Subject: [ggl] Adding support for Curves (and other custom geometry types)
From: Mats Taraldsvik (mats.taraldsvik)
Date: 2011-07-20 15:51:42
Barend,
Fortunately, a simple implementation wasn't that difficult to do.
http://pastebin.com/bQ0Mg5Jd ## circularstring_def.h ## (the definition
of the circularstring, reusing linestring)
cheers,
Mats
### circularstring_def.h ###
#ifndef CIRCULARSTRING_DEF_H_INCLUDED
#define CIRCULARSTRING_DEF_H_INCLUDED
#include <boost/geometry/core/access.hpp>
#include <boost/geometry/core/mutable_range.hpp>
#include <boost/geometry/core/point_type.hpp>
#include <boost/geometry/geometries/concepts/point_concept.hpp>
#include <memory>
#include <vector>
#include <boost/concept/assert.hpp>
#include <boost/range.hpp>
#include <boost/geometry/core/tag.hpp>
#include <boost/geometry/core/tags.hpp>
#include <boost/geometry/domains/gis/io/wkt/write_wkt.hpp>
namespace boost
{
namespace geometry
{
/// Circularstring identifying tag
struct circularstring_tag : single_tag {};
// namespace concept
//
// {
// template <typename Geometry>
// class Circularstring
// {
// #ifndef DOXYGEN_NO_CONCEPT_MEMBERS
// typedef typename point_type<Geometry>::type point_type;
//
// BOOST_CONCEPT_ASSERT( (concept::Point<point_type>) );
// BOOST_CONCEPT_ASSERT(
(boost::RandomAccessRangeConcept<Geometry>) );
//
// public :
//
// BOOST_CONCEPT_USAGE(Circularstring)
// {
// Geometry* ls = 0;
// traits::clear<Geometry>::apply(*ls);
// traits::resize<Geometry>::apply(*ls, 0);
// point_type* point = 0;
// traits::push_back<Geometry>::apply(*ls, *point);
// }
// #endif
// };
// }
namespace model
{
template
<
typename Point,
template<typename,typename> class Container = std::vector,
template<typename> class Allocator = std::allocator
>
class circularstring : public Container<Point,
Allocator<Point> >
{
BOOST_CONCEPT_ASSERT( (concept::Point<Point>) );
typedef Container<Point, Allocator<Point> > base_type;
public :
/// \constructor_default{circularstring}
inline circularstring()
: base_type()
{}
/// \constructor_begin_end{circularstring}
template <typename Iterator>
inline circularstring(Iterator begin, Iterator end)
: base_type(begin, end)
{}
};
} // namespace model
#ifndef DOXYGEN_NO_TRAITS_SPECIALIZATIONS
namespace traits
{
template
<
typename Point,
template<typename,typename> class Container,
template<typename> class Allocator
>
struct tag<model::circularstring<Point, Container, Allocator> >
{
typedef circularstring_tag type;
};
} // namespace traits
#endif
#ifndef DOXYGEN_NO_DETAIL
namespace detail
{
namespace wkt
{
struct prefix_circularstring_par
{
static inline const char* apply() { return
"CIRCULARSTRING("; }
};
}
} // namespace detail
#endif
#ifndef DOXYGEN_NO_DISPATCH
namespace dispatch
{
template <typename Linestring>
struct wkt<circularstring_tag, Linestring>
: detail::wkt::wkt_range
<
Linestring,
detail::wkt::prefix_circularstring_par,
detail::wkt::closing_parenthesis
>
{};
} // namespace dispatch
#endif
} //namespace geometry
} // namespace boost
#endif // CIRCULARSTRING_DEF_H_INCLUDED
## END ##
> Hi Mats,
>
> >/ I'm very interested in using Boost.Geometry in my project, as it looks
> />/ like a better fit than GEOS.
> />/
> />/ In GEOS I need to extract the geometry from my internal geometry
> />/ object/class, and create new geos::geom::Geometry objects, while
> />/ Boost.Geometry -- if I have read the docs correctly -- allows me to
> />/ define my internal geometry object as Boost.Geometry compatible (with
> />/ BOOST_GEOMETRY_REGISTER_*), and get all of the Boost.Geometry
> />/ functionality "for free" directly against my internal geometry
> />/ objects/classes, thus eliminating the additional object I have to
> />/ create with GEOS. Is this correct?
> />/
> /
> Yes, this is correct.
>
>
> >/ In addition to the existing Geometry models in Boost.Geometry, I need
> />/ support for Curves (CIRCULARSTRINGs in PostGIS [1]), which are
> />/ basically a start, end and middle point (PostGIS takes care of
> />/ calculating the actual arc).
> />/
> />/ At the moment, I only need support for writing the Curve to EWKT (a
> />/ PostGIS extension to WKT. Currently, EWKT is a superset of WKT.).
> />/ Could you give me pointers on how to achieve this (i.e. defining a
> />/ Curve Geometry model, and extending the io.wkt module to add some EWKT
> />/ features)?
> />/
> /
> That is not trivial. But it should be doable. We do not yet support
> curves indeed. We do have circles and spheres but they are moved to an
> extension, because we first want to concentrate on linear features. And
> in that extension, WKT is not supported.
>
> What you would need to do to support it (more or less the official way):
> 1) define a tag (e.g. "circularstring_tag"), comparable to linestring_tag
> 2) create a concept (so for circular string it would probably be similar
> to a linestring). A concept is a set of traits classes, plus a concept
> checking class
> 3) create a model, or adapt your existing geometries to this concept
> 4) in write_wkt.hpp, add a dispatch for the tag, and implement the WKT
> writing.
>
> You might postpone a step (e.g. the concept checking class). Anyway,
> because it is more or less comparable to a linestring implementation,
> you can look how these steps are done there.
>
> I'm interested in the results!
>
> Regards, Barend
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