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<p>Dear Marco, DAL,</p>
<p>I may be wrong, but I think that the STC definition of the inside
of a polygon is not compatible with "complex" shapes.</p>
<p>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.<br>
</p>
<p>How to deal easily with such a case while remaining compatible
with the STC definition?</p>
<p><br>
</p>
<p>A polygon is just a sequence of vertices with great-circle arcs
connecting the consecutive vertices (plus the last vertex
connection the first one).</p>
<p>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)).</p>
<p>There is no ambiguity on the inside and we have fast
implementations for polygons having possibly very complex shapes
(ray-tracing method).</p>
<p>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?</p>
<p>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.</p>
<p>Do you agree / disagree?<br>
</p>
<p><br>
</p>
<p>Kind regards,</p>
<p><br>
</p>
<p>François-Xavier<br>
</p>
<p><br>
</p>
<br>
<div class="moz-cite-prefix">Le 12/06/2018 à 17:45, Marco Molinaro a
écrit :<br>
</div>
<blockquote type="cite"
cite="mid:CAFtRUxKmZTHjBB=4BaS5ggop-zDbN_Z3eBfsm4K+P1Evar0NQg@mail.gmail.com">
<div dir="ltr"><span
style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">Dear
DAL members,</span><br
style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">
<span
style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">at
the recent IVOA Interop in Victoria it was pointed out</span><br
style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">
<span
style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">by
Alberto</span><br
style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">
<br
style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">
<a
href="http://wiki.ivoa.net/internal/IVOA/InterOpMayy2018DAL/ivoa_201805_micol_polygons.pdf"
target="_blank" style="color:rgb(17,85,204);font-size:12.8px"
moz-do-not-send="true">http://wiki.ivoa.net/internal/<wbr>IVOA/InterOpMayy2018DAL/ivoa_<wbr>201805_micol_polygons.pdf</a><br
style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">
<br
style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">
<span
style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">that
not all the data providers follow the STC specification</span><br
style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">
<span
style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial;float:none;display:inline">about
the winding direction of polygons stored in their</span>
<div
style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">archives.<br>
<br>
STC states that "The inside of the region is defined as
<div>that part of coordinate space that is encircled by the </div>
<div>polygon in a counter-clockwise sense".</div>
<div>And this is to be considered when looking at the</div>
<div>celestial sphere from the inside.</div>
<div><br>
</div>
<div>The different behaviour of the polygons stored at </div>
<div>different sites creates an interoperability issue.</div>
<div><br>
</div>
<div>This email is a request on data providers to check</div>
<div>on their data and implementations (when dealing</div>
</div>
<div
style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">with
polygons) to solve the presented issue.</div>
<div
style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial"><br>
</div>
<div
style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">James
also put together a python code snippet </div>
<div
style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">implementing
a simple test, it is available here:</div>
<div
style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial"><br>
</div>
<div
style="text-decoration-style:initial;text-decoration-color:initial"><span
style="font-size:12.8px"><a
href="https://gist.github.com/jd-au/45d1cdc0c6e2a7bc848a4be8f46c8958"
moz-do-not-send="true">https://gist.github.com/jd-au/45d1cdc0c6e2a7bc848a4be8f46c8958</a></span><br>
</div>
<div
style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial"><br>
</div>
<div
style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">Thank
you in advance!</div>
<div
style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">Cheers,</div>
<div
style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">
Marco & James</div>
<div
style="font-size:12.8px;text-decoration-style:initial;text-decoration-color:initial">
(your DAL chair & vice)</div>
<br>
</div>
</blockquote>
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