Possible? Single Bone to Drive Two Shapekeys in Opposite Directions?

The title may sound a bit convoluted, so I’ll elaborate…

I have an eye with a pupil.

The pupil has three shape keys:

  • ‘normal’ (average light)
  • dilated (dim light)
  • constricted (bright light)

I’m looking for a way to drive these shapekeys from:

  • normal to constricted when the control bone is rotated counterclockwise,
  • and from normal to dilated when the bone is rotated clockwise.

In other words, is it possible to have the bone’s rotation at twelve o’clock for normal, nine o’clock for constricted, and three o’clock for dilated?

I’m trying to do this without either extreme being the Basis key.

I’ve been playing around with var/-var, but it’s not having the affect I’m looking for. I was also looking at constraints, but don’t see one that would allow me to split the influence of the rotation between two shapekey drivers.

Any ideas?


I figured it out.

It’s pretty much the same thing as Nathan’s heel/ball pivot in the Humane Rigging DVD, a three-bone rig with one being the control for the other two. The main difference is that the driver for dilate needed a modifier with x = -1 and Influence = 1.