[fitsbits] FITS 'P' descriptors: signed or unsigned?

[Preben Grosbol]

On Thursday 16 June 2005 00:21, William Pence wrote:
> At issue is whether to reverse the recent decision to define the 'P'
> variable-length array descriptors in FITS binary tables to be a pair of
> 'signed 32-bit integers', and make them 'unsigned 32-bit integers' instead.
I'm sorry but we cannot 'reverse the recent decision'.  That would violate
the section 9 of the FITS Standard 'Restrictions on Changes'

Technically there is no good argument for  'unsigned 32-bit integers'
as we are discussing 64-bit pointers.  To keep the standard more
symmetric I would argue for signed 64-bit integers.  When we will
need the last bit we should start the discussions on 128-bit integer.

In general, I agree on the need for 64-bit pointers and integer columns
(both signed) in binary tables based on the need for large heaps and
time stamp.

I still have reservation concerning BITPIX=64 for the following reasons:
  1) there seems no good physical reason for 64-bit integer images.  The
      number of photons from astronomical source hardly justifies it
      especially considering their statistical distribution.  Let someone
      present a real, practical case and we should considere it.
  2) The FITS standard is useful because the vast majority of systems
       implements it - that is if one writes a conforming FITS file the
       likelihood of reading it on any system is high.
       Adding BITPIX=64 would require changes at the top level of all readers.
       In order for this to actually be implemented people would have to feel
       the need otherwise it remains empty words.

Preben Grosbol