Visual Completion. Edges are so fundamental to visual experience, in fact, that the mind will construct (perceive) a form even where there is none, as the following example demonstrates.

visual closure in an underdetermined pattern (Kanizsa illusion)

 
In this case the mind is simply presented with three dark, incomplete circles. Yet the appearance of a white triangle standing in front of or on top of the circles is so dramatic that some people will even perceive the triangular area as whiter than the area outside it (the Ehrenstein illusion)! A "literal eye" would simply see what is there.

Benary cross: pattern as form.

Benussi illusion in contrasted patterns.

DeValois checkerboard illusion

pattern contrast

 
The whole thing with pattern contrast.  

Foreground/Background Contrasts. One of the principal tasks necessary for depth perception is to set the separate regions defined in the primal sketch at different distances from the viewer. In effect, the regions are cut out as separate units, then mentally located as near or far from the view in a third dimension, much as planes are separated in the continuous flat surface of a bas-relief carving. This representation has been called the 2.5 dimensional sketch (at right), because it shows depth without being truly three dimensional: it does not define the volume and mass of objects, only their relative distance from the viewer.

As part of this representation, color shifts can be induced by a "foreground vs. background" pattern of the geometrical areas or color shapes in an image (this is called White's effect). The following example shows this clearly.

color shifts in a simple pattern

The astonishing result is that hue shifts emerge from the pattern alone: the squares that interrupt the violet stripes appear lighter and less saturated than the squares that interrupt the green stripes. Visual completion causes the squares to appear as horizontal blue bands, which means the eye interprets the pattern as blue stripes behind the uninterrupted colored bands and in front of the interrupted color, which acts as a simultaneous contrast background. The squares that interrupt (are "in front of") the green background (left) shift toward a violet hue and a darker value, while the squares that interrupt the purple stripes (right) shift toward a green hue and a lighter value.

lightness shifts induced by visual completion

A standard method for showing this surface layout is a circular disk skewered by a short rod and placed on a surface; the length and orientation of the rod shows the tilt and slant of the surface, the size of the disk its distance from the viewer.

This would be fussy and cluttered to use in a drawing, but artists have used a variety of other line conventions to indicate surface orientation and distance. Typically line thickness represents the distance of the edge or object from the viewer (distant objects are more lightly drawn), or the approach of the line to either a dark terminator or shadowed area. A variety of patterned lines, such as the bracelet drawing used by etchers, convey contour by the path of the line across the surface. The spacing of lines in relief drawings shows the orientation of the surface in the spacing of the lines: imitating foreshortening, the more slanted the surface, the more closely the lines are spaced. Finally, in imitation of perspective gradients, lines across the same surface are drawn closer together as they recede from the viewer.  

Light and Shadow. Because saturation and value are easy to confuse with each other, and tend to be disguised or shifted by the hue we are looking at, it's important to observe the distinction between the two when modeling three dimensional forms.  

(a) To begin, we arrange two rows of identical gray bars so that they slightly overlap in their spaces. Note that they appear to be exactly the same lightness — because they are.

(b) Light and dark squares can be added to the ends of these bars, but the apparent lightness of the bars remains largely unchanged. (I see a very slight shift to make the upper bars appear darker than the lower, corresponding to the Craik-O'Brian effect.)

(c) Or, a lighter gray field can be added around the lower bars, but this again does not alter the apparent uniform value of the bars.

(d) However, when the two elements are combined, the mind interprets the continuous horizontal edge created by the small squares and the large gray field to mean there is a coherent spatial relationship among all the color areas: the overall pattern resembles two rows of bars partially obscured at the bottom by a shadow or a translucent filter. This spatial interpretation causes the mind to infer color appearances that can "explain" the value contrasts in the apparent lightness relationships — the upper bars appear much darker than the lower ones.

These effects are not limited to value (lightness) alone. The section on saturation and value demonstrates similar and large shifts in apparent chroma and hue, simply by changing the location of the colors within an apparently three dimensional figure.

