motion tracking - ucl computer science · 2009-01-26 · wii remote & sensor bar optical...
TRANSCRIPT
Motion Tracking Michael Donnerer, Sandeep Rakhra
Overview
Introduction
The ideal motion tracker
Possible errors and problems
Current Technologies
Conclusion
Why do we need motion tracking?
View control for immersive environments
Cave™, head mounted displays
Navigation in a virtual environment
Intuitive object selection and manipulation
Tracked gloves
Instrument Tracking
Computer-aided surgery
Why do we need motion tracking?
Orad TrackVision
Motion Capture for Animation
Movies & video games
Augmented Reality
Add virtual features to real world
X-Ray vision, information
Camera tracking – CGI effects
Movies, news, sports (football – offside)
What would an ideal device look like? Size
Tiny would be nice
Self contained
No other parts on the user or on the environment
6 Degrees of Freedom
Position and orientation
What would an ideal device look like? Accuracy in position and orientation
Low latency & High update rate
For example 1,000 Hz with latency less than 1ms
Tenacity
Track fast and far objects
What would an ideal device look like? Immunity to occlusion
Needing no clear line of sight
Robustness
Light, sound, heat, magnetic fields, radio waves, and other
trackers don’t interfere
What would an ideal device look like? Autonomy
Wireless, battery powered 3 years on coin sized battery
Cost
Cheap
Some Common Problems
Some Common Problems Spatial distortion
Constant difference between reported and actual position/
orientation
Spatial jitter
Changing difference between reported and actual position/
orientation
Special case: Creep – constant increase in error over time
Some Common Problems Slow update rate and/or high latency
Latency Jitter
Cycle to cycle variations in latency
Overcoming Latency Latency can be a big problem
Especially for fast objects
Motion prediction - Azuma 1995[1]
Extrapolate forward from most recent data
“like driving a car by only looking in the rear-view mirror”
Mechanical Sensing Uses in object tracking
Mechanical Trackers Conceptually simple approach
Articulated arm
Counterweights
Physical linkage between tracked target and environment
Use potentiometers, shaft encoders or optical technology
Easy to get target position and orientation
SensAble PHANTOM® 1.5/6DOF
Mechanical Trackers Most common uses
Precision 3D digitization over a small area
Precise measurements in engineering
SensAble PHANTOM® Desktop™ Faro Tech. FaroArms
Mechanical Trackers Very precise and accurate
High update rate and low latency
Can be integrated with force-feedback
Mechanical Trackers Generally limited to single target
Small range of motion
Typically under a cubic meter
Inertial Sensing Uses in object tracking
Inertial Trackers First used in ships, submarines and airplanes in 50’s
Early systems used gyroscopes far too heavy to be attached to a
person
MEMS (microelectronic mechanical systems) inertial sensors in 90’s
Small and lightweight
Xsens MTi
Inertial Trackers Modern systems use “strapdown INS*”:
Three orthogonal angular-rate gyroscopes
Three linear accelerometers
Resulting data double integrated to get position and orientation
Gravity must be accounted for (g)
(*inertial navigation system)
Nintendo Wii Remote
Inertial Trackers Small size
Completely self contained
No line of sight
Very low latency
Typically a couple of milliseconds
High update rate
Thousands of samples per second
Extremely low jitter
Animazoo IGS-190-M mobile
Inertial Trackers Drift
Bias error of just 1 milli-g in accelerometer causes drift of 4.5m
within 30s
1 milliradian error coming from gyroscopes would produce same
effect
Even very good gyroscopes that you wouldn’t wear on your
head drift by a milliradian within a short time
Inertial Trackers Hybrids:
Compass (Xsens MTi)
GPS (Xsens MTi-G)
Camera/Visual
Others… Xsens Mti-G
Demo
Acoustic Sensing Uses in object tracking
Acoustic Trackers Use transmission and sensing of sound waves
Sonar
Commercial system operate by timing the flight duration of a brief
ultrasonic pulsec
Intersense IS-900 Receiver and Transmitter components
(inertia & ultrasonic tracking)
Acoustic Trackers Multi-pathing - direct and indirect signals
Amplitude and phase of the signal can change wildly
Pulsed time-of-flight acoustic overcomes this by waiting until signal
has arrived
Acoustic Trackers Omni-directional system needs small speakers and microphones
But efficiency proportional to the active surface area
Line of sight required
But somewhat tolerant to occlusions
Acoustic Trackers Higher frequencies are beneficial to avoid interference
Most noise sources fall off with increasing frequency
Frequency dependent attenuation of sound in air starts to be
significant at 40kHz and dominant by 80 kHz
Accuracy can be affected by
Wind
Uncertainty in the speed of sound
Temperature, humidity
About one-millimeter error per degree Fahrenheit at one
meter
Acoustic Trackers Update rate limited by reverberation
5 to 100 ms may be necessary because of echoes
Update rates can be as slow as 10 Hz.
