Kleur Digital Multimedia, 2nd edition Nigel Chapman & Jenny Chapman Chapter 6 This presentation © 2004, MacAvon Media Productions

Opdrachten Welke drie kleuren licht gebruikt een computermonitor? Wat kun je zeggen over R=G=B? Waarvoor worden Cyaan, magenta en geel (Y) gebruikt? Kun je die kleuren laten zien? Bereken hoeveel kleuren een plaatjes kan bevatten als je per kleur 8 bits tot je beschikking hebt (3 kleuren) Hoeveel kleuren kan Jpg bevatten? En Gif?

© 2004, MacAvon Media Productions 6 Subtract additive primary from white gives its complement Equivalently, add other two additive primaries C = G+B = W-R M = R+B = W-G Y = R+G = W-B Cyan, magenta and yellow are subtractive primary colours (mixing ink/paint) Complementary Colours 170–171

© 2004, MacAvon Media Productions 6 Require a model that relates subjective sensation of colour to measurable phenomena Spectral Power Distribution (SPD) How intensity varies with wavelength Good model, but too cumbersome to work with in computer graphics Tristimulus Theory Any colour can be produced by mixing different amounts of three additive primaries Colour & Science 157–158

© 2004, MacAvon Media Productions 6 Use standard red, green and blue as primaries Actually no single agreed standard