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Colouran algorithmic approach
Thomas [email protected]
http://www.eecs.qmul.ac.uk/~tb300/pub/PhD/Stage1Report.pptx
PhD Research Topic
part 1: The Colour Model
The Munsell Colour Model
actual mapping to human vision
A colour catalog vs a colour model
color catalog vs colour model• catalog requires selection of colours based on
perceptual matching• partial colour model codes spectrum as systematic
mixing of wavelengths• true colour model codes the color of spectrum
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X=100,Y=100,Z=0
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Colour as information
a theory of information processing.
Colour Reproduction
• true colour code + specs of viewer = image– colour defined by code– viewer can be group or individual– display decides how to create colour from code– gives perceptual predictability
Yellow
Orange
bluish-red
Magenta 0
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The Standard Observerfrom colour matching studies
CIE1931 xy chromaticity diagramprimaries at: 435.8nm, 546.1nm, 700nm
The XYZ sensor response
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Y is defined as luminancedifference from Y is the colour informationThe Math:
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x y z
… 2-d as z is redundant
Understanding CIE chromaticity
White in center
Saturated / monochromatic wavelengths on the periphery
Best understood as a failed colour circle
Everything in between is a mix of white and the colour
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x y z
yy
x y z
Circular colour models are the holy grail of colour theory … so far no one has succeeded!
x and y show difference from Y
Does it match?Problem #1:‘negative primaries’
But does the CIE model work?
Problem #2:no definition of colour
Colour Sensorresponse to monochromatic light
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Wavelength (nm)
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RGB
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700 nm330 nm 400 nm 500 nm 600 nm
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Human
Bird
4 sensors
Equidistant on spectrum
What are these sensors used for?
What information is needed?
my answer is: Wavelength
How to calculate wavelength with 2 poor quality luminance sensors.
Roughly speaking:
Wavelength
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λ-Δ λ λ+Δ
RG
a shift of Δfrom a known reference point
the ideal light stimulus
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Wavelength
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λ-δ λ λ+δ
RGmonochromatic stimulus
Allows wavelength to be measured in relation to reference.
Monochromacy:The reason we see rainbows is because the human visual system works with single wavelength light -- monochromatic light
This is the underlying paradigm!
Problem:natural light is not ideal
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Wavelength
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λ-δ λ λ+δ
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• Light stimulus might not activate reference sensor fully.
• Light stimulus might not be fully monochromatic.
ie. there might be white mixed in
Sens
or V
alue
Wavelength(λ, in nm)
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Solution:
A 3rd sensor is used to measure equiluminance.
Which is subtracted.
Then reference sensor can be normalized
Equiluminance & Normalization – essential to finding wavelength, can also called saturation and lightness
a 4 sensor designS
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Wavelength(λ, in nm)
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2 opponent pairs• only 1 of each pair can be active• min sensor is equiluminance
,R G B y
Human Retina only has 3 sensors!What to do?
We add an emulation layer.
Hardware has 3 physical sensorsbut emulates 4 sensors
Wavelength (nm)
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460 580 640520 550 610490430 670400370 700
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R G B
No maths … just a diagram!
Testing Colour Opponent model
What we should see
What we do see
There is Red in our Blue – the problem of Purple
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Pigment Absorption Data of human cone sensors
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Wavelength (nm)
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Red > Green
Dual Opponency with Circularity
an ideal model using 2 sensor pairs
Senso
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Wavelength(λ, in nm)400300 430 460 490 520 550 580 610 640 670 700
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a circular colour model
We divide colour coding and colour reproduction:
• Codingno need to link to specific observer – ideal observernot linked specifically to human vision
• Displaydecides how best to present colour to observer – making colour anomalies fit
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Part 1 – Coding Colour• fully circular• universal ideal observer
Part 2 – Reproducing Colour• takes knowledge about observer
and optimizes/distorts to the individual/group
• improved or natural reproduction modes
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Wavelength(λ, in nm)400300 430 460 490 520 550 580 610 640 670 700
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Coding Natural Colour
Problem #1:
Real world is not monochromatic
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Spectrum of a common yellow flower
Colour coding … for dual channel opponency
Problem # 1
easy to solve
we simply assume monochromacy
when stimuli are not monochromatic opponent channels simply subtract to 0
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green, yellow and red are active
r-g = 0b = 0
leaving only yellow
stimuli equivalent to monochromatic
Opponent Coding
Only primaries are true coloursall other colours are intermediary
… and can be generated by proportions of primaries!
Accurate colour reproduction … for humans
Any colour may be displayed by a combination of 2 primaries
but the location of primaries can vary between individualsand intermediary locations can be distorted
Problem # 2
Accurate colour reproduction … tuned to the individual
1. primaries must be mapped for the individual
2. mid-points must be mapped
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Provides an individual colour profile … a map of the primaries and intermediary points.
tunable primaries
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Wavelength (nm)
Blue
Green
Yellow
Red
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Part 3testing the theory
• is it sound?• is it useful?• does human vision use it?
Is there empirical evidence to support paradigm + theory?
note: a theoretical model about information is the information itself!
Apparatus
• monochromator
• light sourceequal light across visible spectrum
the stimuli
Transition Colour Matching
• generate subject selectable monochromatic stimuli
• subject selects colour• perceptual primaries are
calculated
Resultsno leading questions -- only “blue”• 4 primaries (pure colours)
naturally resolve to blue, green, yellow and red
• primaries are equidistant• transitions worked for all subjects• Most subjects see peripheral colours
– red in blue– 40% could see “magenta” – blue in red
• potential problem:people treat purple as if it were primary
• some colour blind people can’t see purple
histogram of results
results from mapping colour vision
Application
natural colour reproduction
Current display technology: 0.1 – 100 cd/m2
(currently pushed up to 500, but designed for 100 cd/m2)
DICOM GSDF: 0.05 – 4,000 cd/m2 (defined for grayscale medical imaging only)
Natural environment: 0.01 – 10,000 cd/m2
Luminance: High Dynamic Range
Coding HDR
HDR is here now … using multiple exposure!
… using an absolute lumiance code rather than a relative code
the colour of infra-red (650-750nm)
remove the filter from a digital camera & it will work in the infra-red
Images in the infra-red produced by enthusiasts now!
What is the colour when you go beyond red?
not the stereo-typebut true infra-red – high wavelength light
related work: Dolby
Examples of real world colour?
Colours are often computed, not measured!
… an extreme example
What is the colour?
http://www.eecs.qmul.ac.uk/~tb300/pub/PhD/Stage1Report.pptx
Poynton, 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.
http://blog.dolby.com/2013/12/tv-bright-enough/
Questions?
References
