EARLY PROCESSING OF VISUAL INFORMATION

The 1st step of consequence in human visual information processing is to compute a primitive but rich description of the grey-level changes present in an image. The description is expressed in a vocabulary of kinds of intensity change (edge, shading-edge, extended-edge, line, blob, etc.). Modifying parameters are bound to the elements in the description, specifying their position, orientation, termination points, contrast, size and fuzziness. This description is obtained from the intensity array by fixed techniques, and it is called the primal sketch. For most images, the primal sketch is large and unwieldy. The 2nd important step in visual information processing is to group its contents in a way that is appropriate for later recognition. The necessary grouping of elements in the primal sketch may be achieved by a mechanism that has available the processes inferred from above, together with the ability to select items by 1st order discriminations acting on the elements'' parameters. Only occasionally do these mechanisms use downward-flowing information about the contents of the particular image being processed. Non-attentive vision is in practice implemented by these grouping operations and 1st order discriminations acting on the primal sketch. The class of computations so obtained differs slightly from the class of 2nd order operations on the intensity array. The extraction of a form from the primal sketch using these techniques amounts to the separation of figure from ground. Most of the separation can be carried out by using techniques that do not depend upon the particular image in question. Figure-ground separation can normally precede the description of the shape of the extracted form.