Colour an algorithmic approach Thomas Bangert thomas.bangert@qmul.ac.uk...

Colouran algorithmic approach

Thomas Bangertthomas.bangert@qmul.ac.uk

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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… 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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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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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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• 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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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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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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Red > Green

Dual Opponency with Circularity

an ideal model using 2 sensor pairs

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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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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

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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