Almost all work in image sequence processing begins by attempting to find the vector field which describes how the image is changing with time. Ideally, the projection into the two dimensional image plane of the three dimensional velocity field seen by the camera should be computed. However, this is not just difficult to achieve in practice, it is usually impossible to achieve (perfectly) even in theory. The reasons for this are well understood; Horn and Schunk in  give the example of a rotating sphere with no surface markings which, under constant illumination, causes no changes in the image intensity over time, even though there is motion in the world. Related to this is the well known aperture effect -- for example, when a pole moves behind a window, only the component of motion perpendicular to the pole can be found. These problems have led to careful definitions of optic flow being made; it is frequently necessary to make the distinction between the motion perceived in the image and the theoretical projection of the world velocities into the image. A discussion on this subject can be found in .
With 2D feature-based optic flow, the need for making explicit which sort of optic flow is being discussed does not arise. Neither does the question of whether the full flow is being found, or just a component of it. This is for two reasons. Firstly, optic flow is found by matching two dimensional features. One of the strongest advantages of this approach is the fact that it does not suffer from the problems of the aperture effect, for obvious reasons. (In  Nagel discusses this in detail.) Secondly, the work is largely aimed at producing good results from images of real events, mostly taken outside. Thus there is rarely a difference in practice between the image flow and the projected world motion. Thus the term optic flow will be used freely and generally.
The different methods used for finding optic flow reported here do not always fit neatly into the three categories described below, but an attempt has been made to give a clear discussion of the subject by looking at each approach in turn.