Visual perception is a function of our eyes and brain. We

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• Visual perception is a function of our eyes and
  brain. We see images as a whole rather than in
  parts. However, images can be broken down into
  their visual elements line, shape, texture, and
  color. Together they allow our eyes to see images
  and our brain to recognize them.

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Line

• A line is the path made by a tool, such as a pen,
  crayon, or stick and can be seen as a distinct
  series of points. Line is believed to be the most
  expressive of the visual elements.
• It outlines things and the outlines are key to
  their identity. Most of the time, we recognize
  objects or images only from their outlines.

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• Line is important because it is a primary
  element in writing and drawing, which are
  universal. 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. A line implies action,
  and the impression of movement suggests sequence,
  direction, or force.

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• 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.

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• 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

Paul Klee - Insula Dulcamara, 1938 88 x 176 cm,
Oil and colored past on paper on burlap
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Shape

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

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• 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.

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• 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.
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For example, a work by Theo van Doesburg,
Composition The Card Players 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

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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.

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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.

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• 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).

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Nature of color

• What we perceive as color is primarily the
  wavelength of the light stimulation. The shortest
  viewable wavelength is what we see as BLUE and
  the longest wavelength is what we see as RED.

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• The wavelengths that fall between them are what
  we see as the other colors. However, color
  perception is very subjective. We do not have a
  way of proving that two different people perceive
  the same color.

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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 red
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.
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• The white light from the sun consists of many
  different wavelengths because of Newton's prism
  (shown right).
• 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.

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The dimensions of color

• Wavelength explains differences in the colors we
  see , but 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 refers to the intensity of the light.
  The more intense the light, the brighter that
  object appears. An object's color appears
  brighter in a well-lit room than in a dark one.

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• Saturation is related to the physical dimension
  of spectral purity. It tells us the amount of hue
  that we see in an object.
• If the light is simple, 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 above.
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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.

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• Additive color mixture refers 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.

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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.

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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. There are two main theories that are
  strongly supported.
• They are the trichromatic receptor theory and
  Opponent-Process theory. The trichromatic
  receptor theory 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.

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• There are S cones, which are most sensitive to
  the color blue M cones, which are most sensitive
  to the color green and L cones, which are most
  sensitive to the color red, as shown below.

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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. Therefore,
colors other than green, red, and blue, according
to this theory, activate mixed patterns of cones
in the additive color mixture.
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• Another theory to explain how we perceive color
  is the opponent process theory. We can notice
  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. Also it can be
  observed that there is a distinct pattern in the
  color of the afterimages we see.

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• 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.

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• Like the trichromatic receptor theory, the
  opponent process theory also has three types of
  receptors. 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 and yellow
  light will excite the same receptors toward the
  yellow end. When both blue and yellow lights are
  present together, we will not see any color
  (we'll see gray) because blue and yellow cancel
  each other out.
• The trichromatic receptor theory and the opponent
  process theory are both plausible as our
  color-coding mechanism. Studies have shown that
  both theories might work together in our visual
  system using a two-stage process that combines
  the two theories.

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• 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 at right.

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• 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.
• Also, 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 are
  supported by the physiology of the visual system,
  and recent studies show that both of them work
  together as part of our color-coding system.