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function G=doog2(sig,r,th,N);
% G=doog2(sig,r,th,N);
% Make difference of offset gaussians kernel
% theta is in degrees
% (see Malik & Perona, J. Opt. Soc. Amer., 1990)
%
% Example:
% >> imagesc(doog2(1,12,0,64,1))
% >> colormap(gray)
% by Serge Belongie
no_pts=N; % no. of points in x,y grid
[x,y]=meshgrid(-(N/2)+1/2:(N/2)-1/2,-(N/2)+1/2:(N/2)-1/2);
phi=pi*th/180;
sigy=sig;
sigx=r*sig;
R=[cos(phi) -sin(phi); sin(phi) cos(phi)];
C=R*diag([sigx,sigy])*R';
X=[x(:) y(:)];
Gb=gaussian(X,[0 0]',C);
Gb=reshape(Gb,N,N);
m=R*[0 sig]';
Ga=gaussian(X,m,C);
Ga=reshape(Ga,N,N);
Gc=rot90(Ga,2);
a=-1;
b=2;
c=-1;
G = a*Ga + b*Gb + c*Gc;
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