C o lo ur an algorithmic approach
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Colouran algorithmic approach
Thomas [email protected]
http://www.eecs.qmul.ac.uk/~tb300/pub/PhD/ColourVision2.pptx
PhD Research Topic
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understanding how natural visual systems process information
Visual system: • about 30% of cortex• most studied part of
brain• best understood part
of brain
This research is abut the information produced by the early visual system Information which goes from front of
brain to higher levels at rear of brain
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Image sensors Binary sensor array
monochromatic ‘external retina’
Luminance sensor arraydichromatic colour
Multi-Spectral sensor arraytetrachromatic colour
Sensors: what is measured and what information is sent?
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What is Colour?
Visible Spectrum
Visual system must measure and represent light within this zone.
We start with Luminance – how bright?(we measure how much light)
What information does colour add?How do we code this information.
the stimulus
Any ideas?
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Lets hypothesise … When an astronomer looks at a star, how does he code the information his sensors produce?
It was noticed that parts of spectrum were missing.
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Looking our own star – the sun
• x
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Each atomic element absorbs at specific frequencies …
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We can Code for these elements …
We can imagine how coding spectral element lines could be used for visual perception … by a creature very different to us… a creature which hunts by ‘tasting’ the light we reflect… seeing the stuff we are made of
Colour in this case means atomic structure and chemistry…
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Sensor we build cannot do spectral analysis
Crude system to reproduce colour
3 colour values, usually 8-bit: RGB
What does RGB mean? lights for colour reproduction
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The Standard Observer
CIE1931 xy chromaticity diagramprimaries at: 435.8nm, 546.1nm, 700nm
XYZ – a 3 sensors model of human vision
xxx y z
1 central luminance sensor: Yand colour information are 2 difference measurements ... from YThe Math:
yyx y z
… z is redundant
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Understanding CIE chromaticity
White in center
Saturated / monochromatic colours on the periphery
Best understood as a failed colour circle
Everything in between is a mix of white and the colour
xxx y z
yyx y z
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Does it match?The problem of
‘negative primaries’
But does it blend?
Monochromatic Colours
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The Human Visual System (HVS) does things differently!
?
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Human Visual
System (HVS)
Coding Colour
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The Sensor2 systems: day-sensor & night-sensor
To simplify: we ignore night sensor system
Cone Sensors very similar to RGB sensors we design for cameras
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sensor array
arrangement is random
note:very few blue sensors, none in the centre
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sensor pre-processing circuitry
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First Question: What information is sent from sensor array
to visual system?
Very clear division between sensor & pre-processing (Front of Brain) andvisual system (Back of Brain) connected with very limited communication link
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starting with the sensor:Human Sensor Response
to non-chromatic light stimuli
350 400 450 500 550 600 650 7000
10
20
30
40
50
60
70
80
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100
Wavelength (nm)
Abso
rptio
n (%
)
RGB
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HVS Luminance Sensor IdealizedSe
nsor
Valu
e
Wavelength(λ)λ
0.8
0.6
0.2
0.0
1.0
0.4
λδλ−
A linear response in relation to wavelength.Under ideal conditions can be used to measure wavelength.
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Spatially Opponent
HVS:Luminance is always measured by taking the difference between two sensor values.Produces: contrast value
Sens
or V
alue
Wavelength(λ)λ
0.8
0.6
0.2
0.0
1.0
0.4
λδλ−
Sens
or V
alue
Wavelength(λ)λ
0.8
0.6
0.2
0.0
1.0
0.4
λδλ−
Which is done twice, to get a signed contrast value
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Colour Sensorresponse to monochromatic light
350 400 450 500 550 600 650 7000
10
20
30
40
50
60
70
80
90
100
Wavelength (nm)
Abso
rptio
n (%
)
RGB
370 nm 445 nm 508 nm 565 nm
700 nm330 nm 400 nm 500 nm 600 nm
1.0
0.5
0.0
Human
Bird
4 sensorsEquidistant on spectrum
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if we make a simplifying assumption:our light is monochromatic!
Then:
Wavelength
0.8
0.6
0.2
0.0
1.0
0.4
λ-Δ λ λ+Δ
RG
a shift of Δfrom a known reference point
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the ideal light stimulusSe
nsor
Val
ue
Wavelength
0.8
0.6
0.2
0.0
1.0
0.4
λ-δ λ λ+δ
RG Monochromatic Light
Allows frequency to be measured in relation to reference.
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Problem:natural light is not ideal
Sens
or V
alue
Wavelength
0.8
0.6
0.2
0.0
1.0
0.4
λ-δ λ λ+δ
RG
• Light stimulus might not activate reference sensor fully.
• Light stimulus might not be fully monochromatic.
ie. there might be white mixed in
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Sens
or V
alue
Wavelength(λ, in nm)400300 430 460 490 520 550 580 610 640 670 700
0.8
0.6
0.2
0.0
1.0
0.4
Solution:
A 3rd sensor is used to measure equiluminance.
Which is subtracted.
Then reference sensor can be normalized
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a 4 sensor designSe
nsor
Val
ue
Wavelength(λ, in nm)400300 430 460 490 520 550 580 610 640 670 700
0.8
0.6
0.2
0.0
1.0
0.4
2 opponent pairs• only 1 of each pair can be active• min sensor is equiluminance
,R G B y
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What is Colour?What is the information?• Luminance• Equi-Luminance• Colour
Colour channels are: RGByellow
4 primaries.
Purpose of Colour is to code wavelength!
Information = Luminance + Wavelength
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Any Stimuli can be reduced to:
Equi-Luminance Location on Spectrum Luminance
Complex Spectrum is reduced to very simple equivalent
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Colour often involves further high level processing …Examples of real world colour:
Colours are often computed, not measured!
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… an extreme example
What is the colour?
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http://www.eecs.qmul.ac.uk/~tb300/pub/PhD/ColourVision2.pptx
ReferencesPoynton, C. A. (1995). “Poynton’s Color FAQ”, electronic preprint.http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.html
Bangert, Thomas (2008). “TriangleVision: A Toy Visual System”, ICANN 2008.
Goldsmith, Timothy H. (July 2006). “What birds see”. Scientific American: 69–75.
Neitz, Jay; Neitz, Maureen. (August 2008). “Colour Vision: The Wonder of Hue”. Current Biology 18(16): R700-r702.
Questions?