Newsgroups: comp.compression
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From: ahenden@magnus.ircc.ohio-state.edu (Arne A Henden)
Subject: Astronomical Data Compression
Message-ID: <1991Apr3.150357.20825@magnus.acs.ohio-state.edu>
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Date: Wed, 3 Apr 1991 15:03:57 GMT
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Don Wells indicates he achieved 40 percent compression with
even/odd byte splitting.  For clarification, I assume that
Don was referring to high order / low order byte splitting
for the HST file in question.

You can achieve that kind of compression as long as the
noise level is contained in the low-order byte (i.e., less
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.

I like Don's scheme of splitting a floating-point image
into exponent and mantissa for separate encoding.  Has
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
would give high efficiency.  Also, bit-plane storage of 18-bit
images would certainly be more storage-efficient than 24-bits
per pixel, but at higher CPU cost.

Arne Henden