Cone beam computed tomography (CBCT) projection data consist of views of a moving point (e.g. diaphragm apex). The point is selected in identification views of extreme motion (two inhale, two exhale). The room coordinates of the extreme points are determined by source-to-view ray tracing intersections. Projected to other views, these points become opposite corners of a motion-bounding box. The view coordinates of the point, relative to the box, are used to interpolate between extreme room coordinates. Along with the views' time stamps, this provides the point's room coordinates as a function of time. CBCT-derived trajectories of a tungsten pin, moving 3 cm cranio-caudally and 1 cm elsewhere, deviate from expected ones by at most 1.06 mm. When deviations from the ideal imaging geometry are considered, mean errors are less than 0.2 mm. While CBCT-derived cranio-caudal positions are insensitive to the choice of identification views, the bounding box determination requires view separations between 15 and 163 degrees . Inhale views with the two largest amplitudes should be used, though corrections can account for different amplitudes. The information could be used to calibrate motion surrogates, adaptively define phase triggers immediately before gated radiotherapy and provide phase and amplitude sorting for 4D CBCT.