Week 6 Colour

2 Overview By the end of this lecture you will be familiar with: –Human visual system –Foundations of light and colour –HSV and user-oriented colour models

3 The Human Eye

4 HVS Function Part of the nervous system Light enters through the pupil (2mm-9mm) The lens focuses the light onto the retina Retina cells are of two types: cones and rods Huge dynamic range of around 80dB –30,000 candel/m 2 (white A4 in bright sunlight) –0.03 cd/m 2 (same page in moonlight)

5 The Human Visual System Retina –light-sensitive membrane consisting of three types of colour sensors (cones) –each type is most receptive to wavelengths in either the red, green or blue ranges –perceived colour is a result of the relative excitation of each group of cones Leads to a 3-D representation of colour based on red, green & blue primaries

6 Contrast Sensitivity of the Eye The perception of fine detail is dependent on luminance level The eye is good at identifying sharp boundaries Detail can be missed if the changes are gradual The eye prefers richness in colour than resolution –consider PlayStation or Nintendo The eye can only resolve about 40 grey levels

7 Implications for Multimedia Preserve edge information Consider brightness for fine detail Consider number of colour shades in use Consider when to use inverse video Carefully choose contrasting colours Useful properties for image & video coding...

Colour Models

9 Light A narrow frequency band within the spectrum of electromagnetic energy –the visible spectrum from nm –others are ultraviolet, infrared, microwaves etc. –each wavelength within the visible spectrum produces light of a different colour –400nm=violets, blues, greens, yellows, 700nm=oranges, reds

10 Spectral Density P( ) - The power per unit wavelength of a coloured light P( ) (nm) violet Indigo Blue Green Yellow Orange Red

11 Why we see colours? Materials have different absorption and scattering amounts for the different wavelengths of light Examples: –A yellow object absorbs a lot of blue light, but scatters in the longer (red and green) wavelengths –Black clothing gets very hot in sunlight, because it doesn’t scatter much light (obviously not, as it’s black!) so it absorbs a lot (as heat)

12 Additive Colour Matching Mixing different amounts and wavelengths of light together produces colours Maxwell’s trichromatic colour theory

13 Colour Description Coloured light is described in terms of : –Hue: the “perceived colour” (red/yellow etc) determined by the dominant wavelength. No dominant wavelength = achromatic –Saturation: the purity of the colour –Brightness/Luminance: the perceived intensity of the light The Chrominance of a colour is the combination of hue and saturation

14 Colour Definition Dominant wavelength method –useful for description of colours, but not for precisely obtaining and representing colour values A more precise method is based on the reception of coloured light by the human eye

15 RGB Colour Cube Green Red Blue Green (0,1,0) Cyan (0,1,1) Yellow (1,1,0) White (1,1,1) Black (0,0,0) Blue (0,0,1) Magenta (1,0,1) Red (1,0,0)

16 Using RGB Hardware-oriented model –Equally defined & independent RGB values are well suited to graphics architecture –Greyscales along line where R=G=B –Additive colour specification (additive primaries) colours defined in terms of an addition to black –Linear colour combination operation suitable for colour merging operations, but less useful for colour mixing

17 HSV (Hue, Saturation, Value) Developed by AR Smith (1978) –re-coding of RGB colour cube to generate a user- view of colour –set RGB cube on black vertex and look down from the white vertex –primary and secondary colours are arranged radially around the centre axis

18 The HSV Hexicone Black Red Green Blue Yellow Cyan Magenta White Saturation Value Hue

19 Using HSV First, select the colour (pure hue) which most closely matches the desired colour (hue)

20 Using HSV cont. Lighten the colour by adding white (saturation) Saturation indicates the degree to which the hue differs from a neutral gray. The values run from 0%, which is no color saturation, to 100%, which is the fullest saturation of a given hue at a given percentage of illumination.

21 Using HSV cont. Darken or lighten the colour appropriately by changing the level of illumination. Values run as percentages; 0% appears as black (no light), whilst 100% is full illumination, which washes out the colour.

22 Overview of the HSV cone

23 Other Hardware Oriented Models CMYK –Cyan, Magenta, Yellow, Black –Subtractive primaries colour is specified as a subtraction from white used in printing industry YUV, YIQ –Broadcast standards (YUV=UK, YIQ=USA) –Y=Luminance, UV/IQ are chrominance –RGB re-coded for narrow transmission bandwidth

24 User-Oriented Models RGB-based models are derived from a good fit with hardware requirements –but they do not provide an intuitive means of user colour specification –e.g. how to specify brown, gold, etc.? User-oriented models attempt to view colour using the perceptive terms identified earlier