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J Biomech. 2002 Jun;35(6):823-7.

Estimation of the longitudinal axis of line symmetrical soft bodies by stereophotogrammetry.

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Evolutionary Morphology Group, Institute of Evolutionary and Ecological Sciences, Leiden University, Kaiserstraat 63, 2311 GP, Leiden, The Netherlands.


The three-dimensional (3-D) position of rigid body segments is generally determined by stereophotogrammetry: the position of markers or marker-trees, fixed onto the rigid body, is identified in each of at least 2 images. Soft bodies like the tongue of a snake are not rigid and difficult to attach marker onto. Therefore, a method is developed to estimate the 3-D position of the central axis from the digitised contours. The 3-D finite spatial curve representing the tongue axis is described by a function of a fourth variable t,tin[0,1]. The projection of this line [x(t),y(t),z(t)], a polynomial curve of a user-defined order, was fitted onto minimally 2 recorded images by means of direct linear transformation. The result is an optimally estimated finite spatial curve defined by the polynomial parameters. The method is applicable for all line symmetrical organs or segments.

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