Arrays of geometries?

Markus Demleitner msdemlei at
Mon Sep 18 11:01:58 CEST 2017


On Fri, Sep 15, 2017 at 09:22:03AM -0700, Patrick Dowler wrote:
> An array of 4 points includes those 4 points while a polygon with 4
> vertices incudes *all* the points inside or on the boundary. So the
> polygon xtype says more than just an array of points. It is a sequence
> of points that enclose a portion of the coordinate system (with great
> circles) in a specific winding direction that defines the "inside" and
> "outside". It is a set of all those points - not just the 4 listed.

While I agree with Pat on the conceptual level, I think it's
important to stress that technically, a DALI polygon is *not* an
array of points.  *If* we went ahead with the xtype-array plan, then
I'd say a 4-array of points would be

  arraysize="2x4" xtype="point"

(and a variable-size array of points

  arraysize="2x*" xtype="point").  
A polygon, to VOTable, is just an array of doubles not otherwise
structured.  A 4-polygon in current DALI is

  arrayize="8" xtype="polygon"

-- and I'd stronly argue it should remain that way.

I have to admit that to my C-honed mind the "2x4" spec for the point
array feels a bit weird, but I think that's implied by VOTable
("fastest changing index first").

Or am I missing something?

      -- Markus

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