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From: nitin@ur-laser.uucp (Nitin Sampat)
Newsgroups: net.graphics
Subject: Re: run length encoding
Message-ID: <309@ur-laser.uucp>
Date: Thu, 11-Jul-85 10:00:34 EDT
Article-I.D.: ur-laser.309
Posted: Thu Jul 11 10:00:34 1985
Date-Received: Wed, 17-Jul-85 03:34:55 EDT
References: <964@sdcsvax.UUCP>
Organization: Lab for Laser Energetics, Univ. of Rochester
Lines: 32

Run length coding (RLC) is a data compression technique, used
to reduce the data in large image file, like the ones from LANDSAT.
It is called "error-free" encoding in that the original can be replicated
exactly from the encoded form.

In one-dimensional RLC, the following is done :

Lets take a row of a digital image. As we go along the row, the
sequence of pixels  is mapped into a sequence of pairs ( g1, l1 ),
(g2, l2 )...etc where g is the gray level and l the run length. For example,
if we take a row of a digital image with the following gray levels

	2	2	2	3	3	4	4	4

If this is the first row, we may reduce the data by mapping it into a
sequence of integer pairs. In this case, it would be

	(2,3)	(3,2) 	(4,3)  

	This is then repeated for all the rows.  As is obvious, the 
	process is completely reversible and hence "error-free".

There is also something called two-dimensional RLC which takes vertical
direction also into consideration.

A good explanation of image coding may be found in :

"Digital Image Processing", Gonzales and Wintz, Addison Wesley Pub. Co.,
1977. Mass.

					nitin@ur-laser.uucp
					{seismo,allegra}!rochester!ur-laser!nitin