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Motion Capture: Overview
What it is and what it does.How it works:
Optical Inertial Magnetic Electro Mechanical
Motion Capture in Sports: Hockey Tennis
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What it is and what it does
Process of converting physical motion to digital representation
Motion is measured and the position in 3D space is calculated
Hardware used to measure changesSoftware used to calculate position
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How it works: Optical
Markers are placed on the actors Number of markers varies by
application/accuracy Cameras used to record movement
Generally 8 or more cameras used (depends on size of scene; high-end studios use ~70)
Software detects the markers and triangulates the position of each visible marker relative to camera position
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Optical: Markers
Passive Markers Retroreflective coating
Shines back the light sent from alight source beside the camera(like reflectors on a bike)
Active Markers LEDs used instead of reflectors Camera filters all light except for the LEDs (IR) Synchronized LEDs flash one at a time (at high
speed) this makes identifying LEDs easier
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Optical: Active Markers
RF receiver is worn on the actor
Used to synchronize LED flashes with cameras’ frame rate
1 LED visible per frame(s) Allows faster processing Requires high speed
cameras
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Optical: Cameras
Special cameras High speed
480 FPS High resolution
3600px X 3600px (12-13MP) Large Field-of-View (FOV)
60º FOV
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How it works: Inertial
Uses accelerometers and gyroscopes to measure movement Think Wii Remote with higher accuracy
The more sensors used, the better (more human) the results
Inertial provides 6 Degreed of Freedom (6DoF)
Sensitive to within 1º of rotation 6DoF:-translate X, Y, Z-pitch, yaw, roll
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How it works: Magnetic
Sensors on the actor measure low-frequency magnetic field created by the source
Control Unit correlates the locations of the sensors and source within the field
Benefits: Markers can not be occluded Fewer markers required
Drawbacks: Interference caused by steel (in
building) or other electronic devices (like monitors)
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How it works: Electro Mechanical
Exoskeletal suit is worn by actor Aluminum or plastic rods
connect multiple potentiometers which simulate joints
Gyroscopes on hips and upper back used to measure rotation
Change in voltage is measured (analog to digital) and position is calculated
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Motion Capture in Sports
FoxTrax Hockey Puck
Americans cant follow a hockey puck, so a streak was added.
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FoxTrax Hockey Puck
Standard Hockey Puck Hollowed out and circuit board placed inside Shock Sensor Infrared Emitters Internal Battery (10-30 minute duration)
Puck emitted infrared pulses that were detected by 20 different pulse detectors and 10 modified IR cameras located in the rafters.
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FoxTrax Hockey Puck
Shuttering of IR cameras synchronized with pulse detectors
Each IR camera had a 486-66 PC to process the video locally
These computers then transmitted the coordinates of the puck to an on site production trailer.
This trailer (called the ‘Puck Truck’) used the coordinates to apply a CG trail on the puck, which was then broadcast to the viewers at home.
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Hawk-Eye in Tennis
Does not need to modify the ball to track it.Uses the principles of triangulation using
visual images and timing data from 4+ high speed cameras (60 fps or higher).
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Hawk-Eye in Tennis
On each frame System identifies the ball in every image sent
from each camera Calculates the position of the ball in 3D space
by comparing each image. This is done for every instant of time to create a
path of the ball in 3D space.System can predict the path of the ball and
where the ball will strike the ground using physics.
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Hawk-Eye in Tennis
System is set up before each match with the bounds of the playing area and the rules of the game, thus allowing it to make accurate calls (within 2-3mm)
It uses 4-6 high speed cameras set at different locations around the court.
Each camera feeds into a high speed video processor which runs the ball tracking analysis.
3D coordinates are then passed to a central computer which runs the physics algorithms.
Data is then stored into a database and can be used in recall situations or to track player stats (shot percentages, etc.)
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References
http://www.metamotion.com/motion-capture/motion-capture.htm http://entertainment.howstuffworks.com/king-kong2.htm http://www.metamotion.com/PhaseSpace/optical-motion-capture-
cameras.html http://www.intersense.com/InertiaCube_Sensors.aspx Johnny Lee’s Wiimote Projects: http://johnnylee.net/projects/wii/ http://www.metamotion.com/motion-capture/magnetic-motion-capture-1.htm http://www.metamotion.com/gypsy/gypsy-motion-capture-system-
mocap.htm http://en.wikipedia.org/wiki/FoxTrax http://en.wikipedia.org/wiki/Hawk-Eye
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