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A COMPARISON AND EVALUATION OF MOTION INDEXING ...

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B.2 Motion Capture Technologies<br />

1. Mechanical: In mechanical motion capture the performer wears a very basic<br />

skeleton set made of straight metal pieces which is hooked to the back body<br />

and directly track body joint angles. Hence, it is also known as exo-skeleton<br />

motion capture systems. As the performer moves, these articulated mechani-<br />

cal parts also move with sensors feeling the joints rotation and measuring the<br />

performer’s relative motion. Variations of mechanical mocap systems include<br />

gloves, mechanical arms, or articulated models.<br />

Mechanical mocap systems provide occlusion-free, low cost, and unlimited vol-<br />

umes of capture. These systems are real-time and wireless in nature, but they<br />

often need calibration. They also need extra sensors to determine the perform-<br />

ers direction. Motion captured using mechanical systems are not interfered by<br />

light or magnetic fields. However, the absolute positions are not known and<br />

are estimated from the rotations. Therefore, these type of mocap system is<br />

unaware of ground. Hence, a jumping action cannot be cannot be captured due<br />

to possible feet sliding. The range of the suits used in mechanical mocap lies<br />

in the range of $25,000 to $75,000 which excludes the cost for extra positioning<br />

system.<br />

2. Acoustic: This type of motion capture systems uses a triad of audio receivers<br />

and an array of transmitters. These transmitters are attached to performers<br />

body and are triggered sequentially to produce a click sound. The receivers<br />

measures the time the sound took to reach it from each transmitter. The cal-<br />

culated distance of the three receivers is triangulated to provide a point in 3D<br />

space. This positional data serves as input for an inverse kinematic system to<br />

drive the animated skeleton. This method doesnot suffer from any occlusion is-<br />

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