<div dir="ltr">Dear <span class="" id=":1eb.1" tabindex="-1" style="">François</span>-Xavier,<br><div class="gmail_extra"><br><div class="gmail_quote">2018-06-13 10:05 GMT+02:00 Francois-Xavier <span class="" id=":1eb.2" tabindex="-1" style="">PINEAU</span> <span dir="ltr"><<a href="mailto:francois-xavier.pineau@astro.unistra.fr" target="_blank"><span class="" id=":1eb.3" tabindex="-1" style="">francois</span>-<span class="" id=":1eb.4" tabindex="-1" style="">xavier</span>.<span class="" id=":1eb.5" tabindex="-1" style="">pineau</span>@<span class="" id=":1eb.6" tabindex="-1" style="">astro</span>.<span class="" id=":1eb.7" tabindex="-1" style="">unistra</span>.fr</a>></span>:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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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></div></blockquote><div>I think you're right, but that is what <span class="" id=":1eb.8" tabindex="-1" style="">STC</span> currently states and DALI references to.</div><div>So, apart from changing that, we're stuck.</div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div text="#000000" bgcolor="#FFFFFF">
<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></div></blockquote><div>So you have: or a butterfly with one wing enclosed by its complementary sphere minus the other wing, or you have to use 6 points, doubling the cross-point to define the wings. But both things fall into the discussing of what's a polygon while the intent of the mail was a request depending on current standards statements.</div><div><br></div><div>I'm sure this discussion has to take place, there were already hints of this in Victoria.</div><div>But the discussion I would say does not solve the <span class="" id=":1eb.9" tabindex="-1" style="">interoperability</span> issue raise by Alberto. </div><div><br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div text="#000000" bgcolor="#FFFFFF"><p>
</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></div></blockquote><div>I agree support for complex morphology has to be taken into account.</div><div>For example I'm not sure how a donut-like shape can be represented, or how half a donut with a detached bite would be describable.</div><div><br></div><div>What I wonder is: where do this will lead to. I must say that I usually considered my ~complex uses case of this kind to be solved by a tessellation approach. But if we need an analytical approach, then we have to figure it out.</div><div><br></div><div>I even wonder if this is only DAL matters.</div><div><br></div><div>Cheers,</div><div> Marco</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div text="#000000" bgcolor="#FFFFFF"><p>
</p>
<p><br>
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<p>Kind regards,</p>
<p><br>
</p>
<p>François-Xavier<br>
</p><div><div class="h5">
<p><br>
</p>
<br>
<div class="m_-2971534964882738908moz-cite-prefix">Le 12/06/2018 à 17:45, Marco Molinaro a
écrit :<br>
</div>
<blockquote type="cite">
<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" style="color:rgb(17,85,204);font-size:12.8px" target="_blank">http://wiki.ivoa.net/internal/<wbr>IVOA/InterOpMayy2018DAL/ivoa_2<wbr>01805_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" target="_blank">https://gist.github.com/jd-au/<wbr>45d1cdc0c6e2a7bc848a4be8f46c89<wbr>58</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>
<br>
</div></div></div>
</blockquote></div><br></div></div>