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Fig. 1.3. Perceptual grouping tasks. Perceptual grouping is the process of identifying constituents in the visual scene that together form a coherent object. Perceptual grouping can take place at many different levels, from the very low level (a), to the very high level (c). (a) Grouping by proximity. The two black disks that are close to each other appear to form a unit. Thus, two groups are perceived: one on the left and another on the right. (b) Grouping by good continuation. In the random background of oriented edges (or contour elements), it is easy to notice the long, continuous sequence of edges that runs horizontally from the top-left of the circular area toward the right and slightly down. The task of detecting such contours is known as contour integration. (c) Grouping requiring world knowledge. In this seemingly unintelligible image lurks a Dalmatian dog sniffing on the pavement (a photograph by R. C. James; the dog is in the top right of the image, facing left). Without world knowledge, e.g. experience with dogs, leaves, etc., it would be impossible to group together the dots that form the Dalmatian.