World coordinates cutouts/versus pixel cutouts Re: Multi-dimensional Data Access minimal requirements

[François Bonnarel]

Hi Markus, all
Le 14/03/2014 12:56, Markus Demleitner a écrit :
> On Fri, Mar 14, 2014 at 12:13:28PM +0100, François Bonnarel wrote:
>> Hi Paul, Tom, all,
>>      Of course it would be nice to have this functionnality and it has
>> been discussed in the DAL group vor AccessData version .... 1.1.
>> While it may seem simpler, (and it is as far as syntax definition is
>> concerned maybe) it is actually not true. Because if it is for a
>> pixel cutout query to have any scientific value, some a priori
>> knowledge (even rough) of the Mapping between the pixels and the
>> world coordinates. This knowledge has to be used either by a client
>> or by the service itself to prepare usefull pixel cutouts queries.
> Knowing full well I'm getting on everyone's nerves, I'd still like to
> point out that in the structured parameters approach --
>
> http://www.ivoa.net/pipermail/dal/2013-December/006602.html, chapter
> 6.1 "common parameters"
>
> --, pixel-wise cutouts aren't in any way special and are cheap to
> implement for both clients and servers (although, as I said, I'd now
> use PIX(n) and PIX(n)_WIDTH, although for pixels, MIN and MAX are
> just as appropriate).
Sure it's possible to define easilly pixel cutouts syntax. An 
alternative to the parameters you arev propsing is one parameter 
(PIXCUTOUT or whatever) with cfitsio syntax which is very general for 
all n-d arrays of values. But my point here was about the availability 
of mapping information necessary to build usefull pixel limitations.

Cheers
François
> François is right, though, that for most interesting use cases,
> operating on pixel coordinates requires knowledge of the mapping.  At
> least for common FITS images, that's again easy for clients and
> servers with the proposed mechanism, too: clients would use KIND
> (trivial to implement) and get back the FITS header they can already
> interpret.
>
> Not quite as general as the full DM approach, but very cheap measured
> in implementation effort and, I would claim, effective in terms of
> "Wow!" potential in our clients' users.
>
> Cheers,
>
>           Markus