How to find all local minima in a matrix?

Question

Algorithms

Hello!
There is a matrix of real non-negative numbers approximately 1000*1000 in size. Essentially, these are the values ​​of some more or less smooth function of two variables. The task is to find all local minima of this function. Minima can also be located on the boundaries of the matrix.

Answer 1

Check each element by comparing with 8 or 5 (at the wall) or 3 (in the corner) neighbors.