January 1969
A List of Ecologically Valid Meanings in a Stationary Ambient Optic Array
J. J. Gibson, Cornell University
The World Wide Web distribution of James Gibson’s “Purple Perils” is for scholarly use with the understanding that Gibson did not intend them for publication. References to these essays must cite them explicitly as unpublished manuscripts. Copies may be circulated if this statement is included on each copy.
What do the textures, contours, forms, and other structural variables of an optic array mean? Is there univocal information about the environment in a “frozen” optic array? What is specified therein about the world and its layout, and what is not specified? When is the information ambiguous?
Texture and Kind of Texture
1. A patch of the ambient array containing texture or fine structure specifies a material surface in the world (obstacle, substance, thing) whereas an untextured or homogeneous patch specifies only themedium (air, empty space, sky). This refers to optical texture as such, without consideration of optical contour. For example the upper hemisphere of the ambient terrestrial array (sky) tends to be untextured whereas the lower hemisphere (earth) is textured in special ways.
2. The kind of texture in a patch in the array is fairly specific to the substance of the surface in the world (solid or liquid, rough or smooth, hard or soft, etc. (Cp. Brodatz photos).
Contours
3. A contour in the ambient array (defined as an abrupt intensity-transition or “contrast”) has only an equivocal meaning as such. It may correspond to a dihedral angle in the world (convex or concave) or to an occluding edge in the world (occluding in either direction) or to the margin of an area of pigment on a flat surface.
4. A blurred contour in the ambient array (defined as a gradual intensity-transition) has a fairly specific meaning. It specifies either the penumbra of a cast shadow in the world or an attached shadow on a curved surface.
5. A closed contour in the ambient array, often called a “figure,” is ambiguous as such. (The experiments with closed-contour displays, and the conclusions of Gestalt theory are another matter entirely.) Ecologically if the closed contour is textured inside but not outside it specifies a substantial object in empty space. But if it is untextured inside and textured outside it specifies a window in a surface opening on empty space. (If it is textured both inside and outside, see no. 11 below.)
Forms of Contour
6. The form of a closed contour in the ambient array, as such, is both partly ambiguous and partly specific with respect to the shape of a surface in the world (the face of a polyhedral object). To a frozen form in the array may correspond a whole family of shapes-at-a-slant in the world. But this family is unique, so there is some specificity.
7. Rules could be formulated for what a rectilinear contour in the array specifies, for what an angular contour in the array specifies, what a curved contour in the array specifies, etc. For example, the tilt of a contour in the array relative to the meridians of the array specifies very little about the world. (This should not be confused with the tilt of a line relative to the frame of a picture.) The laws of perspective projection (“inverse projection”) for opaque solid edges and corners, for transparent edges and corners, and for pigment areas on flat surfaces will be somewhat different in these different cases.
8. A pair of adjacent parallel contours in the optic array (that is, a line) is thoroughly ambiguous. It might mean a wire in space, or a tracing on a surface, or a crack in an otherwise continuous surface. A tracing on a surface may “represent” a contour in an array, which can mean either a dihedral angle or an occluding edge. (Experiments in which “line” drawings or “outline” drawings are used to produce artificial optic arrays provide ambiguous information. They often yield variable and fluctuating perceptions.)
Texture Density
9. A contour in the ambient array at which the optic texture approaches infinite density specifies great distance of the terrestrial world (a horizon). A place in the optic array where the structure is large and coarse (and mobile, see below) specifies the body of the observer, which is at the least distance in the terrestrial world.
10. The degree of one-way compression of the optical texture of a patch in the array specifies the slant of a “face” in the world. (But this “foreshortening” of texture is accompanied by the foreshortening of contour and the two are difficult to separate.)
11. An abrupt increase in the density of optical texture at a contour in the array tends to specify an occluding edge in the environment. A special case of the occluding edge is the “cliff” or falling-off place. When there is a closed contour in the optic array the occluding edge may be inside or outside the contour. An increase of texture density from inside to outside usually means an object covering a background. An increase of texture density from outside to inside usually means a window revealing the background. (But the best information to specify an occluded surface is not static; it consists of the progressive accretion or deletion of texture on one side of a contour, caused by movement of the observer or a motion of one of the surfaces in question.)
12. An abrupt increase or decrease in the rate of change of texture density (gradient) at a contour in the optic array specifies quite well a dihedral angle (a corner) in the world, and the direction of change specifies convexity or concavity.