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

From: Giorgino R (giorginor21_at_[hidden])
Date: 2020-06-15 08:07:48

Hi Tinko,

Many thanks for your instant response and for looking at both questions.

You are correct, I missed that bit "*By that we mean they intersect, but
one does not completely contain another*". Do you know which geometric
function is more appropriate for this so as to get true?
Will that be an intersection operation with a resultant multipoint with
size greater than one?

BW
G

On Mon, Jun 15, 2020 at 9:01 AM Tinko Bartels via Geometry <
geometry_at_[hidden]> wrote:

> 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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