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From: wcs@cbnewsh.att.com (Bill Stewart 908-949-0705 erebus.att.com!wcs)
Newsgroups: comp.compression
Subject: Re: info
Message-ID: <1991Jun19.230651.20787@cbnewsh.att.com>
Date: 19 Jun 91 23:06:51 GMT
References: <31316@hydra.gatech.EDU>
Organization: News Busters
Lines: 37

In article <31316@hydra.gatech.EDU> jp49@prism.gatech.EDU (PYLES,JOHN H) writes:
]   A colleague of mine is trying to find ways of compressing digitized map
]data.  The format is a simple 4-bit deep array, and there is some degree
]of redundance (Run Length?) in both directions.  At present, there are no
]restrictions in compression time, but the decompression should be something
]reasonable.  No O(n^n^n!) algorithms, please.  He would (of course) prefer 
]a lossless algorithm, but I would like to know what could be done with
]slightly lossy ones as well.

Depending on what you're trying to do, a LOT of the popular data
compression algorithms discussed here may work well, like compress,
lharc, pack (simple Huffman coding), the various GIF and TIFF formats,
PKZIP, G3 or G4, etc.  Run Length Codes are real simple, but the
Lempel-Ziv coding used in compress will generally pick up run-length
effects effectively.  Most of the common codes decompress rapidly -
the hard part is usually finding the right parameters to compress with.
Also, if you've really got NO restriction on compression time, one
obvious approach is to either find or build a compressor that tries
several different formats and then gives you the smallest result.

The PBM/PPM Portable BitMap / Portable PixMap tools have a lot of
things you can work with.

Most of these are one-dimensional coding, except group 3/4.
You might also experiment with taking row differences,
(after expanding to byte-per-pixel), and seeing how well the
different compression techniques work then.

Also, what do your 4 bits per pixel of data represent?
Gray scale, or color?  If the separate bitplanes mean different
things, there bight be some benefit in tryiong to compress each
plane separately.
-- 
				Pray for peace;		  Bill
# Bill Stewart 908-949-0705 erebus.att.com!wcs AT&T Bell Labs 4M-312 Holmdel NJ
# No, that's covered by the Drug Exception to the Fourth Amendment.
# You can read it here in the fine print.