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# Introduction

Programming the motion of a robotic arm is no small feat. It requires knowledge of trigonometry and kinematics, the study of motion. Moving a robotic arm is complex because you must calculate the angles of each joint required to place the mechanical hand of the robot at its destination. This is typically called forward kinematics because the motion goes forward through the structure. The angle of the shoulder positions the upper arm and elbow, the angle of the elbow positions the forearm and wrist, the angle of the wrist positions the hand, and so on.

Unless you’re a math wiz, this won’t help you much when animating jointed structures in Flash. Fortunately, Flash CS4 Professional introduces an easier approach to the problem; an animation technique called inverse kinematics (IK). Common in higher-end 3D and animation packages, IK is the opposite, or inverse, of forward kinematics. Using IK, you start from the end and work backward. You specify a pose of a jointed structure by positioning its end segment, and let the animation software determine all the joint positions and angles required to create that pose. In Flash, for example, you can drag the hand of an articulated character into position, and Flash then calculates the location and angles of the wrist, forearm, elbow, and shoulder joints.

In the art world, these jointed skeletal structures are called armatures. Armatures are very much like your own skeleton in that they provide the structural ...

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