Re: [Boost-users] boost::geometry: intersection of a point with a linestring

4 Oct 2019

      Hi Mateus,

Many thanks for your descriptive response.
The library is indeed very nice and most of the time we don't have to pull
our hair 😂.
Your example is exactly what I was thinking as a solution. However, I just
wanted to know whether an "optimised split" algorithm has been implemented.
The PostGIS link was very helpful as well.

Many thanks once again.

BW
G

On Thu, Oct 3, 2019 at 6:31 PM Mateusz Loskot via Boost-users <
boost-users@lists.boost.org> wrote:
...
On Thu, 3 Oct 2019 at 18:04, Giorgino R via Boost-users
 wrote:
...
Is there any boost functionality to "split" a linestring with an
intersecting point?
I'm not aware of any such algorithm in Boost.Geometry.
...
I was expecting that boost::geometry::intersection could return a
multi-linestring (of size two) or a vector of linestrings (size two).
That would be a different algorithm, presumably split, but not
intersection according to OGC.
Mind you, the Boost.Geometry documentation at
https://www.boost.org/libs/geometry/doc/html/geometry/reference/algorithms/i...
says "The function intersection implements function Intersection from
the OGC Simple Feature Specification.",
what is similar to PostGIS, https://postgis.net/docs/ST_Intersection.html.
The OGC Simple Feature Specification says the algorithm returns
point set intersection of input geometries.
...
For example, please see a very simple case below which doesn't compile:
...
point_2d pt(0.5, 0);
  linestring_2d ls({ point_2d(0, 0), point_2d(1, 0), point_2d(3, 0),
point_2d(5, 0) });
  std::vector inter;
  bg::intersection(pt, ls, inter);
Is there any way to get inter vector with two linestrings (...) or any
multi geometry container
that will hold the spatial information of two lines connected in this
point?
Or should I create these two lines myself by working out which points of
the linestring
are left and right of the intersecting point?
There is a way and yes you need to build a solution your self,
but it should be not a hair-pulling experience using the building blocks
provided by Boost.Geometry.
For example, here is a quick example with possible solution:
```
#include 
#include <algorithm>
#include <iostream>
#include <vector>
namespace bg = boost::geometry;
using point_2d = bg::model::d2::point_xy<double>;
using linestring_2d = bg::model::linestring;
int main()
{
    point_2d pt(2.5, 0);
    linestring_2d ls({{0, 0}, {1, 0}, {2, 0}, {3, 0}, {4, 0}, {5, 0}});
auto split_begin = std::find_if(bg::segments_begin(ls),
bg::segments_end(ls),
        [&pt](auto const& segment) { return bg::intersects(segment, pt);
});
linestring_2d line1;
    std::for_each(bg::segments_begin(ls), split_begin, [&line1](auto
const& s)
    {
        bg::append(line1, s.first);
    });
    bg::append(line1, split_begin->first);
    bg::append(line1, pt);
linestring_2d line2;
    bg::append(line2, pt);
    bg::append(line2, split_begin->second);
    std::for_each(++split_begin, bg::segments_end(ls), [&line2](auto
const& s)
    {
        bg::append(line2, s.second);
    });
std::cout << bg::dsv(line1) << std::endl;
    std::cout << bg::dsv(line2) << std::endl;
}
```
It does take some time to get familiar with Geometry LEGO, but the docs
are good
and the .cpp files with examples should be useful as well.
Best regards,
--
Mateusz Loskot, http://mateusz.loskot.net
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