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give definition of partial derivative:  lim h->0 [f(x+h,y) – f(x,y)]/h
Work out on board
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Q:  Why might these work better?
A:  more stable when there is noise
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How to fix?
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How many 2nd derivative filters are there?  There are four 2nd partial derivative filters.
In practice, it’s handy to define a single 2nd derivative filter—the Laplacian