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<p>Patrick,</p>
<p>I did not attend the IVOA interops at Capetown and Victoria.</p>
<p>I was not aware of the discussions and decision.</p>
<p><br>
</p>
<p>François-Xavier<br>
</p>
<br>
<div class="moz-cite-prefix">Le 13/06/2018 à 20:19, Patrick Dowler a
écrit :<br>
</div>
<blockquote type="cite"
cite="mid:CAFK8nroPe_MMXv3uoN3hPn4J6gqdoNp=hnuFNtkDikXE8Y8JOw@mail.gmail.com">
<div dir="ltr">
<div class="gmail_default" style="font-size:small">The intent of
DALI was that the butterfly polygon is not valid. We didn't
say it explicitly and maybe should have. <br>
</div>
<div class="gmail_default" style="font-size:small"><br>
</div>
<div class="gmail_default" style="font-size:small">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. <br>
</div>
<div class="gmail_default" style="font-size:small"><br>
</div>
<div class="gmail_default" style="font-size:small">Pat<br>
</div>
<div class="gmail_default" style="font-size:small"><br>
</div>
<div class="gmail_default" style="font-size:small">PS-In my
experience, that construct was always a bug in the polygon
generation s/w</div>
<div class="gmail_default" style="font-size:small"><br>
</div>
<div class="gmail_default" style="font-size:small">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<br>
</div>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On 13 June 2018 at 01:05,
Francois-Xavier PINEAU <span dir="ltr"><<a
href="mailto:francois-xavier.pineau@astro.unistra.fr"
target="_blank" moz-do-not-send="true">francois-xavier.pineau@astro.unistra.fr</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div text="#000000" bgcolor="#FFFFFF">
<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>
<div>
<div class="h5">
<p><br>
</p>
<br>
<div class="m_7475597954916928517moz-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" moz-do-not-send="true">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" moz-do-not-send="true">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>
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</div>
</div>
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<br>
<br clear="all">
<br>
-- <br>
<div class="gmail_signature" data-smartmail="gmail_signature">
<div dir="ltr">
<div>
<div>Patrick Dowler<br>
</div>
Canadian Astronomy Data Centre<br>
</div>
Victoria, BC, Canada<br>
</div>
</div>
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