Motion Capture Hardware

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Motion Capture Hardware

• Craig Maki and Ned Kahvo

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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 changes
• Software 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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Hawk-Eye in Tennis
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References

• http//www.metamotion.com/motion-capture/motion-ca
  pture.htm
• http//entertainment.howstuffworks.com/king-kong2.
  htm
• http//www.metamotion.com/PhaseSpace/optical-motio
  n-capture-cameras.html
• http//www.intersense.com/InertiaCube_Sensors.aspx
  
• Johnny Lees Wiimote Projects http//johnnylee.ne
  t/projects/wii/
• http//www.metamotion.com/motion-capture/magnetic-
  motion-capture-1.htm
• http//www.metamotion.com/gypsy/gypsy-motion-captu
  re-system-mocap.htm
• http//en.wikipedia.org/wiki/FoxTrax
• http//en.wikipedia.org/wiki/Hawk-Eye