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From: warnock@stars.gsfc.nasa.gov (Archie Warnock)
Newsgroups: comp.compression
Subject: Re: Astronomical Data Compression
Message-ID: <4753@dftsrv.gsfc.nasa.gov>
Date: 3 Apr 91 18:51:40 GMT
References: <1991Apr3.150357.20825@magnus.acs.ohio-state.edu>
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In article <1991Apr3.150357.20825@magnus.acs.ohio-state.edu>,
ahenden@magnus.ircc.ohio-state.edu (Arne A Henden) writes... 
>than 255 ADU).  If your mean level is higher than 255, you
>will start getting a lower compression rate.  Also, it
>depends on whether the image is of a stellar field or
>an extended object, where more pixels are above sky.

Yes, but (depending on the angular size of the pixels), the compression 
rate goes down only slowly, because the high-order bytes are still 
relatively "smooth".  The low order bytes should always be noisier than 
the high order bytes.

>anyone attempted such a compression?  With the current
>round of CCDs, the dynamic range is really more like 18 bits
>and I anticipate having to use f.p. for storage of raw data
>as well as reduced images.  On the surface, I'd bet that the
>compression efficiency would be less than bit-plane since
>the mantissa 24bits would be random and only the 8bit exponent

Same argument - looking at the mantissa 8 bits at a time should give you 
reasonable compression except on the lowest order byte.  Note, all this 
assumes that if you can't get reasonable compression directly, try 
running the respective bytes through some sort of run-length or 
running-differences scheme first.  That's where you really take 
advantage of the smoothness.

----------------------------------------------------------------------------
-- Archie Warnock                     Internet:  warnock@stars.gsfc.nasa.gov
-- ST Systems Corp.                   SPAN:      STARS::WARNOCK
-- NASA/GSFC                          "Unix - JCL for the 90s"