Difference in polygon's between SIA2 and ADQL

[Patrick Dowler]

(this time responding to list)

Either are valid ways to define polygons and I asked about this awhile back
(probably ~Madrid interop). The STC definition requires the vertex sequence
to be counter-clockwise while the other doesn't care but then you need to
compute the winding direction.

So, this was intentional (several geometry libraries I have checked or used
do it this way). Whichever was chosen, someone has to do some work: either
computing the winding direction or converting polygons computed by some
library to the standard winding direction...

I assume "complicates our implementation" means "have to compute the
winding direction and maybe reverse vertex order"?

On Wed, Feb 10, 2016 at 1:01 PM, Patrick Dowler <pdowler.cadc at gmail.com>
wrote:

> Either are valid ways to define polygons and I asked about this awhile
> back (probably ~Madrid interop). The STC definition requires the vertex
> sequence to be counter-clockwise while the other doesn't care but then you
> need to compute the winding direction.
>
> So, this was intentional (several geometry libraries I have checked or
> used do it this way). Whichever was chosen, someone has to do some work:
> either computing the winding direction or converting polygons computed by
> some library to the standard winding direction...
>
> I assume "complicates our implementation" means "have to compute the
> winding direction and maybe reverse vertex order"?
>
>
> On Wed, Feb 10, 2016 at 12:09 AM, Alex Szalay <szalay at jhu.edu> wrote:
>
>> STC is mathematically correct and tested carefully. If SIA2 disagrees, it
>> is most likely incorrect.
>>
>> --Alex
>>
>> -----Original Message-----
>> From: dal-bounces at ivoa.net [mailto:dal-bounces at ivoa.net] On Behalf Of
>> Walter Landry
>> Sent: Wednesday, February 10, 2016 3:01 AM
>> To: dal at ivoa.net
>> Subject: Difference in polygon's between SIA2 and ADQL
>>
>> Hello Everyone,
>>
>> Reading the description of polygons in ADQL, it links to the description
>> in STC.
>>
>>   http://www.ivoa.net/Documents/cover/STC-20071030.html
>>
>> That defines a polygon as the points to the left of the edges.  In
>> contrast, in SIA2, a polygon is defined as the region with smaller area.
>> Was that inconsistency intentional?  It complicates our implementation.
>>
>> Thanks,
>> Walter Landry
>>
>
>
>
> --
> Patrick Dowler
> Canadian Astronomy Data Centre
> Victoria, BC, Canada
>



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