matlab - gradient and derivative -


i trying find first derivative of gaussian image (using matlab) , tried 2 ways.one using gradient , 1 calculating derivative results different each other.

  method 1   k=7,s=3% kernel,st.dev   f = fspecial('gaussian', [k k], s)   [gx,gy] = gradient(f)    method 2   k=7,s=3% kernel,st.dev   [x,y] = meshgrid(-floor(k/2):floor(k/2), -floor(k/2):floor(k/2))   g = exp(-(x.^2+y.^2)/(2*s^2))/(2*pi*(s^2))   gn=g/sum(g(:))   gx = -x.*gn/(s^2)   gy = -y.*gn/(s^2)

gx , gy should same 2 methods there difference in values. know why is? expecting same. there preferred way calculate derivative? thank you.

edit: changed g definition per conrad's suggestion problem still persists.

this looks wrong: 

 g = exp(-(x.^2+y.^2)/(2*s^2))/(2*pi*s);

assuming supposed normal density (x,y) x , y independent 0 mean rv equal sds = s, should be:

g = exp(-(x.^2+y.^2)/(2*s^2))/(2*pi*(s^2));

(the term in front of exponential in each component 1/(sqrt(2*pi)*s) , have twice giving 1/(2*pi*s^2) )


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