@mrdoob - yeah that would definitely work.  I don't know though ... do you think that's clearer than `!options.convertUpAxis`?  I think my preference is for the `!` version.  But three.js is your baby, so you get to make the call.  :wink: 

I might suggest that we make that a separate pull request and refactor all of those conditionals at the same time, rather than just this one.  Thoughts?