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Re: Number of control points in DLT

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  • Re: Number of control points in DLT

    This interesting discussion on DLT calibration assumes (I think) that
    the same calibration dataset is used for estimating camera/lens
    nonlinearity (i.e. distortion) as well as the basic 11 DLT parameters.
    It makes sense that many control points are needed in this case. This
    method also requires that the mathematical model chosen for distortion
    modeling is a good fit for the actual distortion pattern. Deviation of
    actual distortion from the model is likely near the edges of the field,
    which can create the extrapolation errors we are all familiar with.

    Some methods employ a seperate procedure using a uniform marker grid
    (typically consisting of several hundred markers covering the entire FOV
    of the camera) to generate a distortion correction mapping specific to a
    given camera/lens combination. This mapping info is then used to remove
    nonlinearities from the 2D images (usually using trilinear interpolation
    from the grid data) before DLT calibration. It would seem that the
    separate distortion correction procedure, if implemented properly, would
    reduce the number of control points required for good accuracy and
    improve calibration near the edges of the field of view.

    I have never seen a comparison of the two different methods. Does
    anyone have any thoughts on the relative merits of the one-step vs
    two-step processes?
    __________________________________________________ ___________________
    Scott Tashman, Ph.D.

    Head, Motion Analysis Section Assistant Professor
    Bone and Joint Center Department of Orthopaedics
    Henry Ford Hospital School of Medicine
    2799 W. Grand Blvd. Case Western Reserve University
    Detroit, MI 48202

    Voice: (313) 876-8680 or 876-7572
    FAX: (313) 556-8812 or 876-8064
    __________________________________________________ ___________________