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Encyclopedia > Motion capture

Motion capture, motion tracking, or mocap is a technique of digitally recording movements for entertainment, sports, and medical applications. This article or section is in need of attention from an expert on the subject. ...

Contents

Methods and Systems

Motion tracking or motion capture started as a photogrametric analysis tool in biomechanics research in the 1970s and 1980s, and expanded into education, training, sports and recently computer animation for cinema and video games as the technology matured. A performer wears markers near each joint to identify the motion by the positions or angles between the markers. Acoustic, inertial, LED, magnetic or reflective markers, or combinations of any of these, are tracked, optimally at least two times the rate of the desired motion, to submillimeter positions. The motion capture computer software records the positions, angles, velocities, accelerations and impulses, providing an accurate digital representation of the motion. Computer animation is the art of creating moving images via the use of computers. ... This article is about computer and video games. ... External links LEd Category: TeX ... Look up position in Wiktionary, the free dictionary. ... This article is about angles in geometry. ... In physics, velocity is defined as the rate of change of displacement or the rate of displacement. ... Acceleration is the time rate of change of velocity and/or direction, and at any point on a velocity-time graph, it is given by the slope of the tangent to the curve at that point. ... In classical mechanics, the impulse of a force is the product of the force and the time during which it acts. ...


In entertainment applications this can reduce the costs of animation which otherwise requires the animator to draw each frame, or with more sophisticated software, key frames which are interpolated by the software. Motion capture saves time and creates more natural movements than manual animation, but is limited to motions that are anatomically possible. Some applications might require additional impossible movements like animated super hero martial arts or stretching and squishing that are not possible with real actors.


In biomechanics, sports and training, real time data can provide the necessary information to diagnose problems or suggest ways to improve performance, requiring motion capture technology to capture motions up to 140 miles per hour for a golf swing.


Optical systems

Optical systems triangulate the 3D position of a marker between one or more cameras calibrated to provide overlapping projections. Tracking a large number of markers or multiple performers or expanding the capture area is accomplished by the addition of more cameras. These systems produce data with 3 degrees of freedom for each marker, and rotational information must be inferred from the relative orientation of three or more markers; for instance shoulder, elbow and wrist markers providing the angle of the elbow... Triangulation can be used to find the distance from the shore to the ship. ...


Optical: Passive Markers

A dancer wearing a suit used in an optical motion capture system
A dancer wearing a suit used in an optical motion capture system

Passive optical system use markers coated with a Retroreflective material to reflect light back that is generated near the cameras lens. The cameras sensitivity can be adjusted taking advantage of most cameras narrow range of sensitivity to light so only the bright markers will be sampled ignoring skin and fabric. This image shows a motion capture system. ... This image shows a motion capture system. ... Retroreflectors are clearly visible in a pair of bicycle shoes. ...


The centroid of the marker is estimated as a position within the 2 dimensional image that is captured. The grayscale value of each pixel can be used to provide sub-pixel accuracy.


An object with markers attached at known positions is used to calibrate the cameras and obtain their positions and the lens distortion of each camera is measured. Providing two calibrated cameras see a marker, a 3 dimensional fix can be obtained. Typically a system will consist of around 6 to 24 cameras. Systems of over two hundred cameras exist. Extra cameras are required for full coverage around the capture subject. Typical eight camera systems are about $150,000 US.


Professional vendors have sophisticated constraint software to reduce problems from marker swapping since all markers appear identical. Unlike active marker systems and magnetic systems, passive systems do not require the user to wear wires or electronic equipment. The markers are usually attached directly to the skin (as in biomechanics), or they are velcroed to a performer wearing a full body spandex/lycra suit designed specifically for motion capture. This type of system can capture large numbers of markers at frame rates as high as 2000fps. The frame rate for a given system is often traded off between resolution and speed so a 4 megapixel system runs at 370 hertz normally but can reduce the resolution to .3 megapixels and then run at 2000 hertz.


