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Motion capture technology is a process by which movement is digitally recorded. The technique was originally used for military tracking purposes and in sports as a tool for biomechanic research which focused on the mechanical functioning of the body, like how the heart and muscles work and move. In the last twenty-five years, motion capture has become an essential tool in the entertainment business, giving computer animators the ability to make non-human characters more life-like. It's a techonolgy used in animated films and television as well as video games.
Historically, motion capture in animated movies was created using an extension of the rotoscoping technique. In this technique, an actor is filmed making certain movements or gestures while wearing markers on specific points of his or her body. Each marker in each frame of film is then manually encoded into the computer. As animation software improved, it became possible to apply an algorithm to the markers that attach them to a 3D object, creating what is now called motion capture.
Motion capture techniques can vary by their input methods; there are four primary input methods: prosthetic, acoustic, magnetic, and optical. Prosthetic, or mechanical, motion capture uses trigonometry to input the data from mechanical devices attached to the performer’s body. Because of the inhibitive nature of the machinery, it is seldom used today.
Acoustic motion capture uses audio transmitters on the actor's body that make a clicking sound when activated by movement. Receivers measure the time it takes for the sound to reach them and triangulate the data to indicate a point on a 3D plane. While the acoustic method doesn't encounter some of the problems of the optical method since a line of sight is not an issue, it does have other potential problems including audio interference affecting the accuracy of the motion capture.
Magnetic motion capture is one of the more commonly used methods. This approach uses a central magnet and several receivers attached to the actor's body. The receivers capture and record the actors movements and save them to the computer. Magnetic motion capture can be hindered by nearby metal objects if they are large enough, and depending on the power of the magnets being used, the capture area may not be as large as one would like.
Today, optical motion capture is probably the most popular motion capture method. This approach uses at least three cameras and proper lighting to recreate the performer's position in a 3D space. Optical motion capture allows for a larger performance area and less inhibited movement than the other methods. Because of the cost of each camera, this approach is likely to be the most expensive of the four.
Movement is the first and fastest way humans perceive visual information, therefore motion capture provides a number of benefits to the filmmaker concerned with authenticity. It is a much faster way to film than rotoscoping, and it can provide real time results. Additionally, because the process records only movement as opposed to physical features, it allows one actor to play many roles. Perhaps most importantly in terms of realism, the accuracy of the data allows complex movements to be replicated with the correct distribution of weight and exchange of forces.