[WIP] GraphicsView: Add General_polygon_2 to QPainterPath converter. …

[chgans]

Summary of Changes

This adds a converter function to convert from General_polygon_2 to QPainterPath.

I couldn't get the template parameter right, eg, express the CGAL::General_polygon_2 concept that depends on the class template parameter K, without having to include CGAL/General_polygon_2.h directly.
I have tried to define it with:

template < template < typename K_> class  T> 
QPainterPath operator()(const typename T<K>::General_polygon_2 &pgn) const;

And using it this way:

typedef CGAL::Epeck                               Kernel;
typedef CGAL::Gps_circle_segment_traits_2<Kernel> Traits_2;
typedef Traits_2::General_polygon_2               Polygon_2;
typedef CGAL::Qt::Converter<Kernel>               QtConvert;

Polygon_2 pgn;
QtConverter convert;
QPainterPath path = convert(pgn);

I have tried other ways to define it, but in the best case, I always ended up with errors along
"template argument deduction/substitution failed". The only other way around was to call the converter this way convert.operator()<...>(); But I think this defeat the purpose of the functor approach of the converter...

I've marked this PR as 'WIP', as I am not sure how to solve this extra template parameter.
As well, do you want me to add an example code to the GraphicsView module? Eg, export to image (PNG), or maybe export to SVG?

Release Management

• Affected package(s): GraphicsView
• Issue(s) solved (if any): fix [RFC] GraphicsView: general polygon to QPainterPath converter #2021

[chgans]

Last thing, the converter itself cannot be used to convert the curve's own points, since they are not CGAL::Point_2<K>, hence all calls to CGAL::to_double()
Maybe this converter function shouldn't even be part of CGAL::Qt::Converter, maybe it should be moved to a specialisation for the circular GPS traits, or a generic 'Polygon converter' class templated on a GPS trait class...

[lrineau]

For the moment, this pull-request triggers compilation errors, because the declaration of CGAL::General_polygon_2 is not known.

I suggest you use a forward declaration

template <typename Traits> class General_polygon_2;

in the CGAL namespace. That should fix the compilation issue.

As for the need for an example: yes, we would need an example, so that the new feature is tested daily, and does not rot in the future.

As far as I can see, your code can only deal with a CGAL::General_polygon_2 templated with CGAL::Arr_circle_segment_traits_2< Kernel >, am I right?

[lrineau]

@chgans Have you seen my message (#2059 (comment)) about a month ago? Do you still have time to deal with this PR?

[chgans]

@lrineau I will update the PR, including the build fix and an example. I just need to find some time.
And yes, the code will only work with CGAL::Arr_circle_segment_traits_2< Kernel >, I tried to make it more general, but i've hit some issues with that (sorry can't remember the details).