Re: [Boost-users] measuring distance between a point and a segment

Subject: Re: [Boost-users] measuring distance between a point and a segment
From: Juliette Pera (juliette.pera_at_[hidden])
Date: 2016-04-21 08:48:29

Thank you very much it works very well now!

I was considering using a ring type at first but I was having a wrong
result too.
I was using the *append* function and then the *correct *one.
Maybe I was missing something to "close" the polygon?

Thank you
Juliette

2016-04-21 14:13 GMT+02:00 Adam Wulkiewicz <adam.wulkiewicz_at_[hidden]>:

> Juliette Pera wrote:
>
> Thanks for your answer.
> Here's some more information.
>
> I am going through the list of vertices of a segment, stored in a vector
> of Eigen::Vector2D and then I am computing the distance from a point stored
> in Eigen::vector2d to a each segment of my polygon.
>
> I calculate the distance using my own function and then I calculate it
> using boost function distance.
> I declare a segment of type  boost::geometry::model::segment<double>  in
> which I add my two vertices that make the segment.
>
>
> You probably mean boost::geometry::model::segment<boost::geometry::model::point<double,
> 2, boost::geometry::cs::cartesian> > ?
>
> Then I display the result for each vertices and I get the same result for
> every segment using boost.
>
> I hope it's more clear now.
>
>
> If xsegment_t is of type boost::geometry::model::segment and
> geom::append() is boost::geometry::append() then it doesn't set the
> endpoints of a segment. It's because a segment is not a container so
> appending to it doesn't change it. If xsegment_t was e.g.
> boost::geometry::model::linestring or instead of bg::append() the
> endpoints of a segment was passed into the constructor it should work.
>
> The behavior of bg::append() is indeed confusing. Anyway, try using the
> constructor of bg::model::segement instead:
>
> xsegment_t segment(geom::make_point(A[0],A[1]),
>                    geom::make_point(B[0],B[1]));
> double distance2 = geom::distance(segment, geom::make_point(P[0],P[1]));
>
> Please let me know if this helps.
>
>
> TL;DR
>
> A side note. If the overall goal was to calculate the distance from a
> point to Polygon2D then you could write your own Ring type (a Polygon
> without holes), e.g. wrap std::vector<Eigen::Vector2d> and adapt your
> type to Boost.Geometry Ring concept. Or simply register
> std::vector<Eigen::Vector2d> as a Ring (though then all std::vectors of
> Eigen::Vector2d points would be treated as Rings). It'd require to adapt
> Eigen::Vector2d type to Boost.Geometry Point concept and since
> Eigen::Vector2d provides operator[] for element access you would be
> forced to adapt it manually specializing traits. If the goal was to
> calculate the closest distance to the edge of Polygon2D then instead of
> adapting std::vector to the Ring concept you could adapt it to the
> Linestring concept. Then Boost.Geometry could calculate the distance for
> you.
>
> Here Boost.Geometry point_xy is adapted:
>
> https://github.com/boostorg/geometry/blob/develop/include/boost/geometry/geometries/point_xy.hpp#L87
>
> You could copy it and replace:
> model::d2::point_xy<CoordinateType, CoordinateSystem> -> Eigen::Vector2d
> CoordinateType                                        -> double
> CoordinateSystem                                      -> cs::cartesian
> return p.template get<Dimension>();                   -> return
> p[Dimension];
> p.template set<Dimension>(value);                     -> p[Dimension] =
> value;
>
> Here is an example of adapting std::vector to the Ring concept:
>
> http://www.boost.org/doc/libs/1_60_0/libs/geometry/doc/html/geometry/reference/adapted/register/boost_geometry_register_ring.html
>
> So you could register your vector like this:
> BOOST_GEOMETRY_REGISTER_RING(std::vector<Eigen::Vector2d>)
>
> And then
> bg::distance(polygon2D, P)
> should return the distance between Polygon and Point.
>
> See also:
>
> http://www.boost.org/doc/libs/1_60_0/libs/geometry/doc/html/geometry/reference/concepts/concept_point.html
>
> http://www.boost.org/doc/libs/1_60_0/libs/geometry/doc/html/geometry/reference/concepts/concept_ring.html
>
> http://www.boost.org/doc/libs/1_60_0/libs/geometry/doc/html/geometry/reference/concepts/concept_linestring.html
>
> http://www.boost.org/doc/libs/1_60_0/libs/geometry/doc/html/geometry/reference/adapted/register/boost_geometry_register_linestring.html
>
> Regards,
> Adam
>
>
>
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