SI23 Introduction to Computer Graphics

1. SI23Introduction to Computer Graphics Lecture 3 – Colour Vision

2. Light and the Spectrum • Light is the visible form of electromagnetic energy 10-6 10-3 10-1 10 103 106 109 1012 (nm) Cosmic rays Gamma rays X-rays UV Infra-red Micro- wave Radar Radio 380 760 nanometres Red Violet Green Blue Yellow

3. Human Visual System – The Eye

4. Rods and Cones

5. 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 Human Eye

6. What do You See?

9. 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 Colour Depth Effects

10. Additive Mixing of Lights

11. Colour Matching

12. 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

13. Sensitivity to Colour

14. Sensitivity 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

17. Union 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

18. Signals from eye to brain

19. From Eye to Brain

20. From Eye to Brain

21. 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 From Eye to Brain

24. Simultaneous 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

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