Optical: Active marker

Active optical systems triangulate positions by illuminating one LED at a time very quickly or multiple LEDs but sophisticated software to identify them by their relative positions, somewhat akin to celestial navigation. Rather than reflecting light back that is generated externally, the markers themselves are powered to emit their own light. Since Inverse Square law provides 1/4 the power at 2 times the distance, this can increase the distances and volume for capture. ILM used active Markers in Van Helsing to allow capture of the Harpies on very large sets. The power to each marker can be provided sequentially in phase with the capture system providing a unique identification of each marker for a given capture frame at a cost to the resultant frame rate. The ability to identify each marker in this manner is useful in realtime applications. The alternative method of identifying markers is to do it algorithmically requiring extra processing of the data.


Optical: Time modulated active marker

A high-resolution active marker system with 3,600 × 3,600 resolution at 480 hertz providing real time submillimeter positions.
A high-resolution active marker system with 3,600 × 3,600 resolution at 480 hertz providing real time submillimeter positions.

Active marker systems can further be refined by strobing one marker on at a time, or tracking multiple markers over time and modulating the amplitude or pulse width to provide marker ID. 12 megapixel spatial resolution modulated systems show more subtle movements than 4 megapixel optical systems by having both higher spatial and temporal resolution. Directors can see the actors performance in real time, and watch the results on the mocap driven CG character. The unique marker IDs reduce the turnaround, by eliminating marker swapping and providing much cleaner data than other technologies. LEDs with onboard processing and a radio synchronization allow motion capture outdoors in direct sunlight, while capturing at 480 frames per second due to a high speed electronic shutter. Computer processing of modulated IDs allows less hand cleanup or filtered results for lower operational costs. This higher accuracy and resolution requires more processing than passive technologies, but the additional processing is done at the camera to improve resolution via a subpixel or centroid processing, providing both high resolution and high speed. These motion capture systems are typically under $50,000 for an eight camera, 12 megapixel spatial resolution 480 hertz system with one actor. Image File history File links Activemarker2. ...


Optical: Semi-passive Imperceptible Marker

One can reverse the traditional approach based on high speed cameras. Systems such as Prakash use inexpensive high speed projectors. The specially built IR projectors optically encode the space. Instead of retro-reflective or active light emitting diode (LED) markers, the system uses photosensitive marker tags to decode the optical signals. By attaching tags with photosensors to scene points, the tags can compute not only their own locations of each point, but also their own orientation, incident illumination, and reflectance.


These tracking tags that work in natural lighting conditions and can be imperceptibly embedded in attire or other objects. The system supports an unlimited number of tags in a scene, with each tag uniquely identified to eliminate marker reacquisition issues. Since the system eliminates a high speed camera and the corresponding high-speed image stream, it requires significantly lower data bandwidth. The tags also provide incident illumination data which can be used to match scene lighting when inserting synthetic elements. The technique is therefore ideal for on-set motion capture or real-time broadcasting of virtual sets.


Non-Optical Systems

Inertial systems

Inertial Motion Capture technology is based on miniature inertial sensors, biomechanical models and sensor fusion algorithms. It's an easy to use and cost-efficient way for full-body human motion capture. The motion data of the inertial sensors (Inertial_guidance_system) is transmitted wirelessly to a PC or laptop, where the full body motion is recorded or viewed. No external cameras, emitters or markers are needed for relative motions. Inertial mocap systems capture the full 6 degrees of freedom body motion of a human in real-time. Benefits of using Inertial systems include; No solving, freedom from studio's as most systems are portable, and large capture areas. These systems are similar to the WII controllers but much more sensitive and having much greater resolution and update rate. They can accurately measure the direction to the ground to within a degree. Base suits tend to be in the $50,000 range, however the cost of a system often relates to the quality of data it produces. This article or section does not adequately cite its references or sources. ...


