function [shape] = shape2d(M);
% SHAPE2(M,S,GIIDs) ----  This function calculates the the 'shape' of each group in 
% field 'M', and modifies the measure so that it will return 1 for a perfect circle,
% and higher values the more the group diverges from a circle. 
%
% See 'shape' for details on the shape index.

d = diameter2d(M);
[k A] = convhull(M(:,2),M(:,3));
shape = d^2/A;

% Since the shape function returns (4/pi) for a circle, the index is now multiplied
% by pi/4 to standardize it to a circle.
shape = shape * (pi/4);