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Visual perception is a function of our eyes and brain. We see images as a whole rather then in parts. However, images can be broken down into their visual elements: line, shape, texture, and color. These elements are to images as grammar is to language. Together they allow our eyes to see images and our brain to recognize them. In this section, we will talk about each of these elements except color, because color perception is a big subject and deserves a section of it own. Therefore we will talk about color perception in the next section. Here we are concerned with line, shape and form, and texture.

Line
A line is the path made by a pointed instrument, such as a pen, a crayon, or a stick. A line implies action because work needs to be done to make it. Moreover, the impression of movement suggests sequence, direction, or force. In other words, a line can be seen as a distinct series of points.

Line is believed to be the most expressive of the visual elements because of several reasons. First, it outlines things and the outlines are key to their identity. Most of the time, we recognize objects or images only from their outlines. Second, line is important because it is a primary element in writing and drawing, and because writing and drawing are universal. Third, unlike texture, shape and form, line is unambiguous. We know exactly when it starts and ends. Finally, line leads our eyes by suggesting direction and movement.

It is not easy to categorize lines because there are so many aspects to them. One can group them by using thickness, smoothness or origin. However, for the purpose of art education and communication, we categorize lines into five groups. There are horizontal lines which run parallel to the ground (figure A), vertical lines which run up and down (figure B), diagonal lines which are slanting lines (figure C), zigzag lines which are made from combining diagonal lines (figure D), and curved lines which do not fall into the first four categories. Curved lines (figure E) are used to express natural movement.

Line has been used a lot in art work. Even though most of the art we see uses line only to form shapes, some artists allow line to call attention for itself in the art piece. One of those artists is Paul Klee (1879-1940). This is a very interesting piece of art that has several lines as the main focus..

 

Insula Dulcamara, 1938 88 x 176 cm, Oil and colored past on paper on burlap
Kunstmuseum, Bern, Paul Klee_Stiftung, Bern

Shape
Shape is related to line. Closed lines become the boundaries of shapes. The shapes that artists create are inspired by many different sources, such as nature and man-made objects. Like with lines, there are many ways of categorizing shapes. We can use their dimensions, for example, distinguishing between two-dimensional shape and three-dimensional shape. Or we can use their style (realism, abstraction, etc), or their origin (organic or geometric)to classify them.

Geometric shapes look as though they were made with a ruler or a drawing tool. The five basic geometric shapes are: the square, the circle, the triangle, the rectangle, and the oval. Organic shapes, which are also called Free Form shapes, are not regular or even. Their outlines are curved or angular, or a combination of both. However, there is no clear-cut line to separate the geometric and organic categories. In the figure below, on the left side is a perfect geometric shape; while on the right side is an organic shape.

Shape, like line, has been used a lot by artists. Sometimes, shape is used by itself to create art works. For example, a work by Theo van Doesburg, Composition: The Cardplayers consists only of geometric shapes. Surprisingly, these shapes are used to represent two men playing cards.

“Card Players,” oil painting by De Stijl artist Theo van Doesburg, 1917;
in the collection of the Haags Gemeentemuseum, The Hague

Texture
Texture is an element of art that refers to the way things feel, or look as though they might feel, if touched. For example, sandpaper looks and feels rough; a cotton ball looks and feels soft. The connection between visual and tactile sensation is very well developed.

The next question is what are the tactile properties of surfaces that enable us to see them. In the other words, why do we see texture? We see texture because of the light-absorbing and light-reflecting qualities of materials. These qualities are together represented by light and dark patterns. The light and dark patterns give us the appearance of texture. Like the other elements discussed above, texture has been used a lot in art work.

Our sensations of colour are within us and colour cannot exist unless there is an observer to perceive them. Colour does not exist even in the chain of events between the retinal receptors and the visual cortex, but only when the information is finally interpreted in the consciousness of the observers (Wright, 1963, p. 20).

