function c = curltopo(f)
%
% curltopo: compute the curl of the 90 degree rotated gradient vectors
% On input:
%   f:  2D image function
% On output:
%   c:  curl of the 90 degree rotated gradient (equivalent to the negative
%       Laplacian
%

[fc,fr] = gradient(f);
[fcc,fcr] = gradient(fc);
[frc,frr] = gradient(fr);

c = -fcc-frr;
