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From: rick@hanauma.stanford.edu (Richard Ottolini)
Newsgroups: comp.dsp,comp.graphics
Subject: Re: Mean Value Filter
Keywords: image processing, filter
Message-ID: <1991Mar20.224801.7413@leland.Stanford.EDU>
Date: 20 Mar 91 22:48:01 GMT
References: <1991Mar20.003425.14767@sunee.waterloo.edu>
Sender: news@leland.Stanford.EDU (Mr News)
Organization: Stanford University, Department of Geophysics
Lines: 12

In article <1991Mar20.003425.14767@sunee.waterloo.edu> rchann@sunee.waterloo.edu (Robert Chann) writes:
>I'm trying to implement a mean value filter.  That is, using an nxn
>window, each picture element is substituted by the mean value of the 
>(nxn) neighbouring picture elements.
>Apparently, by using a "sophisticated" implementation, no more than
>approximately two additions per pel are required for the filtering.
>The number of addition is nearly independent of the filter window
>size and no multiplication is necessary.

Simple.
Use a rolling window.  Add elements of the leading edges and subtract elements
of the trailing edges.