Nature of color
What we perceive as color is primarily the wavelength the light stimulation. The shortest viewable wavelength (about 380 nm) is what we see as blue and the longest wavelength (about 760 nm) is what we see as red. The other wavelengths that fall between them are what we see as other colors, as shown in the figure below. However, color perception is very subjective. We do not have a way of proving that two different people perceive the same color, yet we refer to 760-nm wavelength as RED and 380-nm wavelength as BLUE.

source: http://www.photo.net/photo/edscott/vis00010.htm

We see color in the objects around us because they absorb most of the wavelengths from the sun, called white light; and they reflect only a particular wavelength into our eyes. For example, a red apple absorbs all but the 760-nm wavelength. Therefore, we see it as red in color. Objects that are white in color are objects that do not absorb any viewable wavelengths; while objects that are black absorb almost all viewable wavelengths. We know that the white light from the sun consists of many different wavelengths because of Newton's prism (shown below). Because of the prism's refraction, the white light is split into rays, emitting different colors of light, each of which has a different wavelength. The same phenomenon happens in nature, as we can see in rainbows.

Source: http://home.att.net/~B-P.TRUSCIO/COLOR.htm

 

The dimensions of color
Even though wavelength explains differences in the colors we see around us, color entails more than that. There are three psychological dimensions of color: Hue, Brightness, and Saturation. Hue is what we usually refer to as color, therefore, most people use the two words interchangeably. We recognize a change in hue as color change. The physical dimension of hue is wavelength. Brightness is another psychological dimension that refers to the intensity of the stimulus. The more intense the light, the brighter that object appears. For example, an object's color appears brighter in a well-lit room than in a dark one. Saturation is related to the physical dimension of spectral purity. It tells us the amount of hue that we see in an object. In other words, it refers to how complex the light wave is. If the light is simple (for example, a sine wave light), it is pure and therefore appears to be very saturated. The pure color generated by a single wavelength is called monochromatic color. Examples of effects of hue, brightness, and saturation are shown below.

The mixture of color
Monochromatic color rarely happens. Most of the objects we see around us consist of more than one hue. Their colors are mixtures of wavelengths of light. There are two kinds of color mixtures: additive and subtractive. Additive color mixture referrs to the mixing of the three primary lights: red, blue, and green. When all three colors of light are added, we see the white light (the same as the one from the sun). Subtractive color mixtures, on the other hand, are colors that result from mixing pigments, paint, or dye. The primary colors for subtractive mixtures are magenta, yellow, and cyan.

Memory color
Even though there is a strong relation between what we perceive as color and the physical characteristics of light stimuli, our perception of color is also influenced by other factors. Examples of these factors are familiarity and past experience. For example, Duncker (1938) found that a green paper cut in a leaf shape is perceived to be greener than the same green paper cut in a donkey shape. This is because leaves are typically green but donkeys are not. Therefore, we can conclude that sometimes previous color and form associations have a strong effect on perceived color.

Theories of color perception
Now that we know about visual stimuli or dimensions of color that we can see, the next question is how does our visual system detect color. From the large number of theories that try to explain our perception of color, there are two main theories that are strongly supported. They are the trichromatic receptor theory (or Young-Helmholtz Theory) and Opponent-Process theory. The trichromatic receptor theory was proposed in 1802 by Thomas Young and revised in 1866 by Herman von Helmholtz. It says that there are only three types of color receptors (or cones) in the retina. These receptors are most sensitive to a specific range of wavelength of light. There are S cones, which are most sensitive to 445-nm wavelength, or the color blue; M cones, which are most sensitive to 535-nm wavelength, or the color green; and L cones, which are most sensitive to 570-nm wavelength, or the color red, as shown below.

source: http://www.photo.net/photo/edscott/vis00010.htm

As we see above, there is some overlap between the absorption curves (a small overlap between S and M cones and a larger one between M and L cones). These overlaps show that some wavelengths stimulate more than one type of cone. For example, a 450-nm wavelength light is absorbed almost 91% by S cones, while it is absorbed less than 25% by both M and L cones. Therefore, colors other than green, red, and blue, according to this theory, activate mixed patterns of cones in the additive color mixture.

Another theory that has been used to explain how we perceive color is the opponent process theory, proposed by Ewald Hering in 1920. He made some very interesting observations that could not be explained by the trichromatic receptor theory. He noticed that there are certain pairs of colors one never sees together in the same place and at the same time. For example, we do not see reddish greens or yellowish blues. But we do see yellowish greens, bluish reds, yellowish reds etc. Hering also observed that there was a distinct pattern in the color of the afterimages we see. You can try this "complementary afterimage" experiment by staring at the white dot in the middle of the flag for about 30 seconds. Then, shift your gaze to the black dot on the right picture. The complementary colors will appear, and you should see the American flag.

