Abstract: An object's position and/or motion in three-dimensional space can be captured. For example, a silhouette of an object as seen from a vantage point can be used to define tangent lines to the object in various planes (“slices”). From the tangent lines, the cross section of the object is approximated using a simple closed curve (e.g., an ellipse). Alternatively, locations of points on an object's surface in a particular slice can also be determined directly, and the object's cross-section in the slice can be approximated by fitting a simple closed curve to the points. Positions and cross sections determined for different slices can be correlated to construct a 3D model of the object, including its position and shape. A succession of images can be analyzed to capture motion of the object.