Motion capture
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Motion capture, motion tracking, or mocap is a technique of digitally recording movements for entertainment, sports, and medical applications. In the context of filmmaking (where it is sometimes called performance capture), it refers to the technique of recording the actions of human actors, and using that information to animate digital character models in 3D animation.
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[edit] 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/her 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.
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.
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.
[edit] Advantages
Mo cap offers several advantages over traditional computer animation of a 3D model:
- 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 contain 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 camera angle desired from a scene, including angles that are difficult or impossible to film in live action situations.
- 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.
- Post-shooting experimentation is nearly limitless and devoid of full-crew and on-location requirements, a few examples of which are camera position and motion, lighting and other environmental conditions, wardrobe, and extras.
[edit] 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.
[edit] Applications
Some video games use motion capture to animate athletes, martial artists, and other in-game characters.
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.
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 film Ratatouille, a stamp appears labelling the film as "100% Pure Animation -- No Motion Capture!"
Motion capture has begun to be used extensively to produce films which attempt to simulate or approximate the look of live-action cinema, with nearly photorealistic digital character models. The Polar Express used it to translate the actions of star Tom Hanks to several distinct digital characters (for which he also provided the voices). The 2007 adaptation of the saga Beowulf used it to animate digital characters whose appearances were based in part on the actors who provided their motions and voices. The Walt Disney Company has announced that it will distribute Robert Zemeckis's A Christmas Carol to be produced using this technique.
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. Motion capture technology is frequently used in digital puppetry systems to aid in the performance of computer generated characters in real-time.
Gait analysis is the major application of motion capture in clinical medicine. Organic Motion, makers of markerless motion capture have recently pioneered the ability for clinicians to evaluate human motion, without burdening patients with cumbersome body suits or tracking devices. This ability to allow patients to move freely within a defined area, and uses cameras, not markers, to track range of motion, gait, and several other biometric factors, and streams this information live into analytical software. Because this system removes the markers, patients, physicians and analysts are able to collect quantifiable data in real-time with less patient inconvenience.
[edit] 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.
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.
[edit] Optical systems
Optical systems utilize data captured from image sensors to triangulate the 3D position of a subject between one or more cameras calibrated to provide overlapping projections. Data acquisition is traditionally implemented using special markers attached to an actor; however, more recent systems are able to generate accurate data by tracking surface features identified dynamically for each particular subject. Tracking a large number of 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.
[edit] Optical: Passive Markers
Passive optical system use markers coated with a Retroreflective material to reflect light back that is generated near the cameras lens. The camera's threshold can be adjusted so only the bright reflective markers will be sampled, ignoring skin and fabric.
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 by finding the centroid of the Gaussian.
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 three hundred cameras exist to try to reduce marker swap. Extra cameras are required for full coverage around the capture subject and multiple subjects.
Vendors have 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 rather hundreds of rubber balls with reflective tape, which needs to be replaced periodically. 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. Typical systems are $100,000 for 4 megapixel 360 hertz systems, and $50,000 for .3 megapixel 120 hertz systems.
[edit] Optical: Active marker
Active optical systems triangulate positions by illuminating one LED at a time very quickly or multiple LEDs with 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.
[edit] Optical: Time modulated active marker
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.
[edit] Optical: Semi-passive Imperceptible Marker
One can reverse the traditional approach based on high speed cameras. Systems such as Parkash use inexpensive multi-LED high speed projectors. The specially built multi-LED 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 photo sensors 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.
[edit] Optical: Markerless
Emerging techniques and research in computer vision are leading to the rapid development of the markerless approach to motion capture. Markerless systems such as developed at Stanford, MIT, and Max Planck Institute, do not require subjects to wear special equipment for tracking. Special computer algorithms are designed to allow the system to analyze multiple streams of optical input and identify human forms, breaking them down into constituent parts for tracking. Applications of this technology extend deeply into popular imagination about the future of computing technology.
One commercially available markerless system designed by Organic Motion was recently featured during Intel CEO Paul Otellini's keynote address at the 2008 Consumer Electronics Show in Las Vegas. During the demonstration, singer Steven Harwell of the band Smash Mouth performed live while tracking data generated in realtime by the markerless system were instantaneously fed into the Unreal Engine 3. By using the motion capture system as an input device, the game engine utilized tracking data to animate a virtual Steve located within a garage scene. The demonstration showcases the adaptability of markerless technology in service industries such as patient care wherein a variety of subjects could benefit from motion analysis without the need for extensive user calibrations. These systems work well with large motions, but tend to have difficulties with fingers, faces, wrist rotations and small motions. Some systems require no special suits, while others prefer special colors to identify limbs.
[edit] Non-optical systems
[edit] 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 six degrees of freedom body motion of a human in real-time. Benefits of using Inertial systems include; No solving, freedom from studios 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.
[edit] 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.
[edit] 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.
[edit] 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. However there are a number of systems such as Image Metrics, Mova's Contour and artemdigital's NVisage that offer the ability to capture realistic facial expressions and dialogue without the use of such markers.
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.
[edit] See also
- Video games
- Animation
- Rotoscope
- Match moving, also known as “motion tracking”
- Gollum, a character in The Lord of the Rings movies which was partially animated with motion capture (the motion capture was used as a reference even when the original data wasn't used.)
[edit] External links
- 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 -- As of 2008-02-09, link has been overrun by spammers. Click on HomePage button on left, then "World Records" link to get to good page.
- Free Motion Capture data - Full body and Face MOCAP data
- MoCapData — Over 600 Free Motion Capture data by VICON
Research groups addressing the task of Markerless Motion Capture:
