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Motion Tracking Tutorial

Motion Tracking Technology

According to MEDIALAB's David J. Sturman, motion capture is "The recording of human body movement for analysis and playback. The information captured can be as simple as the position of the body in space or as complex as the deformations of the face and muscle masses. Motion capture for computer character animation involves the mapping of human motion onto the motion of a computer character. The mapping can be direct, such as human arm motion controlling a character's arm motion, or indirect, such as human hand and finger patterns controlling a character's skin color or emotional state."

Tracking systems fall into three classes:

Inside-in - employ sensors and sources that are both on the body. The sensor form factor is usually small and while they capture body movement, are considered obtrusive and therefore generally do not provide 3D world-based information. An example of inside-in technology would be a glove with flex sensors.

Inside-out - employ sensors on the body that sense artificial external sources (coil moving in an externally generated electromagnetic field) or natural sources (mechanical head tracker using a wall as a reference). These systems do provide world-based information, but their workspace is limited due to the use of external sources. Their formfactor is also restricted to medium/large sized bodyparts.

Outside-in - employ an external sensor that senses artificial sources or markers on the body (videocamera based system that tracks the pupil and cornea). These systems are considered the least obtrusive, but they suffer from occlusion (complete obstruction) and a limited workspace.

Technologies for Tracking

Mechanical

Mechanical tracking uses a system in which a mechanical armature connects the top of a helmet to an encoding device on the ceiling. As the user changes head position, the helmet moves the upper device, and data related on that movement is relayed to the computer. While this is the most precise method of tracking, it is, at the same time, very limiting.

Acoustic

Acoustic trackers use high-frequency sound to triangulate a source within the work area. These systems rely on line-of-sight between the source and the microphones, and can therefore suffer from acoustic reflections if surrounded by hard walls or other acoustically reflective surfaces.

Magnetic motion capture systems

This type of tracking system does not rely on line-of-sight observation, as do optical and acoustic systems. Instead they use a source element radiating a magnetic field and a small sensor that reports its position with respect to the source. Systems are very complex and multi-source, multi-sensor systems will track a number of points at up to 100 Hz in ranges from 3 to 20 feet, which are generally accurate to better than 0.1 inches in position and 0.1 degrees in rotation.

Optical motion capture systems

These systems use high contrast video imaging and retro-reflective markers which are attached to an object whose motion is being recorded. The number of cameras used depends on the type of motion capture. For instance, facial motion capture usually uses one or two cameras while full body motion capture may use four or more cameras.

Before motion capture process is begun, a calibration frame (measurement of 3D array of markers) is recorded. This defines the frame of reference for the motion capture session. After a motion capture session, the recorded motion data must be post-processed or tracked. The centroids of the marker images (either computed then, or recalled from disk) are matched in images from pairs of cameras, using a triangulation approach to compute the marker positions in 3D space. Each marker's position from frame to frame is then identified.

Tracking time depends on the quality of the captured data and the fidelity required. For straightforward data, tracking can take only one to two minutes per captured second of data (at 120 Hz). For complicated data, tracking time can take 15 to 30 minutes per captured second.

Ascension Technologies

MotionStar is a magnetic motion-capture system used for character animation and biomechanics. The system can capture the motions of up to 90 sensors mounted on performers interacting on a stage using DC magnetic sensors to overcome blocking and post processing delays. Each sensor is tracked up to 120 times per second to capture and filter motions.

It utilizes a single rack-mounted chassis for each set of 18 sensors. The rack-mounted chassis houses all electronics and a high speed ethernet interface to the host computer. Each sensor is connected to the chassis by a 35-foot cable. MotionStar also includes an extended range transmitter and controller unit.

The system is supported by animation software packages including Alias/Wavefront, Softimage, 3D Studio MAX and is fully supported by Kaydara's FiLMBOX and DreamTeam's Typhoon.

The MotionStar system was recently used by Spectrum Studios and Virtual Ventures at the 2000 Awards Show of the Academy of Interactive Arts and Sciences, to co-produce a live animation in which actor Martin Short interacted with Ms. AIAS, a female 3D animated host. Ms. AIAS' voice and body movements were performed by actress Mary Ann Daniel, who was located on a motion capture stage one floor below the actual show stage. Daniel's motions were tracked by a MotionStar Wireless tracker, which transmitted motion data to a host computer where they were mapped onto the character.

Ascension also has optical tracking systems. Their newest release is called ReActor and is a system that uses 540 digital cameras to cover performers wearing up to 30 active optical markers. The system uses IMRTM (Instant Marker Recognition) to reacquire blocked data by tracking all markers even if momentarily blocked.

Their products are also used in simulation and training, virtual reality, medical imaging and biomechanics applications.

Competitors

InterSense
MotionAnalysis
InMotion Systems
PhoeniX Technologies
Polhemus

Additional sources of information*

Hand Centered Studies of Human Movement, Simon Fraser University
Motion Capture White Paper, Alias/Wavefront and Windlight Studios
Position Tracking and Mapping, National Academy Press
VR News Technology Review

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Page updated 1/24/07
Copyright 4th Wave Inc, 2007