Mechanical motion

Mechanical motion capture systems directly track body joint angles and are often referred to as exo-skeleton motion capture systems, due to the way the sensors are attached to the body. A performer attaches the skeletal-like structure to their body and as they move so do the articulated mechanical parts, measuring the performer’s relative motion. Mechanical motion capture systems are real-time, relatively low-cost, free-of-occlusion, and wireless (untethered) systems that have unlimited capture volume. Typically, they are rigid structures of jointed, straight metal or plastic rods linked together with potentiometers that articulate at the joints of the body. These suits tend to be in the $25,000 to $75,000 range plus an external absolution positioning system.


Magnetic systems

Magnetic systems calculate position and orientation by the relative magnetic flux of three orthogonal coils on both the transmitter and each receiver. The relative intensity of the voltage or current of the three coils allows these systems to calculate both range and orientation by meticulously mapping the tracking volume. Since the sensor output is 6DOF, useful results can be obtained with two-thirds the number of markers required in optical systems; one on upper arm and one on lower arm for elbow position and angle. The markers are not occluded by nonmetallic objects but are susceptible to magnetic and electrical interference from metal objects in the environment, like rebar (steel reinforcing bars in concrete) or wiring, which affect the magnetic field, and electrical sources such as monitors, lights, cables and computers. The sensor response is nonlinear, especially toward edges of the capture area. The wiring from the sensors tends to preclude extreme performance movements. The capture volumes for magnetic systems are dramatically smaller than they are for optical systems. With the magnetic systems, there is a distinction between “AC” and “DC” systems: one uses square pulses, the other uses sine wave pulse. 6 degrees of freedom, also commonly referred to as 6DOF, is a gameplay style wherein there is no gravity, and you are free to move in any 3-dimensional direction. ...


The procedure

In the motion capture session, the movements of one or more actors are sampled many times per second. High resolution optical motion capture systems can be used to sample body, facial and finger movement at the same time.


A motion capture session records only the movements of the actor, not his visual appearance. These movements are recorded as animation data which are mapped to a 3D model (human, giant robot, etc.) created by a computer artist, to move the model the same way. This is comparable to the older technique of rotoscope 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. Rotoscoping is a technique where animators trace live action movement, frame by frame, for use in animated films. ...


If desired, a camera can pan, tilt, or dolly around the stage while the actor is performing and the motion capture system can capture the camera and props as well. 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, as inferred from the 3D position of each marker. If desired, a virtual or real camera can be tracked as well, providing the desired camera positions in terms of objects in the set.


A related technique match moving can derive 3D camera movement from a single 2D image sequence without the use of photogrammetry, but is often ambiguous below centimeter resolution, due to the inability to distinguish pose and scale characteristics from a single vantage point. One might extrapolate that future technology might include full-frame imaging from many camera angles to record the exact position of every part of the actor’s body, clothing, and hair for the entire duration of the session, resulting in a higher resolution of detail than is possible today. Match moving is a special effects technology related to motion capture. ...


After processing, the software exports animation data, which computer animators can associate with a 3D model and then manipulate using normal computer animation software. If the actor’s performance was good and the software processing was accurate, this manipulation is limited to placing the actor in the scene that the animator has created and controlling the 3D model’s interaction with objects.


Advantages

Mo cap offers several advantages over traditional computer animation of a 3D model: Computer animation is the art of creating moving images via the use of computers. ...

  • More rapid, sometimes even real time results can be obtained.
  • The amount of work does not vary with the complexity or length of the performance to the same degree when using traditional techniques.
  • Complex movement and realistic physical interactions such as secondary animation, weight and exchange of forces can be more easily recreated in a physically accurate manner.
  • Mocap technology allows one actor to play multiple roles within a single film.


Advantages over live action


In movies that contains CGI in such large amounts that the actors would have to stay in front of a bluescreen and interact with invisible computer animated characters which is added later, trying to fit into a computer animated world, it is sometimes less problematic to make everything digital, including the actors. This way, all elements would fit together naturally and have the same visual look.

