For each detected face the program finds two base points: central point (point between eyes), and bottom point on the jawline.
For ideal human figure the point between eyes located exactly in the middle of the head. Distance from it to the jawline and to the top of the head is equal. This way we get head height. It is double distance between centre point and bottom point. It works well for ideal figures in ideal conditions but even in real life it is gives quite good approximations.
As far as we know head height we can define size of cropped area in “head heights” as a measurement units. We name it Preset Scale.
Preset Scale - is the size of the crop in relation to the head height. If you set it to “1” - the height of the photo will be equal to calculated head size; “1.50” - height of the crop will be one and half bigger than height of the head; “2” - height of the crop will be twice bigger than height of the head and so on.
Now we know the height of crop. Select aspect ratio to get both height and width of crop rectangle. We define aspect ratio with Width and Height numbers. These values are relative which means if set it as 2x3, 20x30, 4x6, 12x18 they all give you the same 2x3 proportions. It doesn’t matter what numbers you input, it is relation between these numbers that matters.
To define position of the face within a photo we define Face Left and Face Top value. These are relative values. For example “Face Left = 0.50” means centre point of the face in the centre of picture.