7.12. Cross-Correlation

The operation in the previous section — selecting elements from an array and adding them up — is an example of an operation that is so useful, in so many domains, that it has a name: cross-correlation. And NumPy provides a function, called correlate, that computes it. In this section we’ll see how we can use NumPy to write a simpler, faster version of step.

The NumPy correlate function takes an array, a, and a “window”, w, with length N and computes a new array, c, where element k is the following summation:

\[c_k = \sum_{n=0}^{N-1} a_{n+k}*W_n\]

We can write this operation in Python like this:

def c_k(a, w, k):
    N = len(w)
    return sum(a[k:k+N] * w)

This function computes element k of the correlation between a and w. To show how it works, we will create an array of integers:

N = 10
row = np.arange(N, dtype=np.uint8)

[0 1 2 3 4 5 6 7 8 9]

And a window:

window = [1, 1, 1]


With this window, each element, c_k, is the sum of consecutive elements from a:

c_k(row, window, 0)

c_k(row, window, 1)

We can use c_k to write correlate, which computes the elements of c for all values of k where the window and the array overlap.

def correlate(row, window):
    cols = len(row)
    N = len(window)
    c = [c_k(row, window, k) for k in range(cols-N+1)]
    return np.array(c)

Here’s the result:

c = correlate(row, window)

[ 3  6  9 12 15 18 21 24]

The NumPy function correlate does the same thing:

c = np.correlate(row, window, mode='valid')

[ 3  6  9 12 15 18 21 24]

The argument mode='valid' means that the result contains only the elements where the window and array overlap, which are considered valid.

The drawback of this mode is that the result is not the same size as array. We can fix that with mode='same', which adds zeros to the beginning and end of array:

c = np.correlate(row, window, mode='same')

[ 1  3  6  9 12 15 18 21 24 17]

Now the result is the same size as array. As an exercise at the end of this chapter, you’ll have a chance to write a version of correlate that does the same thing.

We can use NumPy’s implementation of correlate to write a simple, faster version of step:

def step2(array, i, window=[1,1,1]):
row = array[i-1]
c = np.correlate(row, window, mode='same')
array[i] = c % 2

In the notebook for this chapter, you’ll see that step2 yields the same results as step.

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