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From: ImageAnalyst <imageanalyst@mailinator.com>
Newsgroups: comp.soft-sys.matlab
Subject: Re: find maximum radius of the object
Date: Wed, 4 Nov 2009 03:37:21 -0800 (PST)
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Once you have your object binarized, use bwboundaries() to get the
boundary coordinates.

Then use bwconncomp (or bwlabel) and regionprops() to get the
centroid.

Then just get the distance between every boundary point to the
centroid for every object, keeping track of the max distance.

This demo may help:
http://www.mathworks.com/matlabcentral/fileexchange/25157