Spectra

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Why is this hard to read
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Unrelated vs. Related Color

• Unrelated color color perceived to belong to an
  area in isolation (CIE 17.4)
• Related color color perceived to belong to an
  area seen in relation to other colors (CIE 17.4)

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Illusory contour

• Shape, as well as color, depends on surround
• Most neural processing is about differences

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Illusory contour
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CS 768 Color Science

• Perceiving color
• Describing color
• Modeling color
• Measuring color
• Reproducing color

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Spectral measurement

• Measurement p(l) of the power (or energy, which
  is power x time ) of a light source as a function
  of wavelength l
• Usually relative to p(560nm)
• Visible light 380-780 nm

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Linearity

• additivity of response (superposition)
• r(m1m2)r(m1)r(m2)
• scaling (homogeneity)
• r(am)ar(m)
• r(m1(x,y)m2 (x,y)) r(m1)(x,y)r(m2)(x,y)
  (r(m1)r(m2))(x,y)
• r(am(x,y))ar(m)(x,y)

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Non-linearity
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http//webvision.med.utah.edu/
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Optic nerve
Light
Ganglion
Amacrine
Bipolar
Horizontal
Cone
Rod
Epithelium
Retinal cross section
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Visual pathways

• Three major stages
• Retina
• LGN
• Visual cortex
• Visual cortex is further subdivided

http//webvision.med.utah.edu/Color.html
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Optic nerve

• 130 million photoreceptors feed 1 million
  ganglion cells whose output is the optic nerve.
• Optic nerve feeds the Lateral Geniculate Nucleus
  approximately 1-1
• LGN feeds area V1 of visual cortex in complex
  ways.

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Photoreceptors

• Cones -
• respond in high (photopic) light
• differing wavelength responses (3 types)
• single cones feed retinal ganglion cells so give
  high spatial resolution but low sensitivity
• highest sampling rate at fovea

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Photoreceptors

• Rods
• respond in low (scotopic) light
• none in fovea
• try to foveate a dim starit will disappear
• one type of spectral response
• several hundred feed each ganglion cell so give
  high sensitivity but low spatial resolution

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Luminance

• Light intensity per unit area at the eye
• Measured in candelas/m2 (in cd/m2)
• Typical ambient luminance levels (in cd/m2)
• starlight 10-3
• moonlight 10-1
• indoor lighting 102
• sunlight 105
• max intensity of common CRT monitors 102
• From Wandell, Useful Numbers in Vision Science
  http//white.stanford.edu/brian/numbers/numbers.h
  tml

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Rods and cones

• Rods saturate at 100 cd/m2 so only cones work at
  high (photopic) light levels
• All rods have the same spectral sensitivity
• Low light condition is called scotopic
• Three cone types differ in spectral sensitivity
  and somewhat in spatial distribution.

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Cones

• L (long wave), M (medium), S (short)
• describes sensitivity curves.
• Red, Green, Blue is a misnomer. See
  spectral sensitivity.

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Receptive fields

• Each neuron in the visual pathway sees a specific
  part of visual space, called its receptive field
• Retinal and LGN rfs are circular, with
  opponency Cortical are oriented and sometimes
  shape specific.

On center rf
Red-Green LGN rf
Oriented Cortical rf
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Channels Visual Pathways subdivided

• Channels
• Magno
• Color-blind
• Fast time response
• High contrast sensitivity
• Low spatial resolution
• Parvo
• Color selective
• Slow time response
• Low contrast sensitivity
• High spatial resolution

• Video coding implications
• Magno
• Separate color from bw
• Need fast contrast changes (60Hz)
• Keep fine shading in big areas
• (Definition)
• Parvo
• Separate color from bw
• Slow color changes OK (40 hz)
• Omit fine shading in small areas
• (Definition)
• (Not obvious yet) pattern detail can be all in
  bw channel

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Trichromacy

• Helmholtz thought three separate images went
  forward, R, G, B.
• Wrong because retinal processing combines them in
  opponent channels.
• Hering proposed opponent models, close to right.

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Opponent Models

• Three channels leave the retina
• Red-Green (L-MS L-(M-S))
• Yellow-Blue(LM-S)
• Achromatic (LMS)
• Note that chromatic channels can have negative
  response (inhibition). This is difficult to model
  with light.

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100
Luminance
10
1.0
Contrast Sensitivity
Red-Green
0.1
Blue-Yellow
0.001
-1
0
1
2
Log Spatial Frequency (cpd)
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Color matching

• Grassman laws of linearity (r1 r2)(l) r1(l)
  r2(l) (kr)(l) k(r(l))
• Hence for any stimulus s(l) and response r(l),
  total response is integral of s(l) r(l), taken
  over all l or approximatelyS s(l)r(l)

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Surround light
Primary lights
Surround field
Bipartite white screen
Subject
Test light
Primary lights
Test light
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Color Matching

• Spectra of primary lights s1(l), s2(l), s3(l)
• Subjects task find c1, c2, c3, such
  that c1s1(l)c2s2(l)c3s3(l)matches test light.
• Problems (depending on si(l))
• c1,c2,c3 is not unique (metamer)
• may require some cilt0 (negative power)

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Color Matching

• Suppose three monochromatic primaries r,g,b at
  645.16, 526.32, 444.44 nm and a 10 field (Styles
  and Burch 1959).
• For any monochromatic light t(l) at l, find
  scalars RR(l), GG(l), BB(l) such that t(l)
  R(l)r G(l)g B(l)b
• R(l), G(l), B(l) are the color matching functions
  based on r,g,b.

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Color matching

• Grassman laws of linearity (r1 r2)(l) r1(l)
  r2(l) (kr)(l) k(r(l))
• Hence for any stimulus s(l) and response r(l),
  total response is integral of s(l) r(l), taken
  over all l or approximatelyS s(l)r(l)

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Color matching

• What about three monochromatic lights?
• M(l) RR(l) GG(l) BB(l)
• Metamers possible
• good RGB functions are like cone response
• bad Cant match all visible lights with any
  triple of monochromatic lights. Need to add some
  of primaries to the matched light

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Surround light
Primary lights
Surround field
Bipartite white screen
Subject
Test light
Primary lights
Test light
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Color matching

• Solution CIE XYZ basis functions

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Color matching

• Note Y is V(l)
• None of these are lights
• Euclidean distance in RGB and in XYZ is not
  perceptually useful.
• Nothing about color appearance