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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.
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