[fitsbits] Re: FITS Bintable proposals

[Arnold Rots]

I concur with Bill's changes, but want to provide one clarification.

Actually, I was not thinking about unsigned 32-bit integers in the P
columns (I suppose Bill's assuming this says something about issues I
am associated with :-), but whether these P columns shouldn't really
contain 64 bit integers (or, if you like, pairs of 32 bit integers).
I realize that that would immediately cause trouble with certain
existing files, so it might be more prudent to define 'Q' as the
64-bit equivalent of 'P'.
Aside from that, I would, of course, be all in favor of using unsigned
integers in the 'P' format, especially since the negative values are
outlawed, as Bill points out.

  - Arnold


William Pence wrote:
> The following comments about the FITS binary table proposals from Arnold 
> Rots and Preben Grosbol may be of general interest, so I'm reposting this 
> here to the wider FITSBITS audience.  I have updated the draft proposals, 
> available from http://fits.gsfc.nasa.gov/bintable_proposals.html with the 
> changes that are discussed here.  -Bill Pence
> ---------------------------------------------------------------
> 
> Arnold Rots wrote:
> ...
> > Re-reading all of this made me realize that, in retrospect, I am
> > uncomfortable with the 32 bit signed restriction.  Here we start
> > worrying about 64 bit integers but we restrict the size of the heap to
> > 2 GB through the second half of the P fields.
> > But I should not reopen the debate :-)
> 
> Some reasons for restricting the array length and offset to signed
> integers are:
>   - there is no precedent in FITS for using unsigned 32-bit integers
>   - use of unsigned integers is problematic in some languages like Fortran
>   - as far as I'm aware, the current software implementations of the heap
>     (e.g. CFITSIO) interpret these values as signed integers
> 
> I share your discomfort about this, however, and think that perhaps in the 
> future we could reverse this decision and redefine these to be unsigned 
> integers.  (This is possible because doing so would not invalidate any 
> existing FITS files and thus would not violate the "once FITS, always FITS" 
> rule).   We need more time to evaluate all the implications before making 
> this decision, so for now I think it is best to restrict these fields to be 
> signed integers, but leave the door open for a change in the future. (see 
> also Preben's related comment, below)
> 
> ...
> -- 
> ____________________________________________________________________
> Dr. William Pence                          William.D.Pence at nasa.gov
> NASA/GSFC Code 662         HEASARC         +1-301-286-4599 (voice)
> Greenbelt MD 20771                         +1-301-286-1684 (fax)
> 
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Arnold H. Rots                                Chandra X-ray Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 67                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head.cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
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