1. An interactive computing system comprising: an input for receiving a first series of depth images from a mobile depth camera when in motion and a second series of depth images from the mobile depth camera when static; a real-time camera tracking engine for tracking a position of the mobile depth camera when in motion; a dense 3D environment modeling system arranged to generate a dense 3D model of the environment from any of the series of depth images and position data from the real-time camera tracking engine; and an integration engine arranged to integrate at least a part of the dense 3D model into an application and to provide user input to the application by tracking real-time movement in a part of the environment using the second series of depth images.