Latency in general under 15ms
Affected by distance
Radio and Microwave tracking works in a similar way
Optical Sensing Uses in object tracking
Rely upon measurements from reflected or emitted light
Consist of two major components: light source and optical sensors
Light sources can be passive or active
Two common sensors used in industry…
Introduction
What is optical sensing in a nutshell? Tracking objects in space by measuring light intensity or processing images.
Analog Photosensors
Fast and Cheap
CCD (Charged Coupled Devices)
Computationally Expensive
Wii Remote & Sensor Bar Optical Sensing your mum’s familiar with…
Capable of tracking hand motion in space
Uses a infrared camera housed in the remote in conjunction with a
sensor bar emitting infrared via LEDs
Inside-out methodology
Houses accelometers, so inertial
tracking too
Uses triangulation
Outside-in methodology used in other
home brew Wii remote applications.
Reference Johnny Lee
Not just a toy…
Image based optical trackers use images from a camera and sample
for a predefined target or search for features
CCDs fall into this category
Microsoft researcher recently launched tracking software based on
this principle
Tracking Image Features
Demo
Advantages & Disadvantages Outside-in or Inside-out? Both possible!
Outside-in requires the tracked object to reflect or emit light. This
is cheaper and light-weight (good for tracking humans)
Tracking volume increased by addition of more sources – fairly
cheap for optical sources
Analog sensors are cheap and with active light sources provide fast
updates and good spatial precision
Targets orientation is difficult to determine
Require a clear line of sight! Multiple tracked objects interfere and
objects in the environment don’t help
Magnetic Sensing Uses in object tracking
Function by measuring the strength of the magnetic fields using
magnetometers or current induced in an electromagnetic coil when a
changing magnetic field passes through the coil
Three coils or magnetometers are embedded in a small unit that is
attached to whatever the system needs to track
Measuring the magnetic fields generated on each of the three
perpendicular coils or magnetometers give rise to a 3D vector
From vector it is possible to determine the position and orientation
of the sending unit
Introduction
What is magnetic sensing in a nutshell?
Tracking objects in space by measuring magnetic fields.
Advantages & Disadvantages May experience interference operating in the vicinity of
ferromagnetic objects
May experience interference operating in the vicinity of metal
object
Need to work in a small volume to obtain accuracy
Possible slow updates as coils subside before continuing
measurements take (not for magnetometers)
No line of sight issues!
One source can excite many sensors hence the ability to track many
objects form single source so cheaper in this sense. Reference Polhemus
Component is small and light-weight
Hybrid Sensing Uses in object tracking
Hybrid Tracking
There is no single solution to tracking that suits all applications.
We can select several solutions and combine their advantages to
produce better tracking results
Example, the Wii Remote using optical and inertial sensing to fully
track hand movements
The Wii Motion Plus provides more accurate tracking for rotational
movements
Many more examples…
Conclusion
Tracking is essential for VE (Virtual Environments) but there is no silver bullet solution.
Questions?
References: [1] R. Azuma, Predictive Tracking for Augmented Reality, PhD dissertation, tech. report TR95-007, Univ. North Carolina,
Chapel Hill, Dept. Computer Science, 1995
General: Greg Welch, Eric Foxlin, "Motion Tracking: No Silver Bullet, but a Respectable Arsenal," IEEE Computer Graphics and
Applications, vol. 22, no. 6, pp. 24-38, November/December, 2002.