Identifying Coordinates of boundary using regionprops and finding curvature in MATLAB -

i newbie in image processing using matlab. have been trying follow thread - http://www.mathworks.com/matlabcentral/newsreader/view_thread/264686. have been successful in doing edge detection , generating pixellist arrays. 3 arrays after run code. code used:

clear all; = 'c:\users\ea\desktop\1.tif';  b = imread(a,'tif');                   b = double(b)/256; bw = edge(b,'sobel'); imshow(bw); l = bwlabel(bw);  stats = regionprops(l,'pixellist'); 

i not able identify coordinates of particular point on image( of bubble) using data have in arrays. how should proceed in order curvature of boundary? couldn't follow thread posted before clearly. helpful if explains better since newbie.

thanks in advance!

the answer depend on details of image using, perhaps using more of analysis available through regionprops. example if have elliptical objects, use majoraxislength , minoraxislength properties determine curvatures.