[Image-SIG] Second Derivative Test

Fredrik Lundh fredrik at pythonware.com
Thu Jun 15 14:18:25 CEST 2006

Ahmad Affzan Abdullah wrote:

> 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)

looks reasonable.

> 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

any reason you cannot use a Python list for this ?


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