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 03:14:48

Hi Adam,

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.

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.

Thank you for your help.
Juliette

        double closestDistance(std::vector<Eigen::Vector2d> polygon2D,
Eigen::Vector2d P)
        {
                double minDist = DBL_MAX;
                for( unsigned int i = 0 ; i < polygon2D.size() ; i++ )
                {
                        Eigen::Vector2d A = polygon2D.at(i);
                        Eigen::Vector2d B;
                        if( i == polygon2D.size()-1 )
                        {
                                B = polygon2D.at(0);
                        }
                        else
                        {
                                B = polygon2D.at(i+1);
                        }
                        Eigen::Vector2d seg = B-A;

                        double U =
(((P[0]-A[0])*seg[0])+((P[1]-A[1])*seg[1]))/(seg.norm()*seg.norm());

                        double distance;
                        if( U < 0 )
                        {
                                distance = (A-P).norm();
                        }
                        else if( U > 1 )
                        {
                                distance = (B-P).norm();
                        }
                        else
                        {
                            Eigen::Vector2d Ph = A + U*seg;
                            distance = (Ph-P).norm();
                        }

                        // testing boos distance here
                        xsegment_t segment;
                        geom::append(segment,geom::make_point(A[0],A[1]));
                        geom::append(segment,geom::make_point(B[0],B[1]));
                        double distance2 =
geom::distance(segment,geom::make_point(P[0],P[1]));

                        std::cout << "Distance comparison : " << distance
<< " / " << distance2 << std::endl;

                        if( distance < minDist )
                        {
                                minDist = distance;
                        }
                }
                return minDist;
        }

Result obtained:

Distance comparison : 36.6926 / 4.37843

Distance comparison : 19.5823 / 4.37843

Distance comparison : 20.6683 / 4.37843

Distance comparison : -nan / 4.37843     --> two vertices are at the same
point

Distance comparison : 12.2823 / 4.37843

2016-04-21 3:51 GMT+02:00 Adam Wulkiewicz <adam.wulkiewicz_at_[hidden]>:

> Hi Juliette,
>
> Juliette Pera wrote:
>
> Dear all,
>
> I am currently trying to use the boost library in order to measure the
> shortest distance between a point and a list of segments.
>
> I am declaring a segment to which I append the two points of my segment
> and the I calculate the distance from my point P to the segment.
>
>    xsegment_t segment;
>    geom::append(segment,geom::make_point(A[0],A[1]));
>    geom::append(segment,geom::make_point(B[0],B[1]));
>    double distance2 = geom::distance(segment,geom::make_point(P[0],P[1]));
>
> The issue is that I measure every time the same distance even if the
> segment is different.
>
> Does anyone knows from where that issue can come from?
>
>
> Sorry, but I don't have enough data to say what's the problem. It'd be the
> best if you could share some small but complete example showing this issue.
>
> Does the algorithm return this result when geometries of types
> bg::model::segment and bg::model::point are passed?
> What coordinate system do you use?
>
> Or do you use your own types adapted to Segment and Point concepts?
> If the answer is yes, then I'd start from checking if the adaptation was
> done correctly.
>
> Regards,
> Adam
>
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