How to mirror geometries?

[Jihadist]

Does boost::geometry supports something for mirroring geometries out-of-the-box? I cannot find something releted

[tinko92]

The algorithm boost::geometry::transform with a scale-strategy with a negative value for one of the dimensions might be what you are looking for.

#include <iostream>
#include <boost/geometry.hpp>
#include <boost/geometry/geometries/point_xy.hpp>

int main()
{
    using namespace boost::geometry;
    using namespace boost::geometry::strategy::transform;
    using point_2d = model::d2::point_xy<double>;
    
    point_2d p(2,5), pm;
    scale_transformer<double, 2, 2> mirror(-1, 1);
    transform(p, pm, mirror);
    std::cout << wkt(pm) << "\n"; //POINT(-2 5)

    return 0;
}

More generally, with matrix_transformer you can have arbitrary affine transformations, this example shows how they can be used and chained: https://github.com/boostorg/geometry/blob/develop/example/06_b_transformation_example.cpp . Here you can see the definitions of various matrix transformers:

geometry/include/boost/geometry/strategies/transform/matrix_transformers.hpp

Line 168 in 6235b8a

class matrix_transformer<CalculationType, 2, 2> : public detail::matrix_transformer::matrix_transformer<CalculationType, 2, 2>

.