The Physical Causes of Optical Textures

March 1965

The Physical Causes of Optical Textures

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 References to these essays must cite them explicitly as unpublished manuscripts.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.

1. Optical texture is one kind of structure in an optic array; it is the relatively fine structure as contrasted with the coarse structure. Coarse levels of structure should probably be called pattern. A special kind of structure characterized by a closed contour should have reserved for it the terms form or figure. A large scale pattern of forms or figures should be called an arrangement.

2. An optic array may have structure at all levels of angular size from the finest texture in fractional seconds of are to the largest arrangement in many degrees of arc (up to 360 degrees of ambient light). A completely homogeneous sphere of ambient light is said to be undifferentiated, i.e., to have no differences in different directions. It is not an array since it has no structure or arrangement.

3. The main kind of difference in ambient light at a station-point is the difference of intensity, but another is a difference of spectral composition. Still another is a difference of polarization, but these are progressively of less importance in constituting stimuli for vertebrate ocular systems. (Ecological Optics, Vision Research, 1961, 1, 253-262.).

4. An intensity difference in ambient light often entails a contour, that is, an abrupt change of intensity with chance of direction. But there exists gradual change of intensity in an array that are not contours. The mathematical definition of contour, and of the other terms loosely synonymous with it or confused with it (margin, border, line, and edge) remains a problem for stimulus-optics. 6. The causes of the structure of ambient light have never been thoroughly inventoried. (An attempt to classify them is made in the revisions of Chapters 9 and 10 of The Perceptual Systems). It is reasonable to assume, however, that visible light can only be produced by sources of radiation and , after that, can only be modulated by differential illumination, reflection (vs. absorption), transmission, refraction, and sometimes diffraction and polarization. Of these types of modulation, by far the most important causes of optical structure are differential illumination of opaque surfaces and differential reflection-absorption by opaque surfaces. Differential transmission through transparent surfaces is much less frequent, and the others are rare. The structuring of ambient light by stars, flames, lamps, and luminescent bodies is a special case.

7. The cause of differential illumination is the fact of different faces and facets of environmental surfaces, relative to the source of illumination (their layout). The cause of differential reflection is the different chemical compositions of different illuminated substances in the environment (their reflectance). An optic array may be caused either by variations in the “facing” of the same substance, or by variations in the reflectance of different substances in the same plane, or by both (of Chapter 9 of manuscript). For example, an optic texture may be caused by a rough surface of different materials (a conglomeration). That is, it may be caused by structural granularity or by pigment granularity, by physical texture or chemical texture. But an optical texture specifies a surface.

8. A form within an optic array, as distinguished from a texture, may be caused by the face of a object, or by the shadow of an object, or by an aperture in a surface, although it may also be caused by a deposit of pigment on a flat surface. A form is therefore ambiguous. A large-scale pattern in an optic array is almost always caused by an environmental layout of different substances, although it may be the result of a flat mural painting or a panoramic picture.

9. It is convenient to divide the causes of an optic array into two types, ecological and artificial, although the dividing line is not sharp. The main ecological causes of a structure and texture have been suggested. The artificial causes need to be classified. I suggest the following types as a preliminary attempt for purposes of discussion. It may be noted that artifacts, unless they are architectural ones, generally yield an optic array of limited scope. (The scope of an array is its visual solid angle.) Only with a room can the complete spherical array of ambient be artificially structured.

Artificial Causes of Optical Structure and Texture

  • A. Method of Production
    • 1. Pigment structuring. The depositing of pigments, dyes, ink, etc. on a surface by any of a variety of methods called “graphic.” Examples are paintings, drawings, printing, pictures, transparencies.
      • a. Chirographic methods (brushes, pencils, pens, etc.)
      • b. Pictographic methods (camera and film, with derivatives).
    • 2. Layout structuring. The indenting of a surface or the shaping of a substance by methods called “plastic.”
      • a. Reliefs (grooving, scratching, embossing, etching. A “trace” can be made either by leaving an indentation or a deposit.)
      • b. Sculptures and models (shaping, cutting, chipping, molding, assembling. An image “in the round” will yield optic arrays at surrounding station-points; other methods will not.
  • B. Intention of Producer
    • 1. Portrayals of surrogates. Artifacts intended to yield a vicarious perception, i.e., to reproduce or represent, by displaying some of the same structures of light as would the original. Examples are photographs and “faithful” pictures, reliefs, and sculptures. (But note that fidelity to some of the original information in an array is all that can be achieved, and that this is selective.)
    • 2. Conventional non-portrayals. Artifacts not intended to represent but displaying agreed-upon graphic symbols. Examples are signals, signs, ideographs, alphabetic letters, numbers, writing.
    • 3. Invented structures. Artifacts not intended to represent anything nor to employ graphic conventions but only to display the possibilities of optical structure as such, considered as stimulation independent of its source. The information in these constructions does not specify or refer; it is information considered as “uncertainty.” As opportunity for pure discrimination, this interests both artists and psychologist. Examples are decorations, design, mathematical construction, modern paintings.
      • a. Regular constructions. Simple or complex repetitive or lawful patterns. Used either to embellish other objectives or simply to be looked at.
      • b. Random or irregular constructions. This includes man generated randomness of pattern or texture or, recently, computer-generated randomness. The perceptions induced are curious and puzzling.

DiscussionThe study of optical texture as stimulus information for visual perception should be pursued from two directions, both for out understanding of the perception and identification of environmental surfaces and for an understanding of the visual discriminative process as such. One can legitimately look to the environment for the causes of textured light or one can invent experimental textures never previously applied to a human eye, using all sorts of gadgetry now available for this purpose. The experimental textures have physical sources, to be sure, but they are invented by the experimenter or the artist for the sole purpose of stimulating an eye, not to be physical objects in the usual sense of that term.

The experimental control of texture and its density, gradients, steps, distributions, mixtures and the parameters of its regularity or irregularity (not to mention its transformations over time, which are not here considered) gives promise of clarifying the perception of “space,” that is, the basis for impression of slant, edge, superposition, depth, concavity, convexity, solidity, transparency, openess, and emptiness.