We can conclude from these and many other specific demonstrations that the mind is continually adjusting our visual experience to clarify the world's spatial structure — specifically, the effects of illumination on three dimensional surfaces. That is, color shifts do not arise "bottom up" through the local contrasts of different color areas, as "color theory" assumes. They are imposed "top down" by the mind, which first generates a three dimensional model of the world and the illumination within it using lightness information alone, then adjusts or "paints over" this three dimensional framework with color appearances appropriate to show the inferred forms and contours.

Perhaps the major shortcoming of "color theory" prescriptions has been the assumption that color effects arise only through the visual proximity of two differently colored surface areas. In the suggestive and very influential color demonstrations of Josef Albers, for example, suitable choices of different values or hues can produce effects of transparency, translucency, spatial ordering, and so on.

Canonical Views. One of the canons of American design theory is an emphasis on the rule of good shapes, which Edgar Whitney stated as "a shape having variety in its length and breadth dimensions, its directional thrust being a dynamic oblique, and with incident at its edges interlocking with negative areas." What exactly that means in a specific painting is not always clear, but one answer may be that a good shape is most informative about the three dimensional form of the object represented.

A relatively unexpected but artistically important aspect of visual recognition is the role of "best" or canonical views of an object. The figure below illustrates these viewpoints for some common objects.

canonical views of six common objects

 
These canonical views were identified by asking experimental subjects to rate several photographs of a single object, each taken from a different point of view, in terms of "how much it looked like the object." Unusual views were downgraded in comparison to more familiar views, and these canonical views rated highest of all. The canonical views turned out to be recognized (named) most quickly when presented to different subjects.

Although it seems obvious that some views are more recognizable or pleasing than others, we'd like to find the rules that define a canonical view for a specific object. It seems the basic rule is, "it depends." Some points of view, such as the clock or telephone, seem determined primarily by the view during function or common use of the object — how it appears when we use it. Others, such as the chair or teapot, seem to provide the best three dimensional information about the object. In most cases, however, because the object is familiar from many different points of view, the canonical view seems to be a three quarters profile viewed slightly from above (the camera and shoe). Interestingly, this viewpoint also seems to create the dynamic oblique directional thrust and incident at edges valued by Whitney.

I suggest that the design requirements for good shape can be thought of as a balance between six competing viewpoints and design goals: (1) the habitual or functional view; (2) the view providing the best information about the object's proportions in three dimensions; (3) the view providing the most informative visual separation among the major parts of a single object; (4) the view producing the most complex edges or outline against a background; (5) the view producing the most dramatic perspective effects of height, recession or profile; and (6) the view producing the most emphatic two dimensional shape within entire image and its format. Balancing these competing criteria allows considerable scope for esthetic and design judgments.  

Behavior Episodes. An equally important aspect of canonical views that artists are rarely taught only emerges across time, in the behavior episodes studied by University of Virginia psychologist Darren Newtson. Think of any common physical activity, such as baking a cake, doing gymnastics or assembling an Ikea bookshelf, as captured on film. Each frame of the film would present a snapshot view of the activity unfolding over time. The unexpected result of Newtson's research is that some frames of this film are far more "canonical" or informative about the activity than others, because they show the key steps or processes of the activity in context. The frames that show the egg dropping from its shell into the mixing bowl, or the batter being poured into the baking pan, are immediately comprehensible; the frames that show the cook looking for a mixing spoon or wiping batter from her chin seem ambiguous and uninformative. Just as objects have canonical views in space, activities have canonical views across time.

It turns out that nearly all human activities, from conversations to team sports, show the same pattern of highly informative views separated by ambiguous intervals of rest, distraction or transition. The artistic challenge is to find the most informative snapshot to represent an entire activity or sequence of events as a single image. Photographers have the luxury of choosing from among many negatives: painters must determine the view through judgment and careful observation.

In athletic images or historical paintings, these canonical views of action can represent a climactic point (as in David's Oath of the Horatii) or can collapse several events in a single view (as in Caravaggio's marvelous The Betrayal of Christ, which shows the traitor kiss, the fleeing disciples and the arresting soldiers as a single moment). But the same principles apply in quiet images: John Singer Sargent was a master at disclosing the vitality and personality of his subjects through a gesturally expressive pose and a subtle displacement of the artist's point of view.

