Praise for the Geometry Constraint, pt 1

I like the Geometry Constraint. A lot.

With a Geometry Constraint you can constrain an object and keyframe it - now when you're rigging, that a bit like having your cake and eating...your cake.

In this example, I set up a very simple eye rig to show you why the Geometry Constraint is worth blogging about...

Create:

• two NURBS circles.
• a NURBS sphere.

Add a ramp texture to the Lambert material and set it up so it looks like an iris and pupil...

The objective here is to control the rotations of the "eyeball" by animating the smaller of the two circles. This is a typical set-up for an eye rig, where the animator has a proxy controller for the character's eyes.

With a Geometry Constraint we will limit the movement of the smaller circle to the confines of the outer circle. 

1) Add a Geometry Constraint:

Typically, a constraint connects directly to attributes of your Transform node. When you're animating, Maya detects incoming connections on these attributes and you cannot keyframe them.

However, the Geometry Constraint ties up none of the keyable attributes on your Transform node. 

1. Select the outer circle.
2. Create a Planar trim (see illustration)
3. Select the Planar surface, then the inner circle.
4. Constrain > Geometry

Your inner circle is confined to the area inside the outer circle.

Finish by linking the rotations of the "eyeball" to the XY translations of the inner circle.

2) Tips:

1. Hide the Planar surface - this makes the inner circle easier to select.
2. Turn off all rendering flags for the Planar surface shape.
3. If you change the shape of your outer circle, you may see the inner circle 'snap' back to the centre. To overcome this, add yout Geometry Constraint to a polygon object instead of a NURBS surface (in this example, convert the Planar trim to a polygon object before adding the constraint).

Coming soon: create an FK/IK leg rig using the Geometry Constraint.

Cheers,

Owen