Polygon CCW winding check request

[Patrick Dowler]

The intent of DALI was that the butterfly polygon is not valid. We didn't
say it explicitly and maybe should have.

More generally, the DALI xtype="polygon" is a simple polygon (convex or
concave) with no crossing segments. We had to decide between picking a
winding direction or for the polygon to be limited to less that half the
sphere. At the Capetown interop the decision was to pick CCW winding
direction and that definition went into DALI-1.1.

Pat

PS-In my experience, that construct was always a bug in the polygon
generation s/w

PPS-Anyone using postgresql+pgsphere should note that winding direction
doesn't matter for spoly because that library chose the "less than half the
sphere" definition

On 13 June 2018 at 01:05, Francois-Xavier PINEAU <
francois-xavier.pineau at astro.unistra.fr> wrote:

> 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_2
> 01805_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)
>
>
>


-- 
Patrick Dowler
Canadian Astronomy Data Centre
Victoria, BC, Canada
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