WD-AccessData-1.0-20140312

[Douglas Tody]

On Thu, 13 Mar 2014, Petr Skoda wrote:

>> Several people have expressed interest in image sections as in IRAF,
>> CFITSIO, etc.  This is a pixel space expression such as "[101:105,*]"
>> (spatial cutout of a 3-cube), "[*,5,*]" (plane 5 of a cube), etc.  In
>> general, WCS-space and pixel-space operations are equally important and
>> the client might prefer either.  AccessData is WCS-space initially as we
>> already have to support this for the discovery query, whereas supporting
>> pixel-space
>
> I see the only application of pixel space for mosaics. Otherwise it might be 
> used for reducing size of images to conserve space - like thumbnails of 
> individual galaxies from SDSS ....
> But it will be always very rough experimental size to select (well if I have 
> CDELT1 I can compute pixel scale - but IMHO all reasonable science request 
> will be driven in WCS space - in physical units ....

Pixel space operations are very common with image data, much less common
with spectra.  For cube visualization for example, it may be much easier
and more precise for the client application to do the subset accessing
and graphics rendering in pixel space.  Another example is cutout
generation, which is always done in pixel space (usually after a
WCS-space computation).

It really just depends upon the client application which is more
natural.  The most universal is World-coordinate space, e.g., one does
not always even have pixels in the input data.

 	- Doug