Inverse kinematics (IK) is a method for animating an object or set of objects in relation to each other using an articulated structure of bones. Bones allow symbol instances and shape objects to move in complex and naturalistic ways with a minimum of design effort. For example, inverse kinematics lets you create character animation, such as arms, legs, and facial expressions much more easily.

You can add bones to separate symbol instances or to the interior of a single shape. When one bone moves, the other connected bones move in relation to the bone that initiated the movement. When animating using inverse kinematics you need only specify the start and end positions of objects. Inverse kinematics lets you create natural motion much more easily.

A shape with an IK bone armature added.

A group of several symbols with an IK bone armature attached.

A chain of bones is called an armature. The bones in an armature are connected to each other in a parent-child hierarchy. An armature can be linear or branched. Branches of an armature that originate at the same bone are called siblings. The point where one bone connects to another is called a joint.

You can use IK in Flash in two ways. The first is to articulate a series of symbol instances by adding bones to connect each instance to another instance. The bones allow the chain of symbol instances to move together. For example, you might have a set of movie clips that each represents different parts of a human body. By linking the torso, upper arm, lower arm, and hand together, you can create a realistically moving arm. You can create a branched armature to include both arms, both legs, and the head.

The second way to use IK is to add an armature to the interior of a shape object. The shape can be created in merge drawing mode or object drawing mode. The bones allow you to move and animate parts of the shape without the need to draw different versions of the shape or create a shape tween. For example, you could add bones to a simple drawing of a snake to enable the snake to move and curve realistically.

When you add bones to symbol instances or shapes, Flash moves the instance or shape and the associated armature to a new layer in the Timeline. This new layer is called a pose layer. Each pose layer can contain only one armature and its associated instances or shape.

Flash includes two tools for working with IK. You add bones to symbol instances and shapes with the Bone tool. You use the Bind tool to adjust the relationships between individual bones and control points of shape objects.

You can animate armatures and their associated symbols or shapes either in the Timeline or with ActionScript 3.0. You animate in the Timeline by defining different poses for the armature in different frames. Flash interpolates the positions of the armature in the frames in between. For information about animating armatures with ActionScript, see the fl.ik classes in the ActionScript 3.0 Language and Components Reference.