Shapes and Morphing - Blender Foundations: The Essential Guide to Learning Blender 2.6 - page 291

A number of things have to happen in the lower portion
of this variable section for everything to work. First, you
have to select the armature object in the Ob/Bone field,
then set the correct bone below it. In this case, it is the
“jaw control” bone, shown in Figure 10.8 . Second, you
have to determine which transform channel you will use
as the actual driver. This will be determined by exactly
how you placed the bone and how you want the control
to feel. In the example, the bone faces forward, and we
would like the jaw to open and close as the bone moves
up and down.

As we did when determining the correct axis to use as a pole
target, enable the transform manipulator on the 3D header
and set it to Normal mode. Select the “jaw control” bone,
and its local axis will display. In the example, the blue ( z )
axis is pointing up, so we know that we should choose Z
Location in the variable panel. Enable Local Space beside
the channel selector, and we're somewhat ready to go. Thus,
moving the bone along its local z axis in Pose mode will
drive the smile value, changing the shape of the mesh.

Figure 10.8 Setting up the variable.

If you actually try to move the bone now, you'll find a
problem. It would be great if moving the bone down
applied the shape key in just such a way that the chin
seemed to follow the bone. With a “plain vanilla” driver
like we've created though, this isn't the case. By default,
drivers map their values on a 1 : 1 basis. This means that as
the bone's Z Location moves from 0 to 1, so does the value
of the property it drives, which in this case is the “jaw
open” shape key. So, to get the jaw to open to the full
extent specified in the shape key (a Value slider of 1.0), we
have to move the chin bone 1.0 units along the local z
axis, placing it way above the character's head.

Although it's a little more work, it's possible to create a
custom mapping for the channel and driver values. This
mapping is created with the curve in the Graph Editor.
Something you may have wondered about when fooling
around with creating the driver variables previously was the
Modifiers panel below it, shown in Figure 10.9 . Like Mesh
modifiers, F-curve modifiers can either change or completely
replace the values found in a standard animation curve.

Figure 10.9 The Modifiers panel of the Drivers view
in the Graph Editor.