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Messages - HighDesert

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Wiimote Desktop VR/Head Tracking / Re: Improving to 6 DOF head tracking
« on: January 24, 2008, 10:18:52 PM »
Unfortunately, the "geometric LED curtain" doesn't work with the wiimote. The device is limited to exactly four points of detection. We should consider this lucky since Nintendo had no reason to detect any more than two.

A little more off topic, but the thing about the digital paper is that the local dot arrangements tell the digital pen exactly where on the paper it is without its seeing the entire paper.  Similar to the temporal chirping of the LEDs, the spatial position of neighoring dots on digital paper is such that an absolute location can be determined. 

I mention the digital paper or "LED curtain" idea as a way to get around the limited field of view of a single wiimote.  Perhaps accurate pointing could still be achieved with a suitable geometric distribution of LEDs.  Sure, it can only see 4 LEDs at a time, but it may be possible to position every set of 4 LEDs in such a way to know exactly which way the wiimote is pointing - absolute pointing being related to the effort to compute absolute position.

Again, this would be quite an elaborate solution.  I think the other approaches are easier to consider for a first attempt at 6DOF positioning.

John.

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Wiimote Desktop VR/Head Tracking / Re: Improving to 6 DOF head tracking
« on: January 24, 2008, 01:43:57 PM »
Oliver has a rudimentary light saber demo.
I know, but the direction of his sword is very restricted, the WiiMote must be pointed at his beacon for 3D tracking.
Yes, I thought of that as soon as I thought more about my own problem (manipulating 3D data) and what would happen if I rotated the wiimote out of view of the LEDs.  It does make the option of leaving the wiimote stationary while rotating a tetrahedral in its view more appealing, except that I'd lose all those buttons for interaction!  In either case, I'd have to solve the point recognition problem.

FreeTrack uses a version of DeMenthon's POSIT algorithm.  I can't help but think that FreeTrack could be adapted by throwing out the initial image analysis step that finds the LEDs and just substituting the output from the wii.
Yes, this might work. As I understand it the Wii implements the image analysis in hardware which is a great advantage.
If I get a chance, I may dig into FreeTrack's code. 

The issue is really just point identification. If we assume that the refresh rate of the CCD is 100Hz (which I sincerely hope it is), then Nyquist[1] says the freq of the LED chirp (50% DC) needs to be less than 50 Hz. Most remote-control IR diodes can handle 40 kHz[2], so no problem there. Some error will occur, but as long as the chirp frequencies are known, it can work. A simple micro mounted inside a sword (I really like that idea) or on the back of a glove[3] can handle this easily.
With a little bandpass filtering you could select the signal from each particular LED.  The trick will be storing enough values to compute the frequencies without introducing too much lag to the response. 

On the other hand, would we need to chirp all the LEDs?  Would a known geometric arrangement with one chirped LED be enough?  Two?

A few more ideas for identifying the LEDs:

1. Fixed temporal intervals: Instead of blinking continuously at separable frequencies, perhaps the LEDs could be blinked in order at fixed intervals with all LEDs off except one and the interval between each LED changing in a fixed pattern.  Then, the interval between measured coordinates indicates which two LEDs fired most recently.  Intuitively, I'd think the chances of an obscured LED erroneously introducing a known interval to the sequence would be small.  You'd know, "Hey, that gap was too large! It's the LED2 - LED4 gap.  LED 3 is obscured."

2. Geometric LED curtain: A more elaborate and expensive geometric arrangement of LEDs similar to digital paper.  Or, a patterned LED reflective material lit by a single infrared source.  The mathematics of the arrangement would probably be much more complicated since the wiimote can change its view of the "field" while a typical digital paper pen always "looks" straight at the paper.

Basically with either the flashing or spatially positioned LEDs, the idea is to impart extra information to the setup that would enable the identification of a particular LED.

John.

P.S. Regarding the digital paper-like solution.  What about projecting an infrared pattern on the wall?  Are there infrared projection systems?  Would there be enough reflection off standard walls or furniture for the wiimote to detect?

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Wiimote Desktop VR/Head Tracking / Re: Improving to 6 DOF head tracking
« on: January 21, 2008, 08:47:42 PM »
I personally, since having seen Johnny's and Olivers videos, have been thinking of a way to track a sword in 6DOF. That's
Oliver has a rudimentary light saber demo.

Oliver Kreylos: "and predicted target point projections are matched with camera observations on a nearest-neighbor basis"

If the ID by radius does not work I think it is impossible to solve the problem with WiiMotes in 6 DOF. One would always have to make restrictions, guess or take the brute-force approach.
Oliver's predictive method seems pretty good.  In Johnny's video on the automatic projector calibration, he mentions that they use predictive modelling when one of the sensors (fiber optics in this case) falls outside the view of the system.

http://www.youtube.com/watch?v=XgrGjJUBF_I

There are apparently other more sophisticated techniques out there, one of which underlies FreeTrack.  Off of FreeTrack's wikipedia page,

http://en.wikipedia.org/wiki/FreeTrack

is referenced Daniel DeMenthon's work on what's called "pose calculation" of 3D objects from 2D projections.  There are oodles of mathematical papers at his site:

http://www.cfar.umd.edu/~daniel/

FreeTrack uses a version of DeMenthon's POSIT algorithm.  I can't help but think that FreeTrack could be adapted by throwing out the initial image analysis step that finds the LEDs and just substituting the output from the wii.

As for identifying the LEDs, how fast is the refresh rate on the WiiMote's CCD?  Would it be possible to chirp each LED with a unique time signature?  Or continuously blink them each with a different blink frequency?

Myself, I'm interested in something like a 6DOF mouse for the  analysis of 3D image data.  I want to be able to move the wii around and have my 3D data rendered following my motion.  There are expensive solutions out there.  This is the first cheap solution that looks practical.

John.

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