Terms used in ecological optics

Terms used in ecological optics. The ecological approach to visual perception Appendix 1 중에서.

• The environment of animals, as distinguished from the physical world, consists of a medium, substances, and the surfaces that separate the substances from the medium.
• The medium for terrestrial animals is air. Air is insubstantial and thus permits locomotion. Locomotion is controlled by the information in the medium.
• Information is provided by sound-fields, by odor-fields, and above all by illumination. Information, in this terminology, is not transmitted but is simply available.
• Illumination is the steady state of reverberating radiant energy such that light is ambient at all points in the medium.
• Substances are solids and liquids that vary in composition, and in resistance to change. Different substances have different affordances. Substances are generally opaque, that is, they reflect and absorb but do not transmit.
• The surface of a substance has a characteristic texture, reflectance, and layout. The ambient light at any point in the medium is structured by the light reflected from surfaces so that these characteristics are specified.
• Surfaces, substances, and the medium manifest both persistence and change, persisting in some respects and changing in others. The changes are environmental events. Animals need to perceive what persists and what changes. A surface goes out of existence when its substance evaporates or disintegrates; a surface comes into existence when its substance condenses or crystallizes.
• Layout refers to the persisting arrangement of surfaces relative to one another and to the ground. Different layouts have different affordances for animals. The perception of layout takes the place of the perception of depth or space in traditional terminology.
• The ground is the basic persisting surface of the environment. It is the surface of support, the terrain, the earth extending out to the horizon. It is normally cluttered.
• Clutter of the environment refers to objects or surfaces that occlude parts of the ground and divide the habitat into semi-enclosures. Semi-enclosures provide vistas.
• A detached object is a substance with a surface that is topologically closed and is capable of displacement. Animals are detached objects.
• An attached object is a substance with a surface that is not wholly closed and is continuous with another surface, usually the ground. It cannot be displaced without breaking the surface.
• An edge is the junction of two surfaces that make a convex dihedral angle.
• A corner is the junction of two surfaces that make a concave dihedral angle.
• An occluding edge is an edge taken with reference to a point of observation. It both separates and connects the hidden and the unhidden surface, both divides and unites them. The same can be said of the far side and the near side of an object. As the point of observation moves in the medium, or as the object moves, the hidden and the unhidden interchange, or the far side becomes the near side and the reverse. For curved surfaces and tangential occluding edges, instead of flat surfaces and apical occluding edges, the rule is the same.
• A point of observation is a position in the medium that can be occupied by an animal. It is stationary only as a limit. A moving point of observation entails a path of observation. Different observers can perceive on the same path of observation. The point of observation in ecological optics should not be confused with the station point of a picture in discussions of artificial perspective.
• Occlusion is one of the three main types of going out of sight. A surface can go out of sight at an occluding edge, at a great distance, or in the dark. In all three cases coming into sight is the reverse of going out of sight, and thus is unlike coming into existence which is not the reverse of going out of existence. All displacements and turns of an observer’s body, or of an object, bring about a change of occlusion. There are two kinds, self-occlusion and superposition.
• Going out of sight at an occluding edge is specified by progressive decrements of structure on one side of a contour in the optic array. Coming into sight at an edge is specified by progressive increments of structure on one side of the contour. Going out of sight in the distance is specified by optical minification of structure to the limit. Going out of sight in darkness is specified by reduction of illumination to the limit.
• The optic array at a moving point of observation is disturbed by what we call changing “perspective” and changing “parallax,” which have never been carefully analyzed. Nevertheless, there is reason to suppose that invariants of the array underlie these changes: ratios, gradients, discontinuities, and other relations in the ambient light that owe their existence to the persisting features of the environment. (The structure of the array is also disturbed by motions and deformations of parts of the environment and by movement of the sun in the sky, but invariants are presumed to underlie these changes also.)
• An arrested optic array at a fixed point of observation has a kind of structure that is somewhat easier to understand. It can be described in terms of visual solid angles that are both densely packed and “nested” up to the hemispheric solid angle of the earth and the spherical angle of the whole ambient array. The envelope of each solid angle intercepts a face of the layout projected to that point, or a facet, or an aperture. Although this description of optical structure is superior to that in terms of rays and pencils of rays, it still cannot cope with shading and transparency, or surface color. But it does emphasize the fact that there is a unique optic array for every fixed point of observation in the environment; no two are identical.
• Disturbance of structure is a general term that will encompass all kinds of change in the optic array. Different disturbances specify different happenings. The term motion, borrowed from mechanics, does not apply to an optic array, and the term transformation, taken from geometry, is not suitable either, because it does not cover a gain or loss of structure.
• Successive overlapping samples of the ambient optic array are picked up by an observer during head movements. The field of view of the head is a sliding sample of the array as the head turns, gaining structure at the leading edge and losing structure at the trailing edge. The field of view of the head consists of the combined fields of view of the two eye-sockets. The amount of simultaneous overlap of the two fields of view differs, being large in the human and small in the horse, but successive overlap is common to all animals. Simultaneous disparity of the overlapping binocular fields has been overemphasized in physiological optics. Note that samples of the ambient array take the place of retinal images in physiological optics.
• Scanning of the field of view is the successive foveating of details of its sample by each eye. The exploratory scanning of a field should not be confused with the exploratory sampling of the ambient array. Some animals do not have foveated eyes and do not scan.
• The visual system is distinguished from the visual sense, from the modality of visual experience, and from the channel of visual inputs. It is a hierarchy of organs and functions, the retina and its neurons, the eye with its muscles and adjustments, the dual eyes that move in the head, the head that turns on the shoulders, and the body that moves around the habitat. The nerves, tracts, and centers of the brain that are necessary for vision are not thought of as the “seat” of vision.