About Us: Motion Capture

A virtual look at a person within the CAREN system.

Intricate hand capture

Motion capture is the name given the technique (or several techniques), which enables to obtain motion data from human performers/subject. It is used for many application areas such as biomechanics, sport performance analysis, tele-robotics, ergonomics, entertainment and so on. Motion capture is based on the idea of tracking key points, usually joints, of the subject. The most common technologies for motion capture are Electro Magnetic (using electro magnetic fields and magnetic sensors) and Optical (Using special markers that reflect back infra-red light coming from special cameras).

Motion capture is widely used in the video game and entertainment industry, to generate realistic character's motion, in an easy and cost effective way. It allows a director to place an actor in a scene that may be too impractical or dangerous to film in real life. Some motion capture systems provide real-time feedback of the animation created (rather like instant rushes on a film set). This allows the director to see immediately whether or not the motion works well for the digital characters. Motion capture can be applied to full body motion, as well as to articulate hand animation, facial animation and real time lip sync.

Motion capture is used in medical, simulation, engineering and ergonomic applications for the creation of generic and special purpose virtual reality character integration. It's also used in the entertainment industry for feature films, advertising, TV and 3D computer games.

MOTEK specializes in several different motion capture methods; Optical, Electro-magnetic, Customized Video based motion capture system for facial animation, Accelerometers and more.

OPTICAL CAPTURE

There are two types of optical capture systems available commercially today - Active Optical and Passive Optical systems. They both use the same underlying principals. A series of cameras placed around the capture space track the positions of markers attached to the body of performers. Triangulation is used to compute the 3D position of a marker at any given sample, from an array of 2D information from every camera. Passive optical systems use retro-reflective markers, while active systems use illuminating elements as markers. Most systems currently use passive markers, that being because they achieve greater speeds. Template matching software is used in order to solve occlusions and marker disappearance problems. MOTEK uses VICON 8 system with 8 to 24 motion capture cameras and Diva software next to VICON's workstation software for data processing. Another Optical system used is the PRIMAS, developed at T. U. Delft in The Netherlands.

MAGNETIC CAPTURE

There are systems using AC fields and others using DC magnetic fields. Sensors, attached to various parts of the performer's body, are usually connected to the recording device via cables, however, some developments constructed systems that were un tethered. Un tethered systems require transmitters, batteries and connecting wires between sensors and transmitter.

The recording device that each sensor is connected to has an interface that can be synchronized so as to prevent a data skew.

The resulting data stream consists of 3D positions and orientations for each receiver.

This data is typically applied to an inverse kinematics system, to drive an animated skeleton. Magnetic system are affected by any sizable areas of metal in vicinity of the capture area.

FACIAL CAPTURE

There are several systems and techniques for facial animation. Some uses special helmet with video camera or infra red camera and small reflective markers on the actors face, some use video-analysis facial systems, voice recognition systems, text recognition systems and so on. MOTEK offers two techniques for facial animation. The first is using VICON motion capture system, where optical markers are being attached to the actors face and the rest is being done using the same techniques as in full body motion capture session. The second system is customized system, which uses special helmet with a lipstick/finger video camera, and customized video tracking software (which does not need any markers or sensors) developed by U. K. based Image-Metrics

MOTION BASE

MOTEK has changed the process of using motion platforms for recording of simulations, through its patented D-Flow^tm technology, using a fast feedback loop involving motion capture and a motion base.

The D-Flow technology changes the nature of the whole process of developing a virtual environment, making it an intuitive, rather than a linear, programming experience. Motion bases are being used for the capture of dynamic virtual environments

D-FLOW

D-Flow has several modules:

• SoftLoad; which imports scenes from Softimage3D, containing the geometry of characters and environments, I/O channel connections, IK skeletons of characters, lighting setup and textures
• Camo; which coordinates the data flow through online human sub-mix, into the various outputs
• Perfly; the SGI-based performer display viewing environment, which allows for extensive debugging and optimizations in run-time

OTHER METHODS

Inertial devices

Inertial measurement includes such devices as accelerometers, gyros, and devices for measuring orientation or acceleration of moving vehicles. Accelerometers are devices for measuring the rate of change in velocity and can provide estimations of distance or be used to detect high forces. Much of the initial research, development and marketing in this area were for military applications.

Range finding devices

Principally there are four basic techniques for distance measurement using electromagnetic radiation. All are used in practice for distance measurement depending on the particular application.

• Pulse Timing - Pulse timing involves measuring the round-trip time for a signal to be transmitted to a reflective surface and return. This is the principle used in Radar, DME for aircraft, LORAN, Satellite Altimetry, Airborne RADAR Altimetry, Lunar Laser Ranging etc. Some of the newer EDM instruments used by surveyors are also using pulse timing and accuracies of +/-5mm are possible. 
  
• Phase Comparison - Phase difference involves the use of a carrier wave which may be modulated at different wavelengths. By measuring the difference in phase between the transmitted signal and the received signal after it has been reflected from the other end of the target, the distance can be determined as an integer number (unknown) of wavelengths plus a fraction of a wavelength which is known from the phase comparison.
  
• Doppler Methods - Doppler techniques were used in the earlier satellite positioning systems. The received frequency of a low orbit satellite is compared with the actual transmitted signal as a function of time. The rate of change of frequency gives the slant range between the satellite and the observer while the instant when the two frequencies are the same gives the point of closest approach.

Force/torque, accelerometers, tactile:

Force measurement provides indications of magnitude and direction of forces for use in manipulation or locomotion. A variety of control schemes have been implemented in force controlled systems to allow smooth and accurate control in situations that would otherwise be precluded without such devices. A number of load cells and acceleration measuring devices exist.

Shape Memory Materials:

Nickel-titanium alloys were first discovered by the Naval Ordinance Laboratory decades ago and the material was termed NiTinOL. These materials have the intriguing property that they provide actuation through cycling of current through the materials. It undergoes a 'phase change' exhibited as force and motion in the wire.

At room temperature Muscle Wires are easily stretched by a small force. However, when conducting an electric current, the wire heats and changes to a much harder form that returns to the "un stretched" shape - the wire shortens in length with a usable amount of force.

Nitinol can be stretched by up to eight percent of their length and will recover fully, but only for a few cycles. However when used in the three to five percent range, Muscle Wires can run for millions of cycles with very consistent and reliable performance.