In motion capture sessions, movements of one or more actors are sampled many times per second, although with most techniques (recent developments from Weta use images for 2D motion capture and project into 3D), motion capture records only the movements of the actor, not his/her visual appearance. This animation data is mapped to a 3D model so that the model performs the same actions as the actor. This is comparable to the older technique of rotoscope, such as the 1978 The Lord of the Rings animated film where the visual appearance of the motion of an actor was filmed, then the film used as a guide for the frame-by-frame motion of a hand-drawn animated character.
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.
Advantages
Motion capture offers several advantages over traditional computer animation of a 3D model:
More rapid, even real time results can be obtained. In entertainment applications this can reduce the costs of keyframe-based animation. For example: Hand Over
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.
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.
Disadvantages
Specific hardware and special programs are required to obtain and process the data.
The cost of the software, equipment and personnel required can potentially be prohibitive for small productions.
The capture system may have specific requirements for the space it is operated in, depending on camera field of view or magnetic distortion.
When problems occur it is easier to reshoot the scene rather than trying to manipulate the data. Only a few systems allow real time viewing of the data to decide if the take needs to be redone.
The initial results are limited to what can be performed within the capture volume without extra editing of the data.
Movement that does not follow the laws of physics generally cannot be captured.
Traditional animation techniques, such as added emphasis on anticipation and follow through, secondary motion or manipulating the shape of the character, as with squash and stretch animation techniques, must be added later.
If the computer model has different proportions from the capture subject, artifacts may occur. For example, if a cartoon character has large, over-sized hands, these may intersect the character's body if the human performer is not careful with their physical motion.
