Geometry Boost List: Re: [geometry] linestring overlap and hash function for linestring

From: Tinko Bartels (tinkobartels_at_[hidden])
Date: 2020-06-15 08:00:28

Hi Giorgino,

this is not a complete answer (I may look at question ii) later), but I
think I may have the answer for question i):

Note, how the postgis-doc you linked specifies "By that we mean they
intersect, but one does not completely contain another." I think,
line_test_1 completely contains line_test_2 so "false" should be the
expected result of the overlaps-check. If you want "true" you may want to
look for another geometric relation.

Kind regards
Tinko

Am Mo., 15. Juni 2020 um 09:23 Uhr schrieb Giorgino R via Geometry <
geometry_at_[hidden]>:

> Hi there,
>
> I have two questions for boost::geometry, any help will be much
> appreciated:
>
> i) I am trying to use overlap function for two linestrings (
> https://postgis.net/docs/ST_Overlaps.html); however, it looks like I am
> doing something wrong (?) .
>
>   using point_2d = bg::model::point<double, 2, bg::cs::cartesian>;
>   using linestring_2d = bg::model::linestring<point_2d>;
>
>   // overlaps
>   linestring_2d line_test_1({ point_2d(0.0, 0.0), point_2d(2.0, 0.0) });
>   linestring_2d line_test_2({ point_2d(0.0, 0.0), point_2d(1.0, 0.0) });
>   bool res = bg::overlaps(line_test1, line_test2);
>
> This results to false. Should I use a specific strategy? I checked also
> tests and I saw that you do test overlap for linestrings.
>
> ii) I have a structure that contains a linestring. I want to use an
> unordered set for storing unique objects with respect to their geometry
> (linestring). Therefore, I will need to write hash functions as below:
>
>
> struct Object_hash
> {
>   std::size_t operator()(const Object& obj) const
>   {
>     Linestring<Point> linestring = obj.get_linestring();
>     std::size_t hash = 0;
>     for (auto iter=linestring.cbegin(); iter!=linestring.cend(); ++iter)
>     {
>       boost::hash_combine(hash, iter->x1());
>       boost::hash_combine(hash, iter->x2());
>     }
>     return hash;
>   }
> };
>
> struct Object_compare
> {
>   bool operator()(const Object& obj1, const Object & obj2) const
>   {
>     return boost::geometry::equals(obj1.get_linestring(),
> obj2.get_linestring());
>   }
> };
>
> As expected, the above is valid for linestrings with the same direction of
> their points. If one linestring is reversed this off course will not be
> valid. Is there any way that I can find a better hashing function? Is there
> any invariant field (for two spatial equaly linestrings) that I could use
> to create hash key?
>
> Many thanks in adavnce,
> BW
> G
>
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