Re: [Boost-users] Enquiry on boost.geometry and intersection()

Hi, Adam thanks for the reply and detailed code and explanation. Yes, it worked. I implemented as per your instructions but encountered a small problem. That is sometimes I realized the intersections points between the linestrings and the polygon are not in a "consistent" order, e.g. as shown in the below image, lingstring 1 has P1->P2; However, instead of P3->P4, linestring 2 is P4->P3. I actually expect linestring 2 is also along P3->P4 direction. Is this how it is supposed to be? Any workaround to solve this problem? Thanks. [image: Presentation1.png] Regards Ethan. On Fri, Nov 16, 2018 at 11:30 AM Adam Wulkiewicz wrote:

Hi and welcome,

Zhang Qun Via Boost-users wrote:

Hi guys, first-time post here. Hello to everyone.

I'm using boost.geometry to implement a project. I need to calculate the intersection between a line and a polygon. I know there is no infinite line concept in boost.geometry. So I try to use the segment to intersect with a polygon, however, it seems not supported yet. Is there any other workaround that I can implement the intersection between a line and a polygon? The line is like y=mx+b or ax + by + c = 0.

Indeed the intersection of segment and polygon does not compile even though it should. Fortunately you can represent a segment as a linestring with 2 points. Something like this:

namespace bg = boost::geometry;

typedef bg::model::point point; typedef bg::model::box<point> box; typedef bg::model::segment<point> segment; typedef bg::model::linestring<point> linestring; typedef bg::model::multi_linestring<linestring> multi_linestring; typedef bg::model::polygon<point> polygon;

// create some polygon, a circle polygon poly; for (double a = 0; a < 2 * 3.14; a += 0.1) bg::append(poly, point(cos(a), sin(a))); // correct data to match compile-time metadata // (clockwise, closed polygon) bg::correct(poly);

// calculate polygon's bounding box which could be used with line // equation to calculate segment's endpoints box b = bg::return_envelope<box>(poly);

// here you could calculate a segment represented as linestring // using min and max coordinates of polygon's envelope // and line's parameters // but for now: linestring ls; bg::append(ls, point(-1, -1)); bg::append(ls, point(1, 1));

// the result of the intersection of linear and areal geometry // is multi-linestring multi_linestring result; bg::intersection(ls, poly, result);

std::cout << bg::wkt(ls) << std::endl; std::cout << bg::wkt(poly) << std::endl; std::cout << bg::wkt(result) << std::endl;

Regards, Adam