  • The director can choose any angle he or she desires from a scene, including angles that would have been hard or impossible in a live action movie.
  • Limitless possibilities for rotating effect.
  • Costumes, make-up, body size and age can be changed to whatever is needed.
  • The characters will blend perfectly in with their digital environments.
  • There is no need to have light, colors and filters in mind when filming the motions, as this will be added digitally later.

Disadvantages

  • Specific hardware and special programs are required to obtain and process the data.
  • The cost of the software and equipment, personnel required can be prohibitive for small productions.
  • The capture system may have specific requirements for the space it is operated in.
  • When problems occur it is sometime 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.
  • Applying motion to quadruped characters can be difficult.
  • The technology can become obsolete every few years as better software and techniques are invented.
  • The 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 represented.
  • 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 are generally not applicable.
  • 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 strangely with any other body part when the human actor brings them too close to his body.
  • The real life performance may not translate on to the computer model as expected.

Applications

Some video games use motion capture to animate athletes, martial artists, and other in-game characters. “Computer and video games” redirects here. ...


Movies use motion capture for CG effects, in some cases replacing traditional cell animation, and for completely computer-generated creatures, such as Gollum, The Mummy, and King Kong. Computer-generated imagery (commonly abbreviated as CGI) is the application of the field of computer graphics (or more specifically, 3D computer graphics) to special effects in films, television programs, commercials, simulators and simulation generally, and printed media. ... This article is about the fictional character. ... The Mummy is the title of: a 1932 movie starring Boris Karloff: see The Mummy (1932 movie) a 1959 movie starring Christopher Lee: see The Mummy (1959 movie) a 1999 movie starring Brendan Fraser: see The Mummy (1999 movie) a novel by Anne Rice: see The Mummy (novel) This is... Peter Jacksons King Kong is a multiplatform videogame based on the forthcoming movie King Kong. ...


Sinbad: Beyond the veil of mists was the first movie made primarily with motion capture, although many character animators also worked on the film.


In producing entire feature films with Computer animation, the industry is currently split between studios that use Motion Capture, and studios that do not. Out of the three nominees for the 2006 Academy Award for Best Animated Feature, two of the nominees ("Monster House" and the winner "Happy Feet") used Motion Capture, and only Pixar's Cars was animated without Motion Capture. In the ending credits of Pixar's latest film "Ratatouille", a stamp appears labelling the film as "100% Pure Animation -- No Motion Capture!" On the other hand, Pixar's parent The Walt Disney Company has announced that it will distribute Robert Zemeckis's A Christmas Carol to be produced using "Performance Capture," the motion-capture approach that Zemeckis first used on The Polar Express. Computer animation is the art of creating moving images via the use of computers. ... The Academy Awards are the oldest awards given to achievements in film; the Academy Award for Best Animated Feature was given the first time for the 2001 film year. ... Pixars studio lot in Emeryville Pixar Animation Studios is an American computer animation studio based in Emeryville, California (USA) notable for its seven Academy Awards. ... Pixars studio lot in Emeryville Pixar Animation Studios is an American computer animation studio based in Emeryville, California (USA) notable for its seven Academy Awards. ... For other uses, see Ratatouille (disambiguation). ... Pixars studio lot in Emeryville Pixar Animation Studios is an American computer animation studio based in Emeryville, California (USA) notable for its seven Academy Awards. ... The Walt Disney Company (NYSE: DIS) is one of the largest media and entertainment corporations in the world. ... A Christmas Carol is an announced film to be directed by Robert Zemeckis and star Jim Carrey in multiple roles. ... The Polar Express is a 2004 feature film based on the childrens book of the same title by Chris Van Allsburg. ...


Virtual Reality and Augmented Reality allow users to interact with digital content in real-time. This can be useful for training simulations, visual perception tests, or performing a virtual walk-through in a 3D environment. This article is about the simulation technology. ... This article or section is in need of attention from an expert on the subject. ...


Gait analysis is the major application of motion capture in clinical medicine. Gait analysis is the process of quantification and interpretation of human locomotion. ... Medicine is the branch of health science and the sector of public life concerned with maintaining human health or restoring it through the treatment of disease and injury. ...


