Get real size of someone's face

Question

I am developing an iOS app where I want to get the size of user's real face so that I can suggest him/her suitable (matched) sized glasses.

I have detected user's face using OpenCV and got various dimensions of eyes, nose, face, etc.

But I want the real size from that dimensions (i.e in millimetres that I am getting in pixels).

I have searched a lot but could not get any solution matching with my requirement.

Has anyone idea how to calculate real size (i.e in millimetres) of the someone's face?

Thank you.

Answer 1

I think there are two ways of doing it.

1. You have an object of known size in the image that you can use to compare with. That object must also be at the same (or known) distance from the camera as the face.

2. If the camera supports depth, you can get the distance to the face from the camera, and using that to calculate the actual size of the face. This option is currently only available on iPhone X. The accuracy of the depth data can vary, so I am not sure how well it might work for you.

Read more about capturing depth data here

Read more on depth data accuracy here

Answer 2

If you have no reference point for size in the image, i guess there is really no way to tell the exact size. You would need at least one length that correlates to your picture to get some sort of a result.

That said, this would only work a 100% accurately on images of plain objects, because objects further away seem to be smaller in an image (like, e.g. here). You would need multiple pictures from different sides (all with a size reference) and there would be a horrendous amount of calculations to do.

The focal length of the camera will distort your image as well, making accurate measurement even harder (see comparision of different focal lengths with different distances to the face).