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- Motion capture is the creation of a 3D representation of a live performance.
- Optical systems triangulate the 3D position of a marker with a number of cameras with high precision (millimeter resolution or better). 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. 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. A newer technique discussed below uses higher resolution linear detectors to derive the one dimensional positions, requiring more sensors and more computations, but providing higher resolutions (sub millimeter down to 10 micrometres time averaged) and speeds than possible using area arrays [1] [2]. Passive optical systems use reflective markers and identify each marker from its relative location, with the aid of kinematic constraints and predictive gap filling algorithms. These systems are popular for entertainment, biomechanics, engineering, and virtual reality applications; tracking a large number of markers and expanding the capture area with the addition of more cameras. Unlike active marker systems and magnetic systems, passive systems do not require the user to wear wires or electronic equipment. Passive markers are usually spheres or hemispheres made of plastic or foam 25 to 3mm in diameter with special retroreflective tape. Manufacturers of this type of system include Vicon-Peak [3], Motion Analysis [4] and BTS [5]. Active marker systems have an advantage over passive in that there is no doubt about which marker is which. In general, the overall update rate drops as the marker count increases; 5000 frames per second divided by 100 markers would provide updates of 50 hertz. As a result, these systems are popular in the biomechanics market. Two such active marker systems are Optotrak by Northern Digital [6] and the Visualeyez system by PhoeniX Technologies Inc.[7]. Image:Activemarker2.jpg Newer active marker systems such as PhaseSpace [8] modulate the active output of the LED to differentiate each marker, allowing several markers to be on at the same time, while still providing the higher resolution of 3,600 x 3,600 or 12 megapixel resolution while capturing at 120 (128 markers or four persons) to 480 (32 markers or single person) frames per second. The advantage of using active markers is intelligent processing allows higher speed and higher resolution at a lower price which competes with magnetic and inertial systems but provides the submillimeter resolution of optical systems. This higher accuracy and resolution requires more processing than older 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. By using newer processing and technology, these motion capture systems are about 1/3 the cost of passive 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, epecially 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 "DC" and "AC" systems: one uses square pulses, the other uses sine wave pulses. Two magnetic systems are Ascension technology and Polhemus. 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. 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.
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