Hi Adam,

I think I've found a little bug in my test program. My polygon type using double types. With double polygon all results are fine. The problems I've had was with polygons of integer types.

typedef boost::geometry::model::polygon<boost::geometry::model::d2::point_xy<int> > polygon;

I've tested again on linux with gcc 4.8.1, gcc 4.9.2 and clang 3.5.0 with and without optimization (-O2) in c++11 and c++14 mode. With polygons of integer type the outputs looking strange like in my original mail.

BTW: Thanks for your information about bg::wkt(..) and bg::svg_mapper(..).

Thanks

Franz

2015-03-17 15:38 GMT+01:00 Adam Wulkiewicz <adam.wulkiewicz@gmail.com>:

Hi,

Franz Alt wrote:

Hi,

I've written a simple test program using the difference algorithm of geometry. I want to build the difference of two simple rectangle polygons (see attachment input.png).

Regardless of whether I subtract polygon A from B or I subtract B from A I got very strange output polygons (see attachments output1.png and output2.png).

I've tested with Boost 1.56, 1.57 and the beta of 1.58. All versions lead to the same results.

Does someone know what is wrong here?

Which compiler exactly are you using and on which platform?
What flags were passed into the compiler?
In particular, was optimization enabled?

I get the correct results (attached) on windows msvc12, mingw4.9.1 and linux gcc 4.8.2.
Using latest develop and 1.57 (though I tested the latter only on msvc12).

The attached svg files were generated with bg::svg_mapper (http://www.boost.org/doc/libs/1_57_0/libs/geometry/doc/html/geometry/reference/io/svg/svg_mapper.html).
In the file green_output2.svg there is green and output2[0] from your code so (blue \ green). In the output1_blue.svg output1[0] (green \ blue) and blue.

What's the output of your program? For me it's:

polygon #0: (15802,253.976) (15943,254) (15943,-1485) (15802,-1485) (15802,253.976)
polygon #1: (-7901,250.024) (-7901,-1485) (-8042,-1485) (-8042,250) (-7901,250.024)
----
polygon #0: (-7901,250.024) (-7901,529) (15802,544) (15802,253.976) (-7901,250.024)

<tldr>

Btw, you can use bg::wkt(), there is no need to manually print coordinates (see: http://www.boost.org/doc/libs/1_57_0/libs/geometry/doc/html/geometry/reference/io/wkt/wkt.html).
E.g. this:

namespace bg = boost::geometry;
std::cout << "green: " << bg::wkt(green) << std::endl;
std::cout << "blue: " << bg::wkt(blue) << std::endl;
std::cout << "output1: " << bg::wkt(output1[0]) << std::endl;
std::cout << "output2: " << bg::wkt(output2[0]) << std::endl;

prints:

green: POLYGON((-8042 -1485,-8042 250,-8042 250,15943 254,15943 -1485,-8042 -1485))
blue: POLYGON((-7901 -1485,-7901 529,-7901 529,15802 544,15802 -1485,-7901 -1485))
output1: POLYGON((15802 253.976,15943 254,15943 -1485,15802 -1485,15802 253.976))
output2: POLYGON((-7901 250.024,-7901 529,15802 544,15802 253.976,-7901 250.024))

Instead of std::vector<polygon> you could use bg::multi_polygon<polygon>. Then it'd be handled by wkt() or svg_mapper automatically.

</tldr>

Regards,
Adam

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