more on FITS and VO

[Doug Tody]

Hi Bill -

The original posting I saw on this talked about NULL images (no pixels),
whereas you talk about degenerate axes below.  I don't have any problem
with NULL images as this has been common practice in FITS usage for
years to handle limiting cases or pass around image metadata using FITS.
Degenerate primary axes on the other hand can greatly complicate the task
of an image viewer or any other analysis program.  If we are to allow
degenerate axes for the image/fits MIME type, then we could compromise and
merely require the first two axes of the (non-null) image be nondegenerate
so that the "primary" image matrix can be accessed by simple software.
Anything else can already be handled by application/fits, which catches
any legal FITS image.  In particular, multi-extension FITS files where
the PHU is not an image are best identified as application/fits.

In short - it is not a simple image unless the first two image axes are
nondegenerate.  An actual NULL image is a special case and is not the same
as a non-NULL image with primary degenerate axes.  We could live without
NULL images (they will cause problems for some programs), but they can
be occasionally useful to handle limiting cases using only FITS syntax.

	- Doug



On Fri, 28 Nov 2003, William Pence wrote:
> The intent of this subtle change is wording is to allow the "image/fits"
> mimetype to be legally applied to FITS files in which the Primary
> array has NAXIS greater than zero, and has one or more NAXISn keywords
> equal to zero, and thus contains 0 pixels,  e.g.,
>   NAXIS  =   2
>   NAXIS1 = 100
>   NAXIS2 =   0
> 
> Take for example a web site or service that returns 2-dimensional FITS
> images to the browser, with mimetype 'image/fits'.   Some people have
> argued that there could be times when the web site would need to return
> a zero pixel length image, e.g., the user might only want the FITS
> header information (metadata) without the image itself, or a zero pixel
> image might be returned if the requested image position was not available
> for some reason (outside of the area covered by a survey).  For whatever
> reason, some people have argued, there *might* be occasions where the 
> web service would need to return a zero pixel image, and in that case
> it would be more convenient, and less confusing to the end user, to assign
> 'image/fits' to the file, just like all the other images that it returns.
> 
> These examples are perhaps a bit contrived, but the advocates for this
> change in wording argue that it would be better to not forbid this
> use of 'image/fits' because more compelling uses for it may arise
> in the future.
> 
> Note that it was not intended that this be applied to the Random Groups FITS
> files (which by convention have NAXIS1 = 0) because Random Groups
> are rarely used to store a single 'image'.  It would be more
> appropriate to classify most Random Groups files as 'application/fits'.
> 
> -Bill Pence