) << std::endl;
}

return 0;
}

-------------------
Thanks!
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