Motion capture technology is frequently used in digital puppetry systems to aid in the performance of computer generated characters in real-time. Digital puppetry is the manipulation and performance of digitally animated 2D or 3D figures and objects in a virtual environment that are rendered in real-time by computers. ...


Related techniques

Facial motion capture is utilized to record the complex movements in a human face, especially while speaking with emotion. This is generally performed with an optical setup using multiple cameras arranged in a hemisphere at close range, with small markers glued or taped to the actor’s face. Facial motion capture is related to body motion capture, but is more challenging due to the subtle expressions possible from small movements of the eyes and lips, requiring even greater resolution and fidelity. ...


Inertial systems use devices such as accelerometers or gyroscopes to measure positions and angles. They are often used in conjunction with other systems to provide updates and global reference, since they only measure relative changes, not absolute position.


RF (radio frequency) positioning systems are becoming more viable as higher frequency RF devices allow greater precision than older RF technologies. The speed of light is 30 centimeters per nanosecond (billionth of a second), so a 10 gigahertz (billion cycles per second) RF signal enables an accuracy of about 3 centimeters. By measuring amplitude to a quarter wavelength, it is possible to improve the resolution down to about 8 mm. To achieve the resolution of optical systems, frequencies of 50 gigahertz or higher are needed, which are almost as line of sight and as easy to block as optical systems. Multipath and reradiation of the signal are likely to cause additional problems, but these technologies will be ideal for tracking larger volumes with reasonable accuracy, since the required resolution at 100 meter distances isn’t likely to be as high.


An alternative approach was developed where the actor is given an unlimited walking area through the use of a rotating sphere, similar to a hamster ball, which contains internal sensors recording the angular movements, removing the need for external cameras and other equipment. Even though this technology could potentially lead to much lower costs for mocap, the basic sphere is only capable of recording a single continuous direction. Additional sensors worn on the person would be needed to record anything more.


A studio in the Netherlands is using a 6DOF (Degrees of freedom) motion platform with an integrated omni-directional treadmill with high resolution optical motion capture to achieve the same effect. The captured person can walk in an unlimited area, negotiating different uneven terrains. Applications include medical rehabilitation for balance training, biomechanical research and virtual reality.


Several research groups around the world tackle the task of Markerless Motion Capture, e.g.

Programs / Systems

  • Animazoo IGS Vs 8.3: IGS 8.3 software is the latest software to be used with Animazoo motion capture products. IGS 8.3 has been develop over 10 years of working with Games Developers, Animation Studio's, and Biomechanics experts.
  • Vicon BLADE: Scalable real-time and systems killer kinematic solving and retargeting full performance motion capture for hands, body and face.
  • Moven Studio: a software application for motion capturing with the Moven suit. Works with Microsoft Windows XP and Windows Vista.
  • Organic Motion: By Organic Motion INC, real-time markerless motion capture. Doesn't require a body suit, markers.
  • Imocap: By ILM, a proprietary on-set computer-vision based full-body motion capture system first used in Pirates of the Caribbean: Dead Man's Chest. The actors wear checkered bands.
  • Mova Contour reality capture: Trackable facial and surface motion capture using phosphorescent makeup applied to skin or dyed into fabric. An array of cameras alternates imaging the phosphor pattern for 3D geometry and natural skin/cloth color for textures.
  • Image Metrics: Proprietary facial motion capture service, no makeup, no spandex suit, no special hardware.
  • PhaseSpace Recap: Motion capture editor, no makeup, black spandex suit, Modulated LED hardware.
  • Pure Data + Pidip: motion capture, open source program
  • Eyesweb: motion capture, freeware
  • Measurand Inc: Portable, durable, wireless, and real-time motion capture
  • NaturalPoint Arena: Full body motion capture solution which generates real time skeleton data. Used with OptiTrack cameras to preprocess images for minimal processor hit.
  • HumanEVA database: Synchronized Video and Motion Capture Dataset for Evaluation of Articulated Human Motion
  • Prakash : Lighting-Aware Motion Capture Using Imperceptible Photosensing Markers and Multiplexed Illuminators
  • [1]: 3D non-camera based motion capture system utilizing the E-Factor 3D Motion Capture Engine
  • I-CubeX: A sensor toolkit specifically designed to capture all kinds of human motions using a variety of technologies

