1. Motion Capture Hardware
Craig Maki and Ned Kahvo

2. Motion Capture: Overview
What it is and what it does.
How it works:
Optical
Inertial
Magnetic
Electro Mechanical
Motion Capture in Sports:
Hockey
Tennis
Motion Capture, Craig Maki and Ned Kahvo

3. 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
Motion Capture, Craig Maki and Ned Kahvo

4. Motion Capture, Craig Maki and Ned Kahvo
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
Motion Capture, Craig Maki and Ned Kahvo

5. Motion Capture, Craig Maki and Ned Kahvo
Optical: Markers
Passive Markers
Retroreflective coating
Shines back the light sent from a light 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
Retroreflective:
-the contrast on the cameras is adjusted to darken out the actor and leave the markers visible
-or infrared light is used and visible light is filtered out
Motion Capture, Craig Maki and Ned Kahvo

6. 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
Motion Capture, Craig Maki and Ned Kahvo

7. Motion Capture, Craig Maki and Ned Kahvo
Optical: Cameras
Special cameras
High speed
480 FPS
High resolution
3600px X 3600px (12-13MP)
Large Field-of-View (FOV)
60º FOV
Motion Capture, Craig Maki and Ned Kahvo

8. Motion Capture, Craig Maki and Ned Kahvo
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
Motion Capture, Craig Maki and Ned Kahvo

9. Motion Capture, Craig Maki and Ned Kahvo
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)
Motion Capture, Craig Maki and Ned Kahvo

10. 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
Motion Capture, Craig Maki and Ned Kahvo

11. Motion Capture in Sports
FoxTrax Hockey Puck
Americans cant follow a hockey puck, so a streak was added.
Motion Capture, Craig Maki and Ned Kahvo

12. Motion Capture, Craig Maki and Ned Kahvo
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.
Motion Capture, Craig Maki and Ned Kahvo

13. Motion Capture, Craig Maki and Ned Kahvo
FoxTrax Hockey Puck
Shuttering of IR cameras synchronized with pulse detectors
Each IR camera had a 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.
Motion Capture, Craig Maki and Ned Kahvo

14. Motion Capture, Craig Maki and Ned Kahvo
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).
Motion Capture, Craig Maki and Ned Kahvo

15. Motion Capture, Craig Maki and Ned Kahvo
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.
Motion Capture, Craig Maki and Ned Kahvo

16. Motion Capture, Craig Maki and Ned Kahvo
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.)
Motion Capture, Craig Maki and Ned Kahvo

17. Motion Capture, Craig Maki and Ned Kahvo
Hawk-Eye in Tennis
Motion Capture, Craig Maki and Ned Kahvo

18. Motion Capture, Craig Maki and Ned Kahvo
References
Johnny Lee’s Wiimote Projects:
Images:
Motion Capture, Craig Maki and Ned Kahvo