Usually you measure the distance between functions.
If you cannot control the position / size / angle of the image - that is, the image of the real world, and not someone sitting in front of the face scanner - then you need to think about how to make all these relationships.
Try to find the eyes (lungs), mouth or nose and get the sides / angle of the triangle. Then you can also make a percentage of the area of the face covered with hair, the ratio of the length and length of the nose, etc.
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