In three-dimensional image-based motion analysis, the direct linear transformation (DLT) is commonly used to measure locations of significant body landmarks. The major drawback of the DLT is that the control points used for calibration must encompass the volume in which the activity occurs. A new procedure is presented where the calibration frame is moved sequentially, permitting calibration of a volume much larger than that encompassed by the calibration frame. A calibration frame with a volume of 0.6 m3 was used to calibrate a volume six times greater, by placing the frame in eight different positions. Reconstruction accuracy was comparable with that for the original frame position. This new multiphase calibration procedure presents the opportunity for calibrating large volumes using a small calibration frame; this may be advantageous, for example, in sporting arenas, where the transportation or manufacture of a sufficiently large calibration frame may be problematic.
John H. Challis is with the Applied Physiology Research Unit, School of Sport and Exercise Sciences, The University of Birmingham, Edgbaston, Birmingham, B15 2TT UK.