Re: boost::geometry: intersection of a point with a linestring

From: Giorgino R (giorginor21_at_[hidden])
Date: 2019-10-04 08:52:53

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_at_[hidden]> wrote:

> On Thu, 3 Oct 2019 at 18:04, Giorgino R via Boost-users
> <boost-users_at_[hidden]> 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/intersection/intersection_3.html
> 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<linestring_2d> 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 <boost/geometry.hpp>
> #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<point_2d>;
>
> 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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