imageMLG

 Calculates maximum likelihood parameters of gaussian that gave rise to image G.

 Suppose G contains an image of a gaussian distribution.  One way to recover the
 parameters of the gaussian is to threshold the image, and then estimate the
 mean/covariance based on the coordinates of the thresholded points.  A better method is
 to do no thresholding and instead use all the coordinates, weighted by their value.
 This function does the latter, except in a very efficient manner since all computations
 are done in parallel over the entire image. 

 This function works over 2D or 3D images.  It makes most sense when G in fact contains
 an image of a single gaussian, but a result will be returned regardless.  All operations
 are performed on abs(G) in case it contains negative or complex values.

 symmFlag is an optional flag that if set to 1 then imageMLG recovers the maximum
 likelihood symmetric gaussian.  That is the variance in each direction is equal, and all
 covariance terms are 0.  If symmFlag is set to 2 and G is 3D, imageMLG recovers the ML
 guassian with equal variance in the 1st 2 dimensions (row and col) and all covariance
 terms equal to 0, but a possibly different variance in the 3rd (z or t) dimension.

 INPUTS
   G           - image of a gaussian (weighted pixels)
   symmFlag    - [optional] see above
   show        - [optional] figure to use for display (no display if == 0)

 OUTPUTS
   mu      - 2 or 3 element vector specifying the mean [row,col,z]
   C       - 2x2 or 3x3 covariance matrix [row,col,z]
   GR      - image of the recovered gaussian (faster if omitted)
   logl    - log likelihood of G given the recovered gaussian (faster if omitted)

 EXAMPLE
   % example 1 [2D]
   R = rotation_matrix2D( pi/6 );  C=R'*[10^2 0; 0 20^2]*R;
   G = filter_gauss_nD( [200, 300], [150,100], C, 0 );
   [mu,C,GR,logl] = imageMLG( G, 0, 1 );
   mask = mask_ellipse( size(G,1), size(G,2), mu, C ); 
   figure(3); im(mask)
   % example 2 [3D]
   R = rotation_matrix3D( [1,1,0], pi/4 ); 
   C = R'*[5^2 0 0; 0 2^2 0; 0 0 4^2]*R;
   G = filter_gauss_nD( [50,50,50], [25,25,25], C, 0 );
   [mu,C,GR,logl] = imageMLG( G, 0, 1 );

 DATESTAMP
   29-Sep-2005  2:00pm

 See also GAUSS2ELLIPSE, PLOT_GAUSSELLIPSES, MASK_ELLIPSE