maxpool.py

def maxpool(image, f=2, s=2):
    ```
    Downsample input `image` using a kernel size of `f` and a stride of `s`
    ```
    n_c, h_prev, w_prev = image.shape
    
    # calculate output dimensions after the maxpooling operation.
    h = int((h_prev - f)/s)+1 
    w = int((w_prev - f)/s)+1
    
    # create a matrix to hold the values of the maxpooling operation.
    downsampled = np.zeros((n_c, h, w)) 
    
    # slide the window over every part of the image using stride s. Take the maximum value at each step.
    for i in range(n_c):
        curr_y = out_y = 0
        # slide the max pooling window vertically across the image
        while curr_y + f <= h_prev:
            curr_x = out_x = 0
            # slide the max pooling window horizontally across the image
            while curr_x + f <= w_prev:
                # choose the maximum value within the window at each step and store it to the output matrix
                downsampled[i, out_y, out_x] = np.max(image[i, curr_y:curr_y+f, curr_x:curr_x+f])
                curr_x += s
                out_x += 1
            curr_y += s
            out_y += 1
    return downsampled