Issue with Inter-eye distance

[fatestayn]

I have the following problem. I’m a beginner of facial landmark detection or so-called face alignment. I see others use Inter-eye distance when they’re doing face alignment. E.g., this: https://github.com/patrikhuber/superviseddescent/issues/56, uses IED for normalization. I wonder what role the IED plays in face alignment methods in general, why and when we need to do it and how we can do it.

May I ask if there’s some good, easy-to-read articles that can help beginners like me understand this question? After some google search I didn’t find much about it. Thanx in advance.