Polygon CCW winding check request

[Patrick Dowler]

Well, I did say "less than half the sphere"... Anyway, we didn't chose that
definition and went with the CCW instead. Now we all have to implement it
correctly. If you mean "what do I do?" I don't have any polygons that are
exactly half the sphere to worry about.

Pat

On 13 June 2018 at 11:34, Arnold Rots <arots at cfa.harvard.edu> wrote:

> So, what do you do when you have exactly half a sphere?
>
> ------------------------------------------------------------
> -------------------------------------------------
> Arnold H. Rots                                          Chandra X-ray
> Science Center
> Smithsonian Astrophysical Observatory                   tel:  +1 617 496
> 7701
> 60 Garden Street, MS 67                                      fax:  +1 617
> 495 7356
> Cambridge, MA 02138
> arots at cfa.harvard.edu
> USA
> http://hea-www.harvard.edu/~arots/
> ------------------------------------------------------------
> --------------------------------------------------
>
>
> On Wed, Jun 13, 2018 at 2:19 PM, Patrick Dowler <pdowler.cadc at gmail.com>
> wrote:
>
>> 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
>>
>
>


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