Polygon CCW winding check request

[Francois-Xavier PINEAU]

Dear Marco, DAL,

I may be wrong, but I think that the STC definition of the inside of a 
polygon is not compatible with "complex" shapes.

Example: if we take the case of a simple 4 vertices's polygon having a 
butterfly shape (i.e. having two crossing great-circle arcs), then the 
inside of one "wing" is in the counter-clockwise sense while the inside 
of the other "wing" is in the clockwise sense.

How to deal easily with such a case while remaining compatible with the 
STC definition?


A polygon is just a sequence of vertices with great-circle arcs 
connecting the consecutive vertices (plus the last vertex connection the 
first one).

The great-circle arc connecting two vertices's is the smallest of the 
two complementary great-arcs (and hence is always <= pi (we have a 
degeneracy with vertices having an angular separation of pi)).

There is no ambiguity on the inside and we have fast implementations for 
polygons having possibly very complex shapes (ray-tracing method).

It is true that then the definition does not allow to describe the 
"exterior" (the complement on the sphere) of a polygon as being itself a 
polygon. Is it a problem in practice?

 From my (biased) point-of-view, it is more important to support complex 
polygons (with a simple and fast algorithm), and to possibly ask for an 
extra boolean parameter in use cases requiring the complement of a polygon.

Do you agree / disagree?


Kind regards,


François-Xavier



Le 12/06/2018 à 17:45, Marco Molinaro a écrit :
> Dear DAL members,
> at the recent IVOA Interop in Victoria it was pointed out
> by Alberto
>
> http://wiki.ivoa.net/internal/IVOA/InterOpMayy2018DAL/ivoa_201805_micol_polygons.pdf 
> <http://wiki.ivoa.net/internal/IVOA/InterOpMayy2018DAL/ivoa_201805_micol_polygons.pdf>
>
> that not all the data providers follow the STC specification
> about the winding direction of polygons stored in their
> archives.
>
> STC states that "The inside of the region is defined as
> that part of coordinate space that is encircled by the
> polygon in a counter-clockwise sense".
> And this is to be considered when looking at the
> celestial sphere from the inside.
>
> The different behaviour of the polygons stored at
> different sites creates an interoperability issue.
>
> This email is a request on data providers to check
> on their data and implementations (when dealing
> with polygons) to solve the presented issue.
>
> James also put together a python code snippet
> implementing a simple test, it is available here:
>
> https://gist.github.com/jd-au/45d1cdc0c6e2a7bc848a4be8f46c8958
>
> Thank you in advance!
> Cheers,
>      Marco & James
>     (your DAL chair & vice)
>

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