Summing up, the common procedure of a markerless tracking system looks as follows: 1. The system detects natural features in the current frame. 2. It compares the features and finds corresponding ones. 3. It approximates the camera’s pose1. It uses the correspondences whose 3D positions are already known for these calculations. 4. It calculates the 3D positions of the correspondences whose 3D position are unknown. The following sections explain each of these steps.