The structure of the optic flow field

The optic flow field is defined as preserving the intensity along flow-lines. Due to singularities in the image at fixed time, poles are created in the optic flow field. In this paper we describe the generic types of flow singularities and their generic interaction over time. In a general analytic flow field, normally the topology is characterised by the points where the flow vanish again subdivided into repellers, attractors, whirls, and combinations hereof. We point out the resemblance, but also the important differences in the structure of a general analytic flow field, and the structure of the optic flow field expressed through its normal flow. Finally, we show examples of detection of these singularities and events detected from non-linear combinations of linear filter outputs.