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SI23Introduction to Computer

Graphics

SI23Introduction to Computer

Graphics

Lecture 3 – Colour Vision

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Light and the SpectrumLight and the Spectrum

Light is the visible form of electromagnetic energy

10-6 1012 (nm)10-3 10-1 10 103 106 109

Cosmicrays

Gammarays

X-rays UV Infra-red Micro-wave

Radar Radio

380 760

Violet Red

BlueGreen

Yellow

nanometres

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Human Visual System – The Eye

Human Visual System – The Eye

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Rods and ConesRods and Cones

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Human EyeHuman Eye

Light enters through cornea, passes through lens and inverted image formed on retina

Cornea is main focus, lens provides the fine tuning

Amount of light entering eye controlled by iris (2-8 mm)

6 million rods, 100 million rods

Cones mainly in fovea, central part of retina, largely absent elsewhere – provide colour perception

Rods in outer part of retina – provide non-colour peripheral vision

160,000 cells per sq mm

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What do You See?What do You See?

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Colour Depth EffectsColour Depth Effects

Light refracted as it passes through the cornea and lens

Normally eye focuses on yellow-green wavelength (560 nm)

Longer red wavelengths converge beyond, blue in front of, retina

To focus on red, we make lens more convex as though object nearer

Effect known as chromostereopsis - works differently for different people (60% see red nearer, no effect for 10%)

Combination of effects including displacement of pupil wrt optical axis of eye – which varies among people

Also depends on background, effect can often reverse

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Additive Mixing of LightsAdditive Mixing of Lights

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Colour MatchingColour Matching

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Additive Mixing of Lights and Colour Matching

Experiments

Additive Mixing of Lights and Colour Matching

Experiments

When two light sources are combined, the result is a simple addition of the sources

Thomas Young (1801) showed that overlapping red, green, blue gave the secondary colours yellow, cyan, magenta; and white where all three overlap

By varying intensities, he was able to match most of the spectral hues

Colour monitors use this principle:– white produced as sum of red, green and blue– both CRT and LCD

Colour matching experiments (CIE, 1931) have given R,G,B values for single wavelength lights, averaged over a number of observers

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Sensitivity to ColourSensitivity to Colour

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Sensitivity to ColourSensitivity to Colour

Three types of cones: spectral absorbtioin curves have peaks at 580, 540 and 440 nm but there is considerable overlap

Each type produces response across range of wavelengths – we determine colour by the combination of the three responses

Relative numbers are:– 40:20:1 in terms of R:G:B– So our sensitivity to blue is much less

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Union JackUnion Jack

Light sensitive elements in cones and rods are proteins known as rhodopsin

By fixating on an image, response is dulled

When replaced by white, we then see the complementary colours only

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Signals from eye to brainSignals from eye to brain

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From Eye to BrainFrom Eye to Brain

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From Eye to BrainFrom Eye to Brain

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From Eye to BrainFrom Eye to Brain

Signals from retina combine into a luminance channel, plus two opponent channels (red-green and yellow-blue differences) [as in colour TV transmission]

Spatial sensitivity of Y-B less than R-G (because few B cones) – so do not show fine detail in blue against black

Further processing goes on as signals leave retina by optic channel to visual cortex

Finally human visual system transforms the signals into a perceptual response – which we are still trying to understand

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Simultaneous Contrast and Coloured SurroundsSimultaneous Contrast

and Coloured Surrounds

Appearance of colour depends on lightness and colour of surrounding region – simultaneous contrast

Colours look smaller and darker against white, lighter and larger against black

Retina takes signals from wider area and does its own image processing

Coloured surrounds can cause a coloured region to be tinged with complementary hue of the surround

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AcknowledgementAcknowledgement

The colour images used in this presentation were prepared by Prof Lindsey MacDonald for the UK Advisory Group on Computer Graphics