hi,
can anyone guide me out to implement the following?

"Nine second order derivatives of Gaussians at scales of(1, 1.414, 2) are used. Each filter output is passed through the nonlinear transformation tanh(ax) where a=0.25... "

I=GRAY IMAGE VALUES
H=fspecial('gaussian',[5 5],1);
hf=imfilter(I,H);
xx=tanh(hf*0.25);

How can i implement nine second order derivatives of Gaussians with three different sigma values if I is my input?

Thanks in advance.