Motion capture


Motion capture is the process of recording high-resolution movement of objects or people into a computer system. It is used in military, entertainment, sports, medical applications, and for validation of computer vision and robots.
In films, television shows and video games, motion capture 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 or voices, 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 their 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.
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.
The first virtual actor animated by motion-capture was produced in 1993 by Didier Pourcel and his team at Gribouille. It involved "cloning" the body and face of French comedian Richard Bohringer, and then animating it with still-nascent motion-capture tools.

Advantages

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 distinct personality that is 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.

    Disadvantages

  • Specific hardware and special software programs are required to obtain and process the data.
  • The cost of the software, equipment and personnel required can be prohibitive for small productions.
  • The capture system may have specific requirements for the space in which it is operated, depending on camera field of view or magnetic distortion.
  • When problems occur, it is easier to shoot the scene again 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 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, oversized hands, these may intersect the character's body if the human performer is not careful with their physical motion.

    Applications

There are many applications of motion capture. The most common are for video games, movies, and movement capture, however there is a research application for this technology being used at Purdue University in robotics development.

Video games

often use motion capture to animate athletes, martial artists, and other in-game characters. As early as 1988, an early form of motion capture was used to animate the 2D player characters of Martech's video game Vixen and Magical Company's 2D arcade fighting game Last Apostle Puppet Show. Motion capture was later notably used to animate the 3D character models in the Sega Model arcade games Virtua Fighter and Virtua Fighter 2. In mid-1995, developer/publisher Acclaim Entertainment had its own in-house motion capture studio built into its headquarters. Namco's 1995 arcade game Soul Edge used passive optical system markers for motion capture. Motion capture also uses athletes in based-off animated games, such as Naughty Dog's Crash Bandicoot, Insomniac Games' Spyro the Dragon, and Rare's Dinosaur Planet.

Robotics

Indoor positioning is another application for optical motion capture systems. Robotics researchers often use motion capture systems when developing and evaluating control, estimation, and perception algorithms and hardware. In outdoor spaces, it's possible to achieve accuracy to the centimeter by using the Global Navigation Satellite System together with Real-Time Kinematics. However, this reduces significantly when there is no line-of-sight to the satellites — such as in indoor environments. The majority of vendors selling commercial optical motion capture systems provide accessible open source drivers that integrate with the popular Robotic Operating System framework, allowing researchers and developers to effectively test their robots during development.
In the field of aerial robotics research, motion capture systems are widely used for positioning as well. Regulations on airspace usage limit how feasible outdoor experiments can be conducted with Unmanned Aerial Systems. Indoor tests can circumvent such restrictions. Many labs and institutions around the world have built indoor motion capture volumes for this purpose.
Purdue University houses the world's largest indoor motion capture system, inside the Purdue UAS Research and Test facility. PURT is dedicated to UAS research, and provides tracking volume of 600,000 cubic feet using 60 motion capture cameras. The optical motion capture system is able to track targets in its volume with millimeter accuracy, effectively providing the true position of targets — the "ground truth" baseline in research and development. Results derived from other sensors and algorithms can then be compared to the ground truth data to evaluate their performance.

Movies

Movies use motion capture for CGI effects, in some cases replacing traditional cel animation, and for completely CGI creatures, such as Gollum, The Mummy, King Kong, Davy Jones from Pirates of the Caribbean, the Na'vi from the film Avatar, and Clu from Tron: Legacy. The Great Goblin, the three Stone-trolls, many of the orcs and goblins in the 2012 film The Hobbit: An Unexpected Journey, and Smaug were created using motion capture.
In the 1990 movie Total Recall, the sequence where Arnold Schwarzenegger and other actors walk through an X-ray machine was shot using motion capture.
The film Batman Forever used some motion capture for certain visual effects. Warner Bros. had acquired motion capture technology from arcade video game company Acclaim Entertainment for use in the film's production. Acclaim's 1995 video game of the same name also used the same motion capture technology to animate the digitized sprite graphics.
The 1999 film Star Wars: Episode I – The Phantom Menace was the first feature-length film to include a main character created, using motion capture. The 2000 Indian-American film Sinbad: Beyond the Veil of Mists was the first feature-length film made primarily with motion capture, although many character animators also worked on the film, which had a very limited release. 2001's Final Fantasy: The Spirits Within was the first widely released movie to be made with motion capture technology. Despite its poor box-office intake, supporters of motion capture technology took notice.
The Lord of the Rings: The Two Towers was the first feature film to utilize a real-time motion capture system. This method streamed the actions of actor Andy Serkis into the computer-generated imagery skin of Gollum / Smeagol as it was being performed.
Storymind Entertainment, which is an independent Ukrainian studio, created a neo-noir third-person / shooter video game called My Eyes On You, using motion capture in order to animate its main character, Jordan Adalien, and along with non-playable characters.
Of the three nominees for the 2006 Academy Award for Best Animated Feature, two of the nominees used motion capture, and only Disney·Pixar's Cars was animated without it. In the ending credits of Pixar's film Ratatouille, a stamp appears labelling the film as "100% Genuine Animation – No Motion Capture!"
Since 2001, motion capture has been used extensively to simulate or approximate the look of live-action theater, with nearly photorealistic digital character models. The Polar Express used motion capture to allow Tom Hanks to perform as several distinct digital characters. The 2007 adaptation of the saga Beowulf animated digital characters whose appearances were based in part on the actors who provided their motions and voices. James Cameron's highly popular Avatar used this technique to create the Na'vi that inhabit Pandora. The Walt Disney Company has produced Robert Zemeckis's A Christmas Carol using this technique. In 2007, Disney acquired Zemeckis' ImageMovers Digital, but then closed it in 2011, after a box office failure of Mars Needs Moms.
Television series produced entirely with motion capture animation include Laflaque in Canada, Sprookjesboom and in The Netherlands, and Headcases in the UK.