How can I compare two faces using facial landmarks?

Question

I'm using first_order_model for an expression transfer between an video and a static image.

Part of the algorithm makes a facial comparison to get the frame in the video where the face is best aligned with the foto.

This is made with:

def normalize_kp(kp):                           # original kp is (step 1)
    kp = kp - kp.mean(axis=0, keepdims=True)    # (step 2)
    area = ConvexHull(kp[:, :2]).volume
    area = np.sqrt(area)
    kp[:, :2] = kp[:, :2] / area                # (step 3)
    return kp

normalized_kp_source      = normalize_kp(get_landmarks(source))
normalized_kp_destination = normalize_kp(get_landmarks(destination))

diff = (np.abs(normalized_kp_source - normalized_kp_destination) ** 2).sum()

What I could not understand is how normalize_kp works. I understand the result of putting the faces roughly in the same "size" and coordinate system.

Below, a comparison between the landmarks in each step of the normalisation:

The question:

How can I compare face alignment. How this normalization works?