[Image-SIG] Second Derivative Test

[Ahmad Affzan Abdullah]

Hi

I'm trying to do a second derivative test to an image for the purpose of 
finding extrema. I just want to make sure whether these are true or not:

Dxx = D(x+1,y) + D(x-1,y) - 2D(x,y)
Dyy = D(x,y+1) + D(x,y-1) - 2D(x,y)
Dxy = D(x+1,y+1) + D(x+1,y-1) + D(x-1,y-1) + D(x-1,y+1) - 4D(x,y)

Secondly, I'm trying to store all the extrema detected in a Nx2 array where 
N is the number of extrema, and 2 being the x and y coordinates of them. 
Since I don't know how many extrema will be detected, can we create an empty 
array in python without specifying the size, and later append the data to 
that array

Thanks a lot