why is this hard to read. unrelated vs. related color unrelated color: color perceived to belong to...
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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() of the power (or energy, which is power x time ) of a light source as a function of wavelength
• Usually relative to p(560nm)
• Visible light 380-780 nm
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Spectral Distribution of daylight
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Normalized to 560 nm
leaf
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Spectra of a red flower and a green leaf
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Normalized to spectrometer max
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Data from http://www.it.lut.fi/research/color/database/database.html
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Retinal line spread function
retinal position
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Linearity
• additivity of response (superposition)
• r(m1+m2)=r(m1)+r(m2)
• scaling (homogeneity)• r(m)=r(m)• r(m1(x,y)+m2 (x,y))=
r(m1)(x,y)+r(m2)(x,y)= (r(m1)+r(m2))(x,y)
• r(m(x,y))=r(m)(x,y)
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Non-linearity
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stimulus
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adaptive architecture provides more sensitivity over smaller range
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http://webvision.med.utah.edu/
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Ganglion
Bipolar
Amacrine
Rod Cone
Epithelium
Optic nerve
Retinal cross section
Light
Horizontal
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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 star—it 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 10^2 From Wandell, Useful Numbers in Vision Science
http://white.stanford.edu/~brian/numbers/numbers.html
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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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Cone Spectral Responses
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Cone Spectral Responses
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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 rf’s are circular, with opponency; Cortical are oriented and sometimes shape specific.
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On center rf Red-Green LGN rf
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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 b&w
– Need fast contrast changes (60Hz)
– Keep fine shading in big areas
– (Definition)
• Parvo
– Separate color from b&w
– Slow color changes OK (40 hz)
– Omit fine shading in small areas
– (Definition)
• (Not obvious yet) pattern detail can be all in b&w channel
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V'() V()
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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-M+S = L-(M-S))– Yellow-Blue(L+M-S)– Achromatic (L+M+S)
• Note that chromatic channels can have negative response (inhibition). This is difficult to model with light.
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RedGreen L-M+S BlueYellow L+M-S
Achromatic L+M+.05*S
Schematic color opponent response
+- +
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V()
wavelength (nm)
RedGreen L-M+S BlueYellow L+M-S
Achromatic L+M+.05*S
Schematic color opponent response
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Log Spatial Frequency (cpd)
Con
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Luminance
Red-Green
Blue-Yellow
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Color matching
• Grassman laws of linearity:()(((
• Hence for any stimulus s() and response
r(), total response is integral of s() r(), taken over all or approximately s()r()
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Primarylights
Test light
Bipartitewhitescreen
Surround field
Test light Primary lights
Subject
Surround light
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Color Matching
• Spectra of primary lights s1(), s2(), s3()
• Subject’s task: find c1, c2, c3, such thatc1s1()+c2s2()+c3s3()
matches test light.
• Problems (depending on si())
– [c1,c2,c3] is not unique (“metamer”)
– may require some ci<0 (“negative power”)
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Color matching
• What about three monochromatic lights?• M() = R*R() + G*G() + B*B()• Metamers possible• good: RGB functions are like cone
response• bad: Can’t match all visible lights with any
triple of monochromatic lights. Need to add some of primaries to the matched light
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Primarylights
Test light
Bipartitewhitescreen
Surround field
Test light Primary lights
Subject
Surround light
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Color matching
• Solution: CIE XYZ basis functions
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CIE 1931 standard colorimetric observer color matching functions
Tris
timul
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alue
wavelength (nm)
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Color matching
• Note Y is V()
• None of these are lights
• Euclidean distance in RGB and in XYZ is not perceptually useful.
• Nothing about color appearance
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3.5Stiles and Burch 1959 10-degree bipartite field color matching functions
primary lights at 645.2 nm 525.3 nmand 444.4 nm
b10() g10()
r10()