Quote - and it seems to me that if the  "sclera + iris + pupil" are one material zone, then talking about edges between the non-existent zones is completely moot.  the edges are whatever you texture them to be.  it's like worrying about the edge of a  mole on the torso.

I wouldn't say completely moot. Those boundaries are in a sense prescribed by the way the geometry is built. There are no sharp edges between zones anymore, so there's a bit of wiggle room there. But if one made the iris or pupil significantly smaller or larger in the texture than they were supposed to be, I'm sure the result would look pretty odd.

-- I'm not mad at you, just Westphalian.