Where ever you click on the following link image, matlab code must find out the two edges and find out the distance between the those two points using [x,y]=ginput(1). http://imageshack.us/photo/my-images/600/imfill.jpg
I gave you code for calling ginput() in http://www.mathworks.com/matlabcentral/answers/57292#answer_69661 that calculates the distance between two points. Recall where it said:
% Calculate diameters: outerDiameter = sqrt((xOuter-xCenter)^2+(yOuter-yCenter)^2) innerDiameter = sqrt((xInner-xCenter)^2+(yInner-yCenter)^2)
Well that's just the Pythagorean theorem. You can replace the two x and y with any any two x and y to get the distance between two points.
threshold. Logically negate so the circle becomes the 1's. bwboundaries that. The result for that image would be a 2 x 1 cell array, each entry of which is an array of row and column coordinates that together form one of the boundaries. Find the minimum of the euclidean distances between all points in the first cell and all points in the second cell, and you will have found the minimum distances between the inner and outer boundaries.
You can toss in a ginput(1) if you really want; you would just ignore the value it returns.