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The action constraint has unlimited potential uses, since it is user defined. As it's name implies, it uses an action (animation) to determine the movement of a bone or object, which is then controlled by another bone or object.
When setting up animations for use with the action constraint, it is a good idea to always return to a 'home' position after each motion. This will help ensure the constraint is always able to blend from one motion to another nicely.
Each bone in the action will have it's own channel, to which the action constraint will refer. This means that it is possible to contain all constraint animation for a character in one action, because each constraint will only refer to one bone at a time and only for the frame range designated in the Start/End values (Example Blend).
Once the constraint has been setup, it is important to detach the action from the armature by pressing the 'x' in the action window. Otherwise the action will influence the armature both through the constraint, and through the timeline.
Options and Settings
OB: The Object to serve as the controller (when rigging, it is the name of the Armature Object which contains the controller bone).BO: The Bone to serve as the controller.
AC: The Action to be used for the motion.
Size/Rot/Loc X/Y/Z rollout This determines which axis and transformation of the control bone will be used to drive the action.
Start/End The frame range to use in the action. If the start value is greater than the end value, the animation will be in reverse.
Min/Max The range of motion the control object uses to drive the action. Negative values can be used, and max can be less than min.
CSpace Determines the coordinate system the constrained bones will be evaluated in.
- World Space means the coordinates will be relative to the world (the grid).
- Pose Space means the coordinates will be relative to the Armature Object orientations (as if the armature was a container full of bones).
- Local Space means the coordinates will be relative to the constrained objects/bones themselves.
- Local Space (With Parent) is the same as Local Space with the addition that the constraint will be evaluated if the control object's parent is transformed, because the controller's transforms are inherited from its parent.
Show Shows the constraint in the IPO window.
Key Sets a keyframe on the current Influence of the constraint at the current frame.
More Resources:
Blender Wiki Page - Action
Paul Caggegi's Tutorial
