[fitsbits] FITS Bintable proposals

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
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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 a
- 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
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
USA http://hea-www.harvard.edu/~arots/
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