Why do Leap Hands have intermediate phalanges on the thumbs instead of metacarpals?

[v-ercart]

As far as I can tell, intermediate phalanges don't exist in human biology. Thumbs have Distal phalanges, proximal phalanges, and metacarpals. From Wikipedia:

But in Leap Motion, thumbs have distal, intermediate, and proximal phalanges, but no metacarpals. The Leap documentation even has a oddly edited version of the same graphic from Wikipedia:

The AttachmentPointFlags enum makes this even less clear, since it uses "Distal Joint", "Middle Joint", and "Knuckle" for fingers, but "Distal Joint", and "Proximal Joint" for thumbs.

I'm confused by this naming convention. What's going on with the naming here? Am I just interpreting it wrong?

[AlexColgan]

Admittedly the naming is rather odd. There is some reasoning behind it

The use of a zero-dimensional metacarpal for the thumb was a function of convenience for the core tracking API; bones are ordered from base to tip, indexed from 0 to 3. It's a lot easier to program interactions focused around the fingertips if you don't have to make a special exception for the thumb all the time.

Unity abstracts a lot of this away, as we provide hands (not just rigged models, but physics models) that handle this sort of thing for you. We reverted to using standard anatomical naming conventions in Unity. This does represent a bit of a disconnect, though most developers don't touch both the low-level tracking API and the high-level Unity assets in the same project.

[v-ercart]

Thanks for the explanation!