allowing
for pixel-perfect registration of virtual objects. Conversely
in open-loop tracking, the system relies only on the
sensed pose of the user and the environmental model.
Using one or two tiny cameras, model-based approaches
can recognise landmarks (given an accurate environmental
model) or detect relative movement dynamically between
frames. There are a number of techniques to detect scene
geometry (e.g. landmark or template matching) and camera
motion in both 2D (e.g. optical flow) and 3D which require
varying amounts of computation.