Image Compression Software

[Jim Heasley <heasley@hoku.ifa.hawaii.edu>]

Andreas,

I'd suggest you take a look at the compression method that was developed
at Yale to deal with the data stream from a 16 CCD drift scan camera on
the 1-m Schmidt telescope in Venezuela.  The paper "Data Acquisition for
a 16 CCD Drift-Scan Survey" by Sabbey, Coppi, and Oemler (1998,
Astronomical Journal, 110, p 1067) outlines the method.  The paper can
be downloaded from the Astronomical Data Service
(http://adsabs.harvard.edu/abstract_service.html) and the software is
available from ftp://www.astro.yale.edu/pub/sabbey/xscroll.tar.gz. 

In this paper you will note there's a reference to a technical note by
Michael Richmond.  The author's claim the method gives on the order of
11% better lossless compression than hcompress.  I've tried this on the
test images they mention in the paper as well as some of my own data
from the Canada-France-Hawaii Telescope.  It does pretty well.  I have
not tried it on the TASS images Michael has made available.  I also
tried comparing with bzip2, which a colleague from computer sciences
tells me is quite good.  However, my recollection is the Sabbey program
and hcompress generally did better.  Unfortunately, the tabulated
results are on a workstation in Germany that is temporarily off line, so
I can only give the "talk-story" version as we say here in Hawaii. 

Regarding Arne's point about having something that will be available
into the future, that depends on having the source code.  The method I
mention above, as well as hcompress, have source available on the web. 
I have successfully compiled hcompress on a Windows machine using the
Cygwin software (you don't need the Cygwin suite to run it, just the
appropriate DLL).  It only required a very trivial change (having to do
with the usual text/binary file business and carriage returns and new
line characters).  I'm sure the same will be true for the code described
by Sabbey and company.  In a previous email I thought someone said that
hcompress might have been patented (and hence no longer freely
available).  Well, I just checked the software web page at STScI and
it's still there. 


Jim Heasley

Institute for Astronomy			heasley@hoku.ifa.hawaii.edu
University of Hawaii			phone: 808-956-6826
2680 Woodlawn Drive			fax:   808-956-9580
Honolulu, HI 96822