[fitsbits] Update to the FITS Tiled Image Compression Convention

[William Pence]

This is to announce that a new version (v2.3) of the document
describing the FITS Tiled Image Compression Convention has been placed
on the FITS Support office web site at

    http://fits.gsfc.nasa.gov/registry/tilecompression.html

The main changes in this new version of the document are:

* A new variant of the GZIP algorithm, called GZIP_2, is supported, in
which the bytes in the pixel array are shuffled into decreasing order of
significance before being compressed with gzip.  This is especially
effective with floating point arrays.

* The algorithm for dithering floating point values before quantizing
them into scaled integers has been expanded and improved.  There is a
new variant in which the pixels that are equal to 0.0 are not dithered,
and instead the value is exactly preserved.

* A new ZDITHER0 keyword is defined, to record the random seed value
that is used to generate the dithering pattern.

* If the compressed file is larger than about 2.1 GB, then the '1Q'
variable length array FITS column format (with 64-bit pointers) must be 
used
instead of the '1P' format.

-Bill Pence
-- 
____________________________________________________________________
Dr. William Pence                       William.Pence at nasa.gov
NASA/GSFC Code 662       HEASARC        +1-301-286-4599 (voice)
Greenbelt MD 20771                      +1-301-286-1684 (fax)