Primal sketch

According to Marr (1982), we can identify two versions of the primal sketch: the raw primal sketch and the full primal sketch. Both sketches are symbolic, meaning that they represent the image as a list of symbols. The raw primal sketch contains information about light-intensity changes in the visual scene, and the full primal sketch makes use of this information to identify the number and outline shapes of visual objects. Why are two separate primal sketches created? Part of the answer is that light-intensity changes can occur for various reasons. The intensity of light reflected from a surface depends on the angle at which light strikes it, and is reduced by shadows falling on the surface. In addition, there can be substantial differences in light intensity reflected from an object due to variations in its texture. Thus, the light-intensity changes incorporated into the raw primal sketch provide a fallible guide to object shapes and edges.

The raw primal sketch is formed from what is known as a grey-level representation of the retinal image. This representation is based on the light intensities in each very small area of the image; these areas are called pixels (picture elements). The intensity of light reflecting from any given pixel fluctuates continuously, and so there is a danger that the grey-level representation will be distorted by these momentary fluctuations. One approach is to average the light-intensity values of neighbouring pixels. This smoothing process can eliminate "noise", but it can produce a blurring effect in which valuable information is lost.

The usual answer to this problem is to assume that several representations of the image are formed which vary in their degree of blurring. Information from these image representations is then combined to form the raw primal sketch. According to Marr and Hildreth (1980), the raw primal sketch consists of four different tokens: edge-segments; bars; terminations; and blobs. Each of these tokens is based on a different pattern of light-intensity change in the blurred representations. One of the limitations of the approach of Marr and Hildreth is that it does not make full use of the intensity-change information contained in the grey-level representation (Watt, 1988).

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