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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 actorsNumber of markers varies by

application/accuracy

Cameras used to record movementGenerally 8 or more cameras used

(depends on size of scene; high-endstudios use ~70)

Software detects the markers andtriangulates the position of eachvisible marker relative to cameraposition

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Optical: Markers

Passive MarkersRetroreflective coatingShines back the light sent from a

light source beside the camera(like reflectors on a bike)

Active MarkersLEDs 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 theactor

Used to synchronize LED

flashes with camerasframe rate

1 LED visible per frame(s)

Allows faster processingRequires 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)

60 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 1 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 bythe source

Control Unit correlates the locations

of the sensors and source within thefield

Benefits:Markers can not be occluded

Fewer markers required

Drawbacks: Interference caused by steel (in

building) or other electronic devices (likemonitors)

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How it works: Electro Mechanical

Exoskeletal suit is worn by

actor

Aluminum or plastic rods

connect multiple potentiometerswhich 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 PuckHollowed out and circuit board placed inside

Shock Sensor

Infrared EmittersInternal Battery (10-30 minute duration)

Puck emitted infrared pulses that were

detected by 20 different pulse detectorsand 10 modified IR cameras located in therafters.

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FoxTrax Hockey Puck

Shuttering of IR cameras synchronized withpulse detectors

Each IR camera had a 486-66 PC to process thevideo locally

These computers then transmitted thecoordinates of the puck to an on site productiontrailer.

This trailer (called the Puck Truck) used the

coordinates to apply a CG trail on the puck,which was then broadcast to the viewers athome.

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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 frameSystem identifies the ball in every image sent

from each camera

Calculates the position of the ball in 3D spaceby comparing each image.

This is done for every instant of time to create apath of the ball in 3D space.

System can predict the path of the ball andwhere the ball will strike the ground usingphysics.

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Hawk-Eye in Tennis

System is set up before each match with the bounds ofthe playing area and the rules of the game, thus allowingit to make accurate calls (within 2-3mm)

It uses 4-6 high speed cameras set at different locationsaround the court.

Each camera feeds into a high speed video processorwhich runs the ball tracking analysis.

3D coordinates are then passed to a central computerwhich runs the physics algorithms.

Data is then stored into a database and can be used inrecall situations or to track player stats (shotpercentages, etc.)

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Hawk-Eye in Tennis

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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 Lees 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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