Shape as primitive with representations.

[CresitelloDittmar, Mark]

This puts a twist in my brain that I'm having trouble straightening out.

I interpret Markus' statement: "shapes are serialised in whatever way the
container formats wants them; in VOTable, use PARAM-s to store literals".  to
mean that VOTable would define the serialization for each shape.. for
example:
   "A GROUP element containing FIELDref/PARAM/PARAMref elements for each
attribute ( center_x, center_y, semi_major, semi_minor, angle )"
   This would *require* UCDs to identify each of the attributes since
FIELDref does not contain names, and the column names probably won't match
the attributes. (eg "RA" vs 'center_x' )

Then there is the DALI serialization spec.. defines value list and order
along with constraints that let the values be a simple list of numbers.
This is more restrictive than the GROUP representation.  I'm not too
familiar with that form, but it seems like it would be something like
       <FIELD name="<colname>" xtype="ellipse" datatype="double"
arraysize=5 />
 where the column value resolves to fill the 5 attributes.

*The goal here is for a Model to allow different representations of the
Ellipse* (or whatever datatype) at different levels of normalization.  DALI
may restrict to the DALI representation, but something more generic (like
FOV/REGION) might want to allow a simple DALI string, STC-S string, MOC, or
more full VOTable GROUP.  The provider could supply whichever satisfies the
needs of their data.
THAT would require being able to identify which representation is being
supplied, so the user knows how to interpet it.

With the above paragraph, I'm thinking also of the Coordinates model to
some degree.  This mechanism would allow a simple representation for the
case of "Sky Coordinate in standard Reference position and reference
Frame", while allowing for a fuller representation for others (e.g. "ccd
coordinates").

Mark


On Wed, Mar 19, 2025 at 11:42 AM Patrick Dowler via dm <dm at ivoa.net> wrote:

> On Wed, 19 Mar 2025 at 06:01, Markus Demleitner via dm <dm at ivoa.net>
> wrote:
> > So, I'd say DM should say not much more than "shapes are serialised
> > in whatever way the container formats wants them; in VOTable, use
> > PARAM-s to store literals".
>
> I agree with this. Serialization may differ depending on the format
> and DALI already
> says how to serialize in VOTable (sometimes referring to other
> standards like ISO8601
> or MOC when appropriate). That advice may or may not be applicable in
> other formats.
>
> --
> Patrick Dowler
> Canadian Astronomy Data Centre
> Victoria, BC, Canada
>
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