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Motion capture

The process of digitally recording an actor's movement in order to apply this movement to a computer-generated object.

Motion capture (sometimes referred to as mo-cap or mocap, for short) is the process of recording the movement of objects or people. It is used in military, entertainment, sports, medical applications, and for validation of computer vision and robotics. In filmmaking and video game development, it refers to recording actions of human actors and using that information to animate digital character models in 2D or 3D computer animation. When it includes face and fingers or captures subtle expressions, it is often referred to as performance capture. In many fields, motion capture is sometimes called motion tracking, but in filmmaking and games, motion tracking usually refers more to match moving.

In motion capture sessions, movements of one or more actors are sampled many times per second. Whereas early techniques used images from multiple cameras to calculate 3D positions, often the purpose of motion capture is to record only the movements of the actor, not his or her visual appearance. This animation data is mapped to a 3D model so that the model performs the same actions as the actor. This process may be contrasted with the older technique of rotoscoping, as seen in Ralph Bakshi's The Lord of the Rings (1978) and American Pop (1981). The animated character movements were achieved in these films by tracing over a live-action actor, capturing the actor's motions and movements. To explain, an actor is filmed acting, and then the recorded film is projected onto an animation table frame-by-frame. Animators trace the live-action footage onto animation cels, capturing the actor's outline and motions frame-by-frame, and then they fill in the traced outlines with the animated character. The completed animation cels are then photographed frame-by-frame, exactly matching the movements and actions of the live-action footage. The result of this is that the animated character replicates exactly the live-action movements of the actor. However, this process takes a considerable amount of time and effort.

Camera movements can also be motion captured so that a virtual camera in the scene will pan, tilt or dolly around the stage driven by a camera operator while the actor is performing. At the same time, the motion capture system can capture the camera and props as well as the actor's performance. This allows the computer-generated characters, images, and sets to have the same perspective as the video images from the camera. A computer processes the data and displays the movements of the actor, providing the desired camera positions in terms of objects in the set. Retroactively obtaining camera movement data from the captured footage is known as match moving or camera tracking.

Motion capture offers several advantages over traditional computer animation of a 3D model:

  • Low latency, close to real-time, results can be obtained. In entertainment applications, this can reduce the costs of keyframe-based animation. The Hand Over technique is an example of this.
  • The amount of work does not vary with the complexity or length of the performance to the same degree as when using traditional techniques. This allows many tests to be done with different styles or deliveries, giving a different personality only limited by the talent of the actor.
  • Complex movement and realistic physical interactions such as secondary motions, weight, and exchange of forces can be easily recreated in a physically accurate manner.
  • The amount of animation data that can be produced within a given time is extremely large when compared to traditional animation techniques. This contributes to both cost-effectiveness and meeting production deadlines.
  • Potential for free software and third-party solutions reducing its costs.

Key Terms

actions
actor
animated character
filmmaking
model
motion
motion capture
motions
movements
process

Additional Resources

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Acronymn

mocap

Synonymns

Motion capture
(none found)

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Sources & Credits

Last modified on November 9 2019
Content adapted from Wikipedia
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