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In-reply-to: Your message of "Mon, 14 Aug 2000 21:25:00 MDT."
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> Compression is generally a loss-less operation on the data, meaning
> NO data is lost. Packing on the other hand generally involves
> removing unnecessary portions of the data, ie low order bits.
COMPRESSION is a general term for encoding a symbol stream so that, on
average, fewer bits are required to represented the encoded symbols than
the unencoded symbols. Compression may be either lossless or lossy.
Since the most popular compression schemes in general use today are
lossy (JPEG images, MPEG multimedia, etc.), it behooves the netCDF
community to be specific whenever lossless compression is being
discussed.
PACKING is a particular kind of compression that reduces the number of
bits per symbol by relaxing the requirment that symbols be aligned with
storage unit boundaries. An example would be packing 3 10-bit AVHRR
samples into 1 32-bit word, as opposed to using 3 16-bit words. Packing
may be either lossless or lossy, although when used without
qualification it usually means lossless packing (I can't imagine a
situation where lossy packing would be the most appropriate compression
scheme.)
Regards,
/Frew
#======================================================================
# James Frew frew@xxxxxxxxxxxxx http://www.bren.ucsb.edu/~frew/
# School of Environmental Science and Management +1.805.893.7356 vox
# University of California, Santa Barbara, CA 93106-5131 .7612 fax
