DynaSight Tracking

We recently borrowed an Origin Instruments DynaSight Sensor to try out with the passive stereo VR system at UB. The DynaSight is an optical tracking device that uses two infrared cameras to track the 3D position of a retroreflective target.

Our Setup

We placed the "transmitter" box immediately below the bottom edge of the screen (about 2 feet above the floor), tilted up at a 25 degree angle. This gave us good coverage of the normal working area. Because this was just a temporary test, we simply placed it on a folding chair; for public use we need to find a more elegant mounting solution. We would've also liked to try it with the transmitter above the top of the screen, looking down, but had no easy way to mount it securely.

The tracking target was a 20 mm disk, attached to a pair of polarized stereo glasses.

Evaluation

Overall we were very pleased with the tracker. Positive points about it are:

• Not having to wear a clumsy sensor + long wire makes the whole system much nicer.
• Latency appears to be very low.
• Accuracy is better than that of the Spacepad.
• It was very good at keeping track of the target. You'd have to move the glasses fairly fast for it to temporarily lose them. But in those cases, or when you look away from the screen & look back, it re-acquires the target almost instantly.
• Coverage was better than expected - it was able to track a user over the full width of the screen, up to about 1.5 feet from the screen, and back to at least 6 feet from the screen.

Some negative points about the tracker are:

• The data are slightly noisy. When standing still, the image of the virtual world would often be seen to jump up & down by 1-2 pixels. We added some smoothing to our tracking daemon, which alleviated this.
• When running at higher reporting rates (60-70 Hz; the default mode is 30 Hz), the noise is worse.
• It only tracks position, not orientation. For head tracking with a single screen, this usually isn't a problem.
• Only one target is tracked. Our tracker daemon for the DynaSight includes an option to merge in data from another daemon, so we were able to use the Spacepad to track the wand along with the DynaSight for the glasses (the latest trackd from VRCO may be able to do something similar).

It should be noted that Origin Instruments also sells an ActiveTarget Adapter that allows you to track up to four targets and get orientation as well as position data. However, this appears to be larger, wired target, and so wouldn't really be an improvement over the Spacepad or Flock of Birds in terms of basic usability. Whether it's more accurate or has lower latency than the electromagnetic trackers is unknown.

Also, the DynaSight clearly cannot be used in a full CAVE system as it stands. One would need to use several transmitters to cover all possible directions that users might look, and would have to come up with some good way to get orientation data (since this is necessary for proper stereo when the user looks at different walls).

Tracker Daemon

We have written two different tracker daemons for the DynaSight. The first is just a very minimal daemon that reads the tracking data and writes it to trackd shared memory. The second adds several features, including smoothing, rotating the transmitter, and adding data from another trackd; this daemon has been tested under both Linux and IRIX 6.5. The source code for both programs is below; they both use the same dyna-trackd.h header file.

dyna-trackd-basic.c
dyna-trackd2.c
dyna-trackd.h

dyna-trackd2 Usage

The command-line options for dyna-trackd2 are as follows:

--shem key

Tells the daemon to use the shared memory segment key number key. The default is 9900.

--port portname

Gives the device name of the serial port that the DynaSight is connected to. The default under Linux is /dev/ttyS0; the default under IRIX is /dev/ttyd2.

--offset x y z

An x/y/z offset to add to the position data. The value should be in feet. The default is 0 0 0.

--xrot angle

The rotation of the transmitter about the X axis (i.e. how much it's tilted up or down). The value is in degrees. The default is 0.

--mode retro

--mode stereosync

--mode stereosync60

Sets the tracker to "Retro", "StereoSync", or "StereoSync60" mode. In Retro mode it will report at 30 Hz; in StereoSync, with no stereo input, it will report updates at 37 Hz; in StereoSync60, with no stereo input, it will report at 64 Hz. See DynaSight manual for further details. The default mode is retro.

--smooth num-readings

Tells the daemon to smooth the data by averaging the last num-readings values on each update. This is off by default.

--lock min-distance

Tells the daemon to lock the reported position when the tracker does not move much. If the distance from the latest reading to the last reported reading is less than min-distance feet, the new data is not reported. The default is 0 (no locking).

--othertracker key

Tells the daemon to also get data from another tracker daemon, whose shared memory segment key number is key. The DynaSight data will be reported as sensor 0; all sensors except sensor 0 will be copied from the other daemon any time they change.

--help

Prints a summary of the command line options.

Last updated 26 April 2002.
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