Vision is the most elaborate sense, and as such has challenged philosophers over many centuries to explain the mystery of its functioning. The advent of the digital computer allowed the development of computational theories that give accounts of the information extracted from the image, and of how it is extracted, processed, and used. One of the last and most influential precomputational theories is that of Gibson [25, 26, 27], which posits direct perception. The crux of this theory is that the environment, by itself, is the repository for information necessary to act. This information is expressed in terms of invariants that are implicitly contained in the optical array. The best known example is that of optic flow, the velocity patterns induced on the optic array by motion of the observer. For instance, if an observer translates in the direction of gaze, the optic flow contains information about

significant events such as the time to collision with surfaces. In Gibson's theory the following two components are most important.