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

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ve P

ow

er

400 450 500 550 600 650 7000

2

4

6

8

10

12

14

Normalized to 560 nm

leaf

flower

Rel

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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

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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

0.2

0.3

0.4

0.5

0.6

0.7

0.8

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

0.5

1

1.5

2

-5 0 50

0.5

1

1.5

2Superposition

-5 0 50

0.5

1

1.5

2

-5 0 50

0.5

1

1.5

2

-5 0 50

0.5

1

1.5

2

reti

nal i

nten

sity

mon

itor

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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