Geons are the simple 2D or 3D forms such as cylinders, bricks, wedges, cones, circles and rectangles corresponding to the simple parts of an object in Biederman's recognition-by-components theory. The theory proposes that the visual input is matched against structural representations of objects in the brain. These structural representations consist of geons and their relations (e.g., an ice cream cone could be broken down into a sphere located above a cone). Only a modest number of geons (< 40) are assumed. When combined in different relations to each other (e.g., on-top-of, larger-than, end-to-end, end-to-middle) and coarse metric variation such as aspect ratio and 2D orientation, billions of possible 2- and 3-geon objects can be generated. Two classes of shape-based visual identification that are not done through geon representations, are those involved in: a) distinguishing between similar faces, and b) classifications that don’t have definite boundaries, such as that of bushes or a crumpled garment. Typically, such identifications are not viewpoint-invariant. Properties of geons There are 4 essential properties of geons: View-invariance: Each geon can be distinguished from the others from almost any viewpoints except for “accidents” at highly restricted angles in which one geon projects an image that could be a different geon, as, for example, when an end-on view of a cylinder can be a sphere or circle. Objects represented as an arrangement of geons would, similarly, be viewpoint invariant. Stability or resistance to visual noise: Because the geons are simple, they are readily supported by the Gestalt property of smooth continuation, rendering their identification robust to partial occlusion and degradation by visual noise as, for example, when a cylinder might be viewed behind a bush. Invariance to illumination direction and surface markings and texture. High distinctiveness: The geons differ qualitatively, with only two or three levels of an attributes, such as straight vs. curved, parallel vs. non parallel, positive vs. negative curvature. These qualitative differences can be readily distinguished thus rendering the geons readily distinguishable and the objects so composed, readily distinguishable. Derivation of invariant properties of geons Viewpoint invariance: The viewpoint invariance of geons derives from their being distinguished by three nonaccidental properties (NAPs) of contours that do not change with orientation in depth: Whether the contour is straight or curved, The vertex that is formed when two or three contours coterminate (that is, end together at the same point), in the image, i.e., an L (2