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Re: Your "Black Holes"

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  • Re: Your "Black Holes"

    On Thu, 10 Oct 1996, judith d redling biom fac/staff
    >Greetings fellow researchers:
    >We our utilizing the Vicon 370 3D motion analysis system to acquire kinematic
    >data and have recently encountered a problem known as a "black hole".
    >While each camera (our system has six) can see a given marker, this marker is
    >"lost" in the reconstruction process. I don't know the details,
    >but apparently it has to do with the cancellation of intersecting rays in the
    >reconstruction algorithm. I was wondering if anyone had encountered a similar
    >problem and perhaps could offer some insight.
    >Thank you,


    I'm not sure that I fully understand the conditions under which
    your problem is occuring, but at the very least it is wrong to
    describe what you are seeing as a "cancellation of intersecting
    rays". What you MAY be experiencing is a special (geometric)
    condition which produces an infinite number of "solutions" for
    the intersection of two (or more rays). It is very unlikely
    that this will occur when three or more cameras can see the
    target (this would require the front nodes of all three
    cameras to lie on a single straight line.) However, a problem
    can arise when a target lies on the line joining the front
    nodes of two cameras and no other camera can provide
    information. Under these conditions there are insufficient
    constraints to define a single "point of intersection"--in
    mathematical terms the equations become "underdetermined".
    >From a geometric standpoint [it's a lot easier to
    understand this ;-) ] the two rays which project out from
    the two available views become collinear--so the target can
    occupy any position along the line joining the two lenses.
    >From a practical standpoint, you can "see" when this is
    likely to occur when a target occupies a position in the
    image which is similar to that of the lens of the "other"
    camera. When a third (fourth, etc.) camera can see the
    target the problem "goes away"--because the rays from
    these cameras do intersect the "coincident" rays from
    the first two cameras at a finite point.

    Apart from the problem I have described, and camera
    geometries which produce very similar conditons (i.e.,
    very slowly converging rays), I know of no reason why
    any reconstruction algorithm would fail. The problem
    may be in the software--or the algorithm used to solve
    the system of equations.

    Hope all this helps.

    By the way, just a reminder for friends and colleagues
    working in biomechanics, the 22nd International
    Congress on High-Speed Photography and Photonics
    will be taking place later this month ...

    October 27 to November 1, in Santa Fe, New Mexico

    If anyone would like additional information, please
    drop me a note.

    Jim Walton
    Chairman, SPIE Working Group on High-Speed Photography,
    Videography and Photonics
    Exhibit & Commercial Chair, 22nd International Congress
    on High-Speed Photography and Photonics.

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