From BlenderWiki

[edit] Introduction

A shape key can be made to happen by something happening to another object. Animating that other object causes the shape key to exert its influence on the mesh. This other object can be any kind of object, but is commonly a mesh object that has been modeled to be be a custom shape that indicates what it does. For example, a mesh that controls the eyelids of a face may be a crescent shape.

As an analogy, think of a robot on a platform, with a control panel nearby. As you move a lever on the control panel, the servos in the robot activate and the robot moves its arm. In this example, the driver is the lever, and the shape key is the robot's arm. By moving or rotating or scaling the driver, the shape key activates.

The relationship between the change in the driver's location/rotation/scale and the influence on the shape key is through the Shape's IPO curve. With the base object's shape key selected,

• go to the IPO window and change the type to Shape.
• In the IPO window, press N to bring up the properties panel.
• Click the tan button called Add Driver.
• In the OB: field, enter the name of the object that you wish to use to control the influence of the shape key. In this example, the object is called "Cube"
• Select what animation aspect of the driver you wish to use to control the shape key influence. In this example we have used the "Rot X". Depending on what you choose, the units of the IPO window will change to either Blender Units or Degrees.
• CtrlLMB click to add IPO control points. A height (Y) indicates the percent influence that shape key has; 0 is nothing, and 1.0 is 100 percent. The X value indicates what value of the driven channel (Rot X in this example) corresponds to the Y value.

In this example, if Cube is rotated about the X axis by less than -90 degrees or more than 90 degrees, the Basis shape will have 100% influence. For values between -90 and 90, the influence changes from 1 to 0 and back up to 1 again. If the RotX of the cube is 0, the basis shape has no influence. As the cube rotates from 0 to 90, the Basis shape will exert more and more influence.

Now, in your animation, add IPO keys for the Cube, and rotate it about the X axis. As you do, the shape key on the other mesh will activate and deform the mesh. Keep the driver mesh out of camera view, or on a hidden layer, or transparent so that it does not render.

You can see that a single driver object can drive up to 9 different shape keys individually; one for each LocXYZ, RotXYZ and ScaleXYZ

[edit] Driven Shape Keys

A Driven Shape Key is the controlling of the Key Value of a relative Shape Key by the movement of another object.

If one would like to animate relative Shape Keys in Blender, this unfortunately, cannot be done with Actions. (See Action Editor)

Instead one uses an "auxiliary construction", Ipo Drivers. With an Ipo Driver you control the value of an Ipo curve by moving another object. So e.g. the color of an object can be dependent on the rotation of another object. Equally the Key Value - the influence strength - of a shape can be dependent on the rotation or movement of another object. This is especially useful, because one can make e.g. the form of a muscle dependent on the amount of rotation of the underlying bone.

Driven Shape Keys are a typical Blender function: built up from several elements, which unfold their power only in the correct combination. In this case that is the vertex animation by relative Shape Key, and the composition of actions in the NLA for Armatures. We do not have to use Armatures (and/or Bones) as "drivers" for the Shape Keys necessarily, but the animation possibilities with Poses are best. Therefore the entire functionality is already described in other sections, it's just that it's not too obvious to combine the elements.

By using driven Shape Keys you're working on increasingly higher levels of abstraction.

• The simplest thing is to work on the level of the Shape Keys, like "Left Eyebrow Down", "Right Eye Open", "Sneer Left".
• The next level is to drive one or more Shapes with the same armature bone, like "Eyelid" + "Eyebrow". Or "Bulge muscle if arm is rotated".
• The next level is to combine the movement of several bones together in an action, like "Happy", "Sad", "Wink", "Frown" ...
• The last level would be to combine the actions in the NLA Editor.

You don't have to use these things, but they may make your life and workflow easier.

[edit] Subpages

1. Example
2. PyDrivers