Motion Capture or Motion tracking or mocap are terms used to describe the process by which movement is digitally recorded.
This 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. It is also used for validation of computer vision and robotics. 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. In filmmaking it refers to recording actions of human actors, and using that information to animate digital character models in 2D or 3D computer animation.
Historically, motion capture in animated movies was created using an extension of the rotoscoping technique. In this technique, movements of one or more actors are sampled many times per second, while wearing markers on specific points of his or her body. Motion capture records only the movements of the actor, not his visual appearance. Each marker in each frame of film is then manually encoded into the computer algorithm and mapped to a 3D model so that the model performs the same actions as the actor.
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.
Motion capture technology is a good example of how digital techniques are being applied to the movie and related industries to allow more convincing visualizations of imaginary or composite images. For motion capture you use human actors who are dressed in a leotard with integral reflective or magnetic markers. The actor performs the actions that are required, and the digital cameras - or array of cameras - capture the motion of the reflective markers. 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, and 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 capturing techniques are very effective, but the computer processing needs much human intervention, and if there is any error in the data, you can find it more effective to re-shoot the whole scene rather than correct the data. However, motion capture technology is so much more effective and realistic than traditional techniques, and ultimately less time consuming, that its future looks assured in movies and in video games.
The recorded data is sent computer alogorithm that converts this motion data into a composite figure. You then modify this composite figure by normal computer animation software and human intervention. The end product gives the effect of an animated character acting directly with human actors. Giving an absolutely life-like image of a composite character.
It's a techonolgy used in animated films and television as well as video games.
Motion capture techniques can vary by their input methods, there are four primary input methods:
- Prosthetic motion
- Acoustic motion
- Magnetic motion
- Optical motion
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.
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.