Source: Schiffman (2000) Sensation and Perception, Wiley: NY

Like the trichromatic receptor theory, the opponent process theory also has three types of receptors. However, each type is responsible for a pair of opponent color processes: a blue-yellow, a green-red, and a white-black, with one color on one end and the other on the other end. For example, blue light will excite the blue-yellow pair toward the blue end; while yellow light will excite the same receptors toward the yellow end. When both blue and yellow lights are present simultanously, we will not see any color (we'll see gray) because blue and yellow cancel out the perception. The trichromatic receptor theory and the opponent process theory are both plausible as our color-coding mechanism. More important than that, recent studies have shown that both theories might work together in our visual system. In 1955, Hurvich and Jameson suggested a two-stage process that combines the two theories. According to Hurvich and Jameson, three types of cones (S, M, and L), in the first stage, peak at different wavelengths and send the signals to color-opponent cells of the second stage. A model of this theory is shown below.

Source: http://www.handprint.com/HP/WCL/color1.html

In conclusion, we know that we perceive different dimensions of physical characteristics of light (wavelength, intensity, and spectral purity) as different psychological dimensions of color (hue, brightness, and saturation). We also know that our major source of light, the sun, produces light that consists of all visible wavelengths that can be broken down using a spectrum. Moreover, all of the colors that we see are made from three primary colors using either additive or subtractive color mixtures. There are two major theories that are used to explain our color-coding mechanism. Both of them are supported by the physiology of the visual system. However, they are not mutually-exclusive. In fact, the most recent studies show that both of them work together as part of our color-coding system.

The whole is different than the sum of its parts.

At the beginning of the twentieth century, the school of Gestalt psychology emerged in Germany as a reaction to structuralism, another school of thought. The Gestalt school of thought believed that our perception is the result of the relation between stimuli, rather than the existence of the stimuli themselves. The word Gestalt means "form," "shape," or "whole configuration" in German. For example, the figures below illustrate Gestalt perception. In each of them, the perception we get is the result of the relation of the existing dots, lines, and shapes to one another rather than the sum of their individual sensory effects. The idea of Gestalt perception applies not only to the visual sense but also to other senses such as hearing.

Source: Schiffman (2000) Sensation and Perception, Wiley: NY

Gestalt Grouping Principles
We will not cover many aspects of Gestalt psychology here because the subject is rather large. Instead we will discuss the Gestalt principles that explain our perception. Gestalt grouping principles are the classifications of the pictorial properties that allow us to perceive different forms.

Proximity or Nearness
The principle of proximity or nearness enables us to group what we see according to closeness. Visual stimuli that are close together are grouped together. In the figure below, the circles are seen as arranged in pairs.

 

 

Similarity
If the distances between elements are the same, the ones that are physically similar will be grouped together, according to the principle of similarity. Therefore, green and red dots in the following figure seem to be organized in columns (in Figure A) and in rows (in Figure B). The similarity between elements can also group them in terms of form (shown in Figure C) and size (shown in Figure D).

 

Uniform Connectedness
We perceive elements as a single unit if they are connected to one another, according to the principle of uniform connectedness. This principle sometimes overrules the principle of proximity and the principle of similarity as shown below on the left and right sides respectively.

 

Good Continuation
According to grouping on the basis of good continuation, elements that appear to follow the same direction are grouped together. Directions can be a straight line or a curve. Two examples of this grouping principle are shown below. In Figure A, we tend to see two curves from A to B and from C to D, rather than from A to D or A to C. In figure B, we tend to see two separate lines, rather than separate unfamiliar shapes.

 

Common Fate
Elements moving in the same direction and at the same speed tend to be grouped together. This principle is similar to the similarity principle except it works for moving elements. One example of this is the "wave" created by the arm movement of sports fans. Similarly, the figure below illustrates the principle of common fate.

Symmetry
Grouping on the basis of symmetry refers to the perception of the more natural, balance, and symmetrical figure as the same unit. The figures below show that perceptual organization follows the symmetrical pattern.

Source: Schiffman (2000) Sensation and Perception, Wiley: NY

Closure
The enclosure of complete figures occurs even though the stimuli are incomplete, according to the grouping principle of closure. For example, we tend to see complete figures from fragmentary ones, such as those displayed below.

