Achromatic and Colored Light

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Achromatic and Colored Light

• CS 288
• 9/17/1998
• Vic

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Introduction

• It is crucial for modern computer graphics
  students to understand the theory and application
  of light and color.
• Color adds impact to images. Without the proper
  use of color, even the most interesting scene can
  be rather dull and boring.

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Color and Light

• The color of an object depends not only on the
  object but also the light source illuminating the
  object, the color of the surrounding area, and on
  the human visual system.
• Certain objects reflect light (walls, desk,
  paper) while other objects also transmit light
  (cellophane, glass).

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Achromatic Light

• Achromatic light is literally the absence of
  color.
• Can be seen as a black-and-white TV or monitor.
• An observer of achromatic light experiences none
  of the sensations we associate with red, green,
  blue, yellow, and so on.

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Achromatic Light

• The only attribute of achromatic light is
  Quantity of light.
• Quantity of light can be discussed in the physics
  sense of energy, in which case we use the terms
  intensity and luminance, or in the
  psychological case, the term brightness is used.

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Achromatic Light

• Intensity can be thought of as a scalar such as 0
  defined as black and 1 defined as white
• Luminance is the amount, or intensity of light
• Brightness is the perceived intensity of an
  object
• Black-and-white TVs can produce many different
  intensities at a single pixel position

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Varying Intensity Levels
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Intensity (4 bit)
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Intensity (1 bit)
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Chromatic Color

• Presence of color!!!
• Color perception involves Hue, Saturation, and
  Brightness.
• Other terms are tint, shade, and tone.

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Hue

• Hue distinguishes between colors.
• It is the pure color of the object.
• Hue allows us to perceive the differences between
  red, green, blue, purple, and yellow.

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Saturation

• Saturation refers to how far a color is from a
  gray of equal intensity.
• Unsaturated colors include more white light than
  do the vivid, saturated colors.
• Red is highly saturated.
• Pink is relatively unsaturated.
• Royal blue is highly saturated
• Sky blue is relatively unsaturated.

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Brightness

• Brightness refers to the perceived intensity of a
  self-luminous object (such as a light-bulb or the
  sun).
• Think of brightness as being associated to a
  light emitting object rather than a reflecting
  object.

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HSB Color Selector
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Tints, Shades, and Tones

• A tint results when white pigment is added to a
  pure pigment, thereby decreasing saturation.
• A shade comes from adding a black pigment to a
  pure pigment, thereby decreasing brightness.
• A tone is the consequence of adding both black
  and white pigments to a pure pigment

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Psychophysics

• Color mixing methods are very subjective -- they
  depend on the human observers ability to
  perceive the varying colors, as well as the
  lighting, surrounding color, size of the sample,
  and the overall lightness of the environment.
• Colorimetry is the objective, quantitative branch
  of physics to measure color.

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Colorimetry

• Dominant Wavelength is the wavelength of the
  color we see when viewing the light. Corresponds
  to the perceptual notion of hue.
• Excitation Purity is the proportion of pure light
  of the dominant wavelength and of the white light
  needed to define the color. Corresponds to
  saturation.

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Colorimetry

• Luminance is the amount or intensity of light.
  Corresponds to brightness.
• A completely pure color is 100 saturated and
  thus contains no white light. White light and
  hence grays are 0 saturated, containing no color
  of any dominant wavelength.

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Tristimulus Theory of Color Perception

• The tristimulus theory of color perception is
  based on the hypothesis that the retina has three
  kinds of color sensors (cones) with peak
  sensitivity to Red, Green, and Blue.

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Luminous Efficiency function of the Eye

• The luminous efficiency function is the eyes
  response to light of constant luminance. Our
  peak sensitivity is to yellow-green of wavelength
  around 550 nm.

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Typical Spectral Energy distribution of a light
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What Are Complementary Colors

• Complementary colors are those colors that
  combine to make white light.

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Color Gamut

• The color gamut is all the colors that a device
  can display.