Unrelated vs. Related Color Unrelated color: color perceived to belong to an area in isolation (CIE...
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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)
Illusory contour
• Shape, as well as color, depends on surround
• Most neural processing is about differences
Illusory contour
CS 768 Color Science
• Perceiving color
• Describing color
• Modeling color
• Measuring color
• Reproducing color
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
350 400 450 500 550 600 650 700 75040
60
80
100
120
Spectral Distribution of daylight
Rel
ati
ve P
ow
er
400 450 500 550 600 650 7000
2
4
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14
Normalized to 560 nm
leaf
flower
Rel
ati
ve P
ow
er
Spectra of a red flower and a green leaf
400 450 500 550 600 650 7000
0.2
0.4
0.6
0.8
Normalized to spectrometer max
leaf
flower
Abs
olu
te P
ow
er
Data from http://www.it.lut.fi/research/color/database/database.html
-5 -4 -3 -2 -1 0 1 2 3 4 50
0.1
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0.9
1
Retinal line spread function
retinal position
rela
tive
inte
nsit
y
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)
-5 0 50
0.5
1
1.5
2
-5 0 50
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1
1.5
2
-5 0 50
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1.5
2Superposition
-5 0 50
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-5 0 50
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1.5
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-5 0 50
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reti
nal i
nten
sity
mon
itor
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nsit
y
Non-linearity
0 10 20 30 40 50 60 70 80 90 100
0.5
0.55
0.6
0.65
0.7
0.75
0.8
0.85
0.9
0.95
1
stimulus
resp
ons
e
adaptive architecture provides more sensitivity over smaller range
http://webvision.med.utah.edu/
Ganglion
Bipolar
Amacrine
Rod Cone
Epithelium
Optic nerve
Retinal cross section
Light
Horizontal
Visual pathways
• Three major stages– Retina
– LGN
– Visual cortex
– Visual cortex is further subdivided
http://webvision.med.utah.edu/Color.html
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.
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
Photoreceptors
• Rods– respond in low (scotopic) light– none in fovea– one type of spectral response– several hundred feed each ganglion cell so give
high sensitivity but low spatial resolution
Rods and cones
• Rods saturate at 100 cd/m2 so only cones work at high (photopic) light levels
• All have same spectral sensitivity
• Low light condition is called scotopic
• Three cone types differ in spectral sensitivity and somewhat in spatial distribution.
Cones
• L (long wave), M (medium), S (short)– describes sensitivity curves.
• “Red”, “Green”, “Blue” is a misnomer. See spectral sensitivity.
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.
--
----
--
-+
-
-
On center rf Red-Green LGN rf
+
++ +
+
+
+
+
-
-
-
Oriented Cortical rf
Visual Pathways
• Channels (pathways)• 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