Arrays of geometries?

[Dave Morris]

Hi DAL,

Are POLYGONs arrays of POINTs ?

The current email thread is considering this:

     <!-- Four points, each made up of two floats -->
     <FIELD datatype="float" arraysize="2x4" xtype="point"...>

to mean an array of four POINTs. Where the arraysize is split, with the 
first number, 2, applied to the 'float' part and the 4 applied to the 
'point' part.

What does that mean for POLYGONs ?

At the moment POLYGONs are handled as 1D arrays of floats:

     <!-- A single polygon made up from 8 floats -->
     <FIELD datatype="float" arraysize="8" xtype="polygon"...>

where the first part of the arraysize, 8, applies to the 'float' part 
and the 'polygon' part is unmodified.

Are these equivalent ?

     <!-- Four points, each made up of two floats -->
     <FIELD datatype="float" arraysize="2x4" xtype="point"...>

     <!-- A single polygon made up from 8 floats -->
     <FIELD datatype="float" arraysize="8" xtype="polygon"...>

What happens if we push the model to handle multiple shapes ?

     <!-- A single polygon made up from 8 floats -->
     <FIELD datatype="float" arraysize="8" xtype="polygon"...>

     <!-- Three polygons, each made up from eight floats -->
     <FIELD datatype="float" arraysize="8x3"   xtype="polygon"...>

     <!-- Four points, each made up of two floats -->
     <FIELD datatype="float" arraysize="2x4" xtype="point"...>

     <!-- Three shapes, each made up of four points, each made of two 
floats -->
     <FIELD datatype="float" arraysize="2x4x3" xtype="point"...>

I *think* this works.

If we serialize polygons as arrays of points, do we need the polygon 
xtype?

It might help avoid the ambiguity caused by an incomplete polygon:

     <!-- An incomplete polygon made up from 3 floats -->
     <FIELD datatype="float" arraysize="3" xtype="polygon"...>


Any thoughts ?

-- Dave

--------
Dave Morris
Research Software Engineer
Wide Field Astronomy Unit
Institute for Astronomy
University of Edinburgh
--------

On 2017-09-05 12:24, Markus Demleitner wrote:
> Dear DAL,
> 
> While implementing DALI 1.1 with a view to TAP 1.1, one of my test
> cases lead to the encoding of a 2-array of points to a
> 
> <FIELD datatype="float" arraysize="2x2" xtype="point"...>
> 
> This made me realise that, as far as I can see, it's unclear what
> that means by DALI.  And it leads to the more fundamental question of
> whether xtype amends the datatype -- which would support such usage
> -- or whether it amends (datatype, arraysize) -- which probably would
> outlaw things like these.
> 
> So:
> 
> Is anyone strongly in favour of such usage?  Note that another,
> perhaps more common, use case is arrays of timestamps, which *could*
> play a role in certain time series serialisations?
> 
> ...or is anyone strongly opposed to it?
> 
> Either way, I think it's one of these corner cases where we should
> somehow agree upon a policy.  Should xtype+array be allowed?
> Forbidden?  Undefined at this point?
> 
>           -- markus