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Message: <blockquote data-quote="sihina_lahiru" data-source="post: 7400840" data-attributes="member: 149577">Several markers are placed at specific points on an actor's face during facial optical motion capture   Passive optical system use markers coated with a <a href="http://en.wikipedia.org/wiki/Retroreflective" target="_blank">retroreflective</a> 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 <a href="http://en.wikipedia.org/wiki/Gaussian" target="_blank">Gaussian</a>.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. Instead, hundreds of rubber balls are attached with reflective tape, which needs to be replaced periodically. The markers are usually attached directly to the skin (as in biomechanics), or they are <a href="http://en.wikipedia.org/wiki/Velcro" target="_blank">velcroed</a> 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 balanced between resolution and speed: a 4-megapixel system normally runs at 370 hertz, 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.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.The TV series ("Stargate SG1") episode was produced using an active optical system for the VFX. The actor had to walk around props that would make motion capture difficult for other non-active optical systems.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.Time modulated active marker  <a href="http://en.wikipedia.org/wiki/File:Activemarker2.PNG" target="_blank"><img src="http://upload.wikimedia.org/wikipedia/commons/thumb/6/6d/Activemarker2.PNG/300px-Activemarker2.PNG" alt="" class="fr-fic fr-dii fr-draggable " style="" /></a>  <a href="http://en.wikipedia.org/wiki/File:Activemarker2.PNG" target="_blank"><img src="http://bits.wikimedia.org/skins-1.5/common/images/magnify-clip.png" alt="" class="fr-fic fr-dii fr-draggable " style="" /></a> 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. <a href="http://en.wikipedia.org/wiki/File:PrakashOutdoorMotionCapture.jpg" target="_blank"><img src="http://upload.wikimedia.org/wikipedia/en/thumb/e/ec/PrakashOutdoorMotionCapture.jpg/300px-PrakashOutdoorMotionCapture.jpg" alt="" class="fr-fic fr-dii fr-draggable " style="" /></a>  <a href="http://en.wikipedia.org/wiki/File:PrakashOutdoorMotionCapture.jpg" target="_blank"><img src="http://bits.wikimedia.org/skins-1.5/common/images/magnify-clip.png" alt="" class="fr-fic fr-dii fr-draggable " style="" /></a> <a href="http://en.wikipedia.org/wiki/Infrared" target="_blank">IR</a> sensors can compute their location when lit by mobile multi-LED emitters, e.g. in a moving car. With Id per marker, these sensor tags can be worn under clothing and tracked at 500 Hz in broad daylight.    Semi-passive imperceptible marker One can reverse the traditional approach based on high speed cameras. Systems such as Prakash 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 appears ideal for on-set motion capture or real-time broadcasting of virtual sets but has yet to be proven.Markerless Emerging techniques and research in <a href="http://en.wikipedia.org/wiki/Computer_vision" target="_blank">computer vision</a> are leading to the rapid development of the markerless approach to motion capture. Markerless systems such as those developed at <a href="http://http//www.stanford.edu/group/biomotion/" target="_blank">Stanford</a>, 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. Several commercial solutions for markerless motion capture have also been introduced.Inertial systems Inertial Motion Capture technology is based on miniature inertial sensors, biomechanical models and sensor fusion algorithms. The motion data of the inertial sensors (<a href="http://en.wikipedia.org/wiki/Inertial_guidance_system" target="_blank">inertial guidance system</a>) is often transmitted wirelessly to a computer, where the motion is recorded or viewed. Most inertial systems use gyroscopes to measure rotational rates. These rotations are translated to a skeleton in the software. Much like optical markers, the more gyros the more natural the data. 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, portability, and large capture areas. Disadvantages include lower positional accuracy and positional drift which can compound over time. These systems are similar to the Wii controllers but are more sensitive and have greater resolution and update rates. They can accurately measure the direction to the ground to within a degree. The popularity of inertial systems is rising amongst independent game developers, mainly because of the quick and easy set up resulting in a fast pipeline. A range of suits are now available from various manufacturers and base prices range from $25,000 to $80,000 USD.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. Performers 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 absolute 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. The sensor output is <a href="http://en.wikipedia.org/wiki/6DOF" target="_blank">6DOF</a>, which provides useful results 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.Related techniques [<a href="http://en.wikipedia.org/w/index.php?title=Motion_capture&action=edit&section=17" target="_blank">edit</a>] Facial motion capture Main article: <a href="http://en.wikipedia.org/wiki/Facial_motion_capture" target="_blank">Facial motion capture</a>Most traditional motion capture hardware vendors provide for some type of low resolution facial capture utilizing anywhere from 32 to 300 markers with either an active or passive marker system. All of these solutions are limited by the time it takes to apply the markers, calibrate the positions and process the data. Ultimately the technology also limits their resolution and raw output quality levels.High fidelity facial motion capture, also known as performance capture, is the next generation of fidelity and is utilized to record the more complex movements in a human face in order to capture higher degrees of emotion. Facial capture is currently arranging itself in several distinct camps, including traditional <a href="http://en.wikipedia.org/w/index.php?title=Vicon&action=edit&redlink=1" target="_blank">Vicon</a> based motion capture data, blend shaped based solutions, capturing the actual topology of an actor's face, and proprietary systems.RF positioning 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.Non-traditional systems An alternative approach was developed where the actor is given an unlimited walking area through the use of a rotating sphere, similar to a <a href="http://en.wikipedia.org/wiki/Hamster_ball" target="_blank">hamster ball</a>, 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. Another alternative 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.In popular culture In <a href="http://en.wikipedia.org/wiki/The_Simpsons" target="_blank">The Simpsons</a> episode "<a href="http://en.wikipedia.org/wiki/HOMR" target="_blank">HOMR</a>," <a href="http://en.wikipedia.org/wiki/Homer_Simpson" target="_blank">Homer</a> rashly invests all his savings in the fictitious motion capture corporation Animotion, for no reason other than that he enjoyed demonstrating it while it was exhibited at an animation festival that he was visiting.</blockquote>

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