1. A method of processing a digital video sequence, the method comprising: detecting a foreground object in an image captured by a depth camera; determining three-dimensional (3D) coordinates of the foreground object; and comparing the 3D coordinates to a 3D video tripwire to determine if the foreground object has crossed the 3D video tripwire, wherein the 3D video tripwire comprises a user interface and wherein the user interface is further configured to receive two-dimensional (2D) coordinates of a first point in the scene selected by the user; and determine 3D coordinates of a first pixel corresponding to the first point using depth information corresponding to the scene.