It is also very instructive to study sports or wildlife photographs, paparazzi celebrity shots, the photojournalism of Weegee (Arthur Fellig) or Henri Cartier-Bresson, advertising images, and in particular the fabulous collections of time series photographs by Eadweard Muybridge.

muybridge photos of a trotting horse

 
Although these twelve photos show different aspects of the same animal activity, clearly some of the snapshots are more informative or interpretable as "a trotting horse" than others. Image 4 seems to show the horse lurching to a halt, while image 12 suggests the wooden mount on a merry go round. Image 11 conveys a greater sense of speed than image 2; image 7 is monumental and static where image 9 is dainty and flowing. Careful comparisons such as these, using different kinds of photodocumentation, can help you understand why some images are more representative or characteristic than others, and to design figures and groups with a greater feeling for moment and gesture.

One caveat: science can clarify, but it can't prescribe. "Canonical" images are hardly the only view of the world. One of the main attributes of Diane Arbus' photographs is her preference for catching her subjects within transition moments, inside the detritus of action, creating an illusion of spiritual disconnectedness, imbalance, and a lack of human warmth. Her contact sheets show her decisions clearly.

Photographs structure the world in specific ways. They can help you study more effectively the motionless form of water ripples or running horses, but they cannot suggest the best way to represent water or horses in a painting. I discuss this issue in the page on aids to drawing, but the main point is simple: don't let dogmas dictate your representations.

Beliefs and Details. This is perhaps the hardest problem in perception. We cannot see the total individuality of everything in the world, or rather we see it but believe it does not matter to the general beliefs we hold about the world. So we float in a realm largely structured of ideas, interspersed with gleams and pools of direct experience.

Speech is a classic and insightful example. Our speech perception is entirely categorical, which means we can be confused by even small variations from the sound patterns we have been habituated to believe are important in speech production. Our perception focuses on specific sound details and discards the rest, and this constitutes our "normal" perception of speech. It is thrown into slight disarray by speech impediments, dialectics, or foreign accents, but at the borderline we are completely deaf to what is said. All of us have had the experience of asking someone to repeat something we don't understand, then once we "get" the misheard word reacting with embarrassed recognition. "Oh, Irene, I thought you said I seen."

Embarrassment is an emotion of misplaced categories. When we paint or draw we have similar difficulties finding the right graphical equivalent for a mental state, we are trying to write down the communications of the world without complete knowledge of the means to do so.

Attention during painting must be placed on the good fit of the new marks, given what came before.

In fact, our visual sense of the world is largely based on a psychological reconstruction of the world, rather than a cameralike documentation of optical events. Visual effects such as complementary color contrasts are demonstrated with color stimuli that affect a very small part of the total visual field. Yet this larger visual experience is usually involved when viewing a painting, and always when viewing a landscape or figure, so its structure is important to understand.

Let's start with a simple illustration of the optical facts presented to us by our eyes. The image below was made by superimposing two separate images taken about 4" apart, to simulate the binocular view created by both eyes.

binocular parallax and depth of field in a visual field

Second, our mind replaces peripheral optical facts with concepts — of a doorway, a shower stall, a window reflection. These ideas cohere in our sense of being in a hallway before a bathroom, and the distances between our view and the objects around us. These ideas shift toward detailed perception only when we shift our direction of view — which we do frequently and continuously — to explore the details of a new object.

The basic difference between optical facts and visual experience is easiest to illustrate through the problem of parallax, but other optical constraints contribute as well: the lens of the eye cannot focus as efficiently along the sides of the curved retina, and light falling on this part of the eye strikes at an oblique angle, rather than head on, blurring the response across neighboring cones.

To cope with all these visual problems, the visual field is divided into three zones, as shown in the figure (looking downward on a viewer looking forward).

In contrast to saccades, pursuit eye movements are voluntary, tandem eye movements that smoothly track a moving object in the visual field. They primarily function to stabilize moving objects on the retina, which allows us to see the object in better detail. However, they typically require a moving object to elicit, and are not under voluntary control. So they do not affect our view of a painting.