A typical Windows XP desktop. ... Windows Vista is a line of graphical operating systems used on personal computers, including home and business desktops, notebook computers, Tablet PCs, and media centers. ... Industrial Light + Magic is a motion picture special visual effects company, founded in July 1975 by George Lucas. ... Pure Data with many patches open (netpd project) Pure Data (or Pd) is a graphical programming language developed by Miller Puckette in the 1990s for the creation of interactive computer music and multimedia works. ...

In fiction

  • In the 2001 television film How to Make a Monster, a motion capture suit is brought to life and becomes a monster as a result of the lightning strike on the AI chip, and starts hunting the programmers in a deadly game.

How to Make a Monster is 2001 sci-fi action thriller TV b-movie which stars Steven Culp, Clea DuVall, Tyler Mane, Jason Marsden, Karim Prince, Danny Masterson and Colleen Camp. ...

See also

This article is about computer and video games. ... The bouncing ball animation (below) consists of these 6 frames. ... Rotoscoping is a technique where animators trace live action movement, frame by frame, for use in animated films. ... Match moving is a special effects technology related to motion capture. ... This article is about the fictional character. ... The Lord of the Rings film trilogy comprises three live action fantasy epic films; The Lord of the Rings: The Fellowship of the Ring (2001), The Lord of the Rings: The Two Towers (2002) and The Lord of the Rings: The Return of the King (2003). ...

External links

  • Free BVH Files Free BVH files from Inertial MoCap Market Leaders Animazoo
  • Inition Motion Capture Resellers of motion capture systems and experts in motion capture technology
  • Inner Esteem Motion Capture Studios Motion Capture files or library from Motioncapturestudios.com
  • Biomechanics Yellow Pages - Motion Capture & Analysis International biomechanics community's repository of products for 3D motion capture and analysis.
  • Human motion analysis for Motion Capturing Paper of introduction to the beginning of Mocap technologies.
  • VirtualCinematography.org—Several papers on Universal Capture use in Matrix films
  • CGW Article on motion capture
  • Motion Capture Resources
  • Free Repository of Motion Capture Data
  • Free Data, membership and information on Motion Capture
  • The World Records of Motion Capture
  • Free Motion Capture data - Full body and Face MOCAP data
  • Over 600 Free Motion Capture data by VICON
  • mastudios.com— A mocap studio that captures animal movements

  Results from FactBites:
 
Motion capture - Wikipedia, the free encyclopedia (2646 words)
Motion tracking or motion capture started as an analysis tool in biomechanics research, and expanded into education, training, sports and recently computer animation for cinema and video games as the technology has matured.
Motion capture equipment costs from fifty thousand dollars for 8 camera active marker systems to millions of dollars for passive marker systems, for the digital video cameras, lights, software, and staff to run a mocap studio, and this technology investment can become obsolete every few years as better software and techniques are invented.
Motion capture technology is frequently used in digital puppetry systems to aid in the performance of computer generated characters in real-time.
Motion Capture Research (8851 words)
Motion capture systems which involves the process of recording a live motion event and translating it into usable mathematical forms are now being widely used in many applications such as medicine (1-5), sports (6, 7), entertainment industry, and in the study of human factors (8).
Motion Capture is the process of recording a live motion event and translating it into usable mathematical terms by tracking a number of key points in space over time and combining them to obtain a single 3D representation of the performance [10].
Motion Capture is the process of recording a live motion event and translating it into usable mathematical terms by tracking a number of key points in space over time and combining them to obtain a single 3D representation of the performance [1, 47, 48].
  More results at FactBites »

 
 

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