Chapter 4. Principles of Perceptual Grouping

In the first half of the twentieth century, a group of German psychologists (known as the Gestalt psychologists) created demonstrations and conducted experiments aimed at clarifying the principles of perceptual grouping. Their strategy was to present arrays of very simple elements and to manipulate various relations among the elements in order to discover how those relations affect the perceived grouping of the elements. The relations that were investigated and the principles discovered included:

• Proximity: Elements that are close together group more easily than elements that are far apart.

• Similarity: Elements with similar features tend to group together. This principle refers to features such as color, size, shape, and orientation.

• Common motion: Elements that move in unison tend to group together.

• Symmetry and parallelism: Elements that are symmetrical or parallel tend to group together.

• Good continuation: Two edges that would meet if extended tend to group together and are perceived as a single edge that has been partially occluded. For example, in the image on the right, the top and bottom edges of the two green shapes would meet if extended, as illustrated by the dashed lines in the image on the right, so we tend to group the two green shapes into a single shape partly hidden by the blue shape.

These principles are thought to guide perceptual grouping in complex real scenes as well as in simplified arrays like those used by the Gestalt psychologists.