Polygon CCW winding check request

[Francois-Xavier PINEAU]

Patrick,

I did not attend the IVOA interops at Capetown and Victoria.

I was not aware of the discussions and decision.


François-Xavier


Le 13/06/2018 à 20:19, Patrick Dowler a écrit :
> 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 
> <mailto: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_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
>>     <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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