Source: Schiffman (2000) Sensation and Perception, Wiley: NY

 

Depth Perception
From the two-dimensional images that fall on our retinas we somehow are able to see three-dimensional objects. Seeing objects in three dimensions, or depth perception, allows us to estimate distances between those objects and us. It allows us, and many other animals, to calculate the height of a cliff or the distance of danger at a glance. Have you ever wondered how our eyes (or brains) do that? How do we transform two-dimensional retinal images into three-dimensional perceptions?

We have depth perceptions because the optic array projected on the retinas conveys information that allows us to do so. Some of this extra information, or cues, only require one eye (monocular cues), while others require two eyes (binocular cues). However, we only use monocular cues when we view two-dimensional images, such as paintings and drawings. Binocular cues such as convergence and binocular disparity, on the other hand, are information gained from real three-dimensional objects such as sculptures. Therefore, we will only focus on monocular cues in this website.

Pictorial Perception
Depth perceptual cues that do not require both eyes to observe at the same time are called monocular cues. Most of them work when both the viewer and the scene are stationary (called pictorial cues), while others are only useful to us when the observer, the scene, or both are moving (sometimes called dynamic cues). Dynamic cues include motion parallax, motion perspective, accommodation, and familiar size. Again, because the main object of this website is to understand visual perception of two-dimensional art works; we will only focus on pictorial cues.

Pictorial cues consist of interposition, aerial perspective, shading and lighting, elevation, linear perspective, texture gradients, and relative size. More than one of these cues is usually present simultaneously by artists who intend to imply three-dimensional perceptions from two-dimension images. If you take a look at the painting by Clande Lorrain (1600-1682), a seaport at sunset, you will find that the artist used a number of pictorial cues to imply depth. Move your mouse over the painting to see several depth cues.

 

Linear Perspective Interposition Elevation Shading and Lighting Texture Gradients Aerial Perspective Relative Size

 

Interposition
Interposition (or partial occlusion) happens when objects are overlapping. The object that is partially covered by another one appears to be in the back. For example, the blue star is in front of the pink bar, while the yellow moon is not. We know that because the yellow moon is partially covered by the pink bar and the pink bar is partially covered by the blue star.

Aerial Perspective
Aerial perspective (or clearness) refers to the difference in how clearly we can see objects that are far away and those that are near. Far objects are less clear because light rays passing through them travel through more of the atmosphere than light rays passing through nearer objects. Moreover, far objects have lower contrast with the background than do near objects. This is, again, because light rays loose more energy while traveling from far objects than they do from near objects. The figure below shows how artists use aerial perspective in their work to imply three-dimensional images.

Source: http://www.dia.org/art/comping/1961_1980_300ppi/76.89.jpg

 

Shading and Lighting
Because the closest surface of an object to the light is brightest, we know from the pattern of shading and lighting about the depth and shape of an object. Artists have used this technique to illustrate depth in two-dimensional images. An example of such artworks is by Vermeer (1665), shown below. He used lighting and shading to cue for depth.

Source: Schiffman (2000) Sensation and Perception, Wiley: NY

 

Elevation
The elevation of objects above the horizon in our visual field is an important cue to their depth. Objects located higher in the field are farther away. Sometimes this cue is called relative height. This cue is very important to artists in simulating depth in their paintings. We perceive the top car to be far and the bottom one to be near.

Source: The New Yorker Album of Drawings, The Viking Press, NY, 1975

 

Linear Perspective
Linear perspective (often referred to only as perspective) is one of the most commonly used in two-dimensional artwork to give the perception of three dimensions. According to linear perspective, far objects are systematically smaller in size than nearer objects. Thus, parallel tracks appear to converge at a point called the vanish point. An example of this is shown below.

Source: http://www.bertnienhuis.nl/images/o4.jpg

 

Texture Gradients
When we look at any textured surface, the elements comprising the texture appear smaller and denser as the distance increases. Texture gradients, together with linear perspective, are used a lot in artwork. As seen in the figure below, textural changes give us a strong cue about what we see. Differences in the way the lines are drawn give us different perceptions.

Source: The New Yorker Album of Drawings, The Viking Press, NY, 1975

 

Relative Size
Relative size refers to a cue applied when two identical objects with different sizes are shown. The larger one is usually seen as being closer to the viewer than the smaller one. This idea is used to draw a simple wired cube, shown below.

Source: The New Yorker Album of Drawings, The Viking Press, NY, 1975

 

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