The tracking algorithm essentially estimates the camera ego-motions on the mobile device. The problem of global motion estimation has been well studied in the past. The tracking algorithm in our system is an accelerated area-matching method, which extracts a set of point features from an image and performs local motion estimation on patches centered on these feature points. On a Nokia N95 mobile phone, which has a 330MHz ARM11 CPU, the tracking algorithm works at around 30fps.