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Subject: [ggl] Understanding get_turns for linestring/polygon overlay
From: John Swensen (jpswensen)
Date: 2011-02-01 15:06:33

On Feb 1, 2011, at 11:57 AM, Barend Gehrels wrote:

> Hi John,
> Welcome to the list!
> On 1-2-2011 16:25, John Swensen wrote:
>> I apologize if this email seems rudimentary. I am just getting into both Boost and Boost.Geometry. From the mailing list, I have gleaned that LineString/Polygon boolean operations are not implemented yet.
> That is right.
>> So, I am trying to implement it myself from some of the examples. I am able to successfully create a polygon and a linestring and follow the overlay example to get the list of turn points describing where they intersect. However, I would like to take the list of turn points and create two new polygons from that are divided by the turn points.
> So basically you are wishing a "cut operation", cut the polygon into pieces. Very interesting.
>> So my questions are:
>> 1) Is there a way to insert the turn point into the outer ring easily?
> Yes, relatively easy. For each turn point you know the coordinate (.point), and at which segment it intersects the polygon (operations[0].seg_id). It must be able to insert a point there, though this is not a Boost.Geometry standard way. Assuming you use the Boost.Geometry polygon, you take the ring (if the ring_id is -1 it is the outer ring), and using std:: you insert that point there.
>> 2) Is there a way to find the line segments of the outer ring that are divided by the turn points?
> Yes, this is related and actually answered above.
> The basic case might be relatively easy to solve. However, as soon as intersection points overlay on the vertices of the input polygons, it becomes more complicated (or you can simply ignore those duplications). The sample shows "entering" and "leaving", but as soon as the linestring touches the polygon and does not intersect, it also becomes more complex. But not unsolvable.
>> If either of these are easy to find, then splitting my polygon into two polygons should be straightforward. I'm just not sure if there is a "right" way to do this. I could always go back and walk my way around the outer ring doing the computation to see of the turn point lies on the line segment and then splitting accordingly, but want to know if this is already implemented and I just can't find it.
> No, it is not implemented, and you have the right approach. In case you succeed and want to contribute this, I'm interested in the results.
> Regards, Barend

Thanks for pointing me in the right direction, however I think there is something wrong with how I am trying it. When I add the following debug output to my application, I don't get what I expected.

BOOST_FOREACH(turn_info const& turn, turns)
        std::cout << action << " polygon at " << boost::geometry::dsv(turn.point) << std::endl;
        std::cout << action << " polygon at " << boost::geometry::dsv(turn.point) << "with seg_id: " << turn.operations[0].seg_id.segment_index << std::endl;
                std::cout << "================" << turn.operations[0].seg_id.source_index << " " << turn.operations[0].seg_id.multi_index << " " << turn.operations[0].seg_id.ring_index << " " << turn.operations[0].seg_id.segment_index << std::endl;

I would have expected to see the ring_index be (-1), but I didn't expect for the segment_index to be (0) for both turns. Just to give some background, I have created the following simple triangle polygon
OriginalPolygon: (((100, 100), (200, 200), (200, 100)))

I then got the turns from a line that crossed it:
entering polygon at (168.749, 168.749)
================0 -1 -1 0
leaving polygon at (200, 151.114)
================0 -1 -1 0

I would have expected the segments to be numbered clockwise also and that the "entering polygon" turn would be in segment 0 and the "leaving polygon turn" would be in segment 1.

Any suggestions?


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