Physiology of Perception

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Physiology of Perception

• Light

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Light

• Light is the stimulus for vision
• The energy within the EM spectrum is
  characterised by its wavelength
• The wavelength of visible light is associated
  with colour
• Light - as a wave or as a stream of particles
  called photons (1 photon 1 packet of light
  energy)

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Light

• An electromagnetic phenomenon, like television
  waves, infrared radiation, and x-rays
• Light means those waves that lie in a narrow band
  of wavelengths in the visible spectrum

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EM spectrum
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Colour Blindness
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Colour Blindness

• 8 Normal vision
• 3 Red Green colour blind
• No number no colour vision!
• Color blindness is not a single problem but a
  variety of problems that affect color vision
• Only a very small portion of people do not see
  color at all, but view the world in shades of
  gray
• The most common type of color blindness is
  red/green, which has two forms
• Some people are missing red receptor cones
• Others are missing green receptor cones
• Approximately eight percent of males and one
  percent of females

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Choosing Colour Schemes for People with Colour
Blindness

• There are four general guidelines to be
  considered when designing computer interfaces
• Design in black and white, adding color for
  emphasis.
• Color should never be the only visual cue for
  anything.
• Distinguish between features that a user must be
  able to see in order to use the application and
  those that would be nice to see.
• Test whether color contrast is sufficient to be
  read by people with color deficiencies or by
  those with low-resolution monitors, print pages
  on a black and white printer (with backgrounds
  and colors appearing in grayscale).
• Sufficient contrast for reading is a key factor
  when considering accessible colours for
  applications
• Therefore, particular colour combinations to be
  avoided are red-green, green-yellow, and
  blue-yellow

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Colour

• We need tools to describe and control colour in
  applications
• Colour depends on interactions between the
  physics of light radiation and the eye-brain
  system
• How are colours described in numerical terms, and
  how do these descriptions relate to everyday ways
  of describing colour?

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Pure Spectral Light

• The eye responds to light with wavelengths
  between approximately 400 and 700 nm
• Some light sources emit light of essentially a
  single wave-length (e.g. lasers)
• Here are some spectral densities for pure
  spectral light, and how we perceive them

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Perceived Colour

• The light from most sources, however, does not
  consist of only one wavelength
• It consists of power of various amounts over a
  continuous set of wavelengths
• Their spectral densities (Spectra) cover a band
  of wavelengths
• An enormous variety of spectra is perceived by
  the eye as having the same colour

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Example Spectra

• The total power of the light in any band of
  wavelengths is found as the area under the
  density curve over the band of visual light (400
  and 700 nm)

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Describing Colour

• How can a a particular colour be precisely and
  unambiguously described?
• Verbal descriptions such as Dark blue, Bright
  red, Slimy green are too broad
• Description of its spectral density curve, by
  specifying its level at a number of wavelengths
  is awkward, and too specific, as many different
  spectral shapes produce the same perceived colour

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Numeric Colour Description

• Ideally, every colour should be described
  uniquely in some numeric way
• How many numbers are required to define a colour?
• What coding scheme can be used to map colours
  into numbers, and vice versa?
• There are several different conventions for
  coding colours, what are they, and how do they
  relate to each other?
• E.g The light sources and their colour are
  described by the International Commission on
  Illumination (CIE)

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Tri-stimulus colour theory

• Tri-stimulus colour theory the colour we see is
  the result of our cones response to red, green
  and blue light
• We do not unlike insects have extra cones that
  react to other colour frequencies

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Tri-stimulus Therory

• Any colour can be constructed as a linear
  combination of three primary colours
• e.g. C n1R n2G n3B (n1, n2, n3 scalars)
• e.g. (0,1,0) would be pure green, (.2,.3,.5)
  would be a yellow

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Hue, Luminance, Purity

• When we view a source of light our eyes respond
  to 3 basic sensations
• Hue
• The colour of light we perceive
• Purity or Saturation
• How washed out or how pure the colour appears
• eg. pastels are described as less pure
• Brightness or Luminance
• Perceived intensity of light
• A.K.A. Hue Saturation Brightness (HSB)

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Dominant Wavelength Theory

• The energy density of the dominant light
  component is labelled D
• red in this example
• The brightness of the light is the area under the
  curve
• Purity depends on the difference between D and A

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Dominant Wavelength Theory

• The dominant wavelength, luminance and saturation
  fully define a colour
• The larger the energy D of the dominant frequency
  compared to the white-light component A the more
  pure the light
• Purity or saturation of 100 when A 0
• Purity or saturation of 0 when D A
• Pastel colours contain much white light, and are
  therefore unsaturated
• The eye can distinguish about 128 different hues,
  and about 20 different saturations (depending on
  the hue).

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Dominant Wavelength Theory

• Luminance (total power in the light)
• L (D - A)B AW
• Hue (location of the dominant wavelength)
• Colour of the main pure light present
• Saturation (purity of the light)
• the of luminance that resides in the dominant
  component

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3-dimensional Colour Spaces

• Saturation, luminance and hue are useful concepts
  for describing colour
• However, not very easy to measure these values
  when presented with a sample colour
• It does, however, illustrate the fact that colour
  perception is three-dimensional,
• i.e. that any colour may be described uniquely by
  exactly three numbers
• Any colour can be represented as a point in a
  three-dimensional colour space

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Colour Models

• The most common colour model for CRT monitors is
  the RGB model, an additive colour system
• The RGB space is a cube

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Colors Computing (Java)

• Several different approaches to specifying
  colours
• red, green, blue
• hue, saturation, brightness
• luminance and two chrominance
• Create Color objects as needed, specify rgb
  values (0...255)
• myColor new Color(135, 206, 235) // light blue

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Colors in Java

• Class Color has some predefined color constants
• black, blue, cyan, darkGray, gray, green,
  lightGray, magenta, orange, pink, red, white,
  yellow
• SystemColor class has colors used by default for
  things like color of window frame, border color
  for caption text, ...