
Geometry : 
Subject: Re: [geometry] Fwd: Some updates [union of multi_polygon]
From: Carl Wellington (carlw_at_[hidden])
Date: 20140805 01:09:50
I'm the originator of this question that Andrew posted to this list. First, I
apologize for the rant about boost documentation. That was written in a
moment of frustration to my coworker Andrew and wasnâ€™t really intended to
be included in a request for help to a mailing list. Second, thank you for
all the helpful responses (especially given the tone of my documentation
rant). The responses provided were very helpful and are exactly the kind of
thing that I would love to see more of in boost documentation. Having a
concrete example is extremely helpful and then I can go back to the concept
definition and understand how to generalize things, but without an example
it can be hard to get started. To be specific, I would love to have seen the
responses in this thread on the model::multi_polygon page of the
documentation  that would have saved me hours.
Anyway, back to my problem at hand. I believe I understand what is causing
my problem but I do not understand why it is a problem since it seems to me
like it should be a valid use case. If I take a union of two polygons and
place the result in a multi_polygon, that succeeds. However, if I take a
multi_polygon and then repeatedly union the polygons into that multi_polygon
it fails.
Below is an adaptation of the code example for union
(http://www.boost.org/doc/libs/1%5f55%5f0/libs/geometry/doc/html/geometry/reference/models/model_multi_polygon.html)
that demonstrates my problem:

// Display some info about a multi_polygon
void check(const multi_polygon& mp)
{
std::cout << " Size: " << mp.size() << std::endl;
for(int i = 0; i < mp.size(); ++i)
{
std::cout << " Num points: " << boost::geometry::num_points(mp[i])
<< std::endl;
}
std::cout << " Intersects: " << boost::geometry::intersects(mp) <<
std::endl;
}
int main()
{
polygon green, blue;
// Using polygons from example  details are unimportant, any overlapping
polygons would do
boost::geometry::read_wkt(
"POLYGON((2 1.3,2.4 1.7,2.8 1.8,3.4 1.2,3.7 1.6,3.4 2,4.1 3,5.3
2.6,5.4 1.2,4.9 0.8,2.9 0.7,2 1.3)"
"(4.0 2.0, 4.2 1.4, 4.8 1.9, 4.4 2.2, 4.0 2.0))", green);
boost::geometry::read_wkt(
"POLYGON((4.0 0.5 , 3.5 1.0 , 2.0 1.5 , 3.5 2.0 , 4.0 3.5 , 4.5 2.0 ,
6.0 1.5 , 4.5 1.0 , 4.0 0.5))", blue);
// Try 1 [WORKS]: Take union of two polygons and put result into
multi_polygon
multi_polygon output1;
boost::geometry::union_(green, blue, output1);
std::cout << "output1 = union(green,blue)" << std::endl;
check(output1);
std::cout << std::endl;
// Try 2 [FAILS]: Build up multi_polygon by repeatedly unioning in each
polygon
multi_polygon output2;
boost::geometry::union_(output2, green, output2);
std::cout << "output2 = union(output2,green)" << std::endl;
check(output2);
boost::geometry::union_(output2, blue, output2);
std::cout << "output2 = union(output2,blue)" << std::endl;
check(output2);
return 1;
}

Result (Using Boost 1.55):
output1 = union(green,blue)
Size: 1
Num points: 27
Intersects: 0
output2 = union(output2,green)
Size: 1
Num points: 17
Intersects: 0
output2 = union(output2,blue)
Size: 2
Num points: 17
Num points: 27
Intersects: 1

It appears that when I repeatedly union the polygons in one at a time, then
it does not do what I expect. I would expect the above two cases to be
equivalent, but when I add polygons one at a time, then the resulting
multipolygon has multiple parts and keeps the first polygon in addition to
the union. Therefore it selfintersects and cannot be used for further
calls.
So, is this behavior expected? If so, is there an alternative formulation I
can use to build up a union of many polygons?
Thanks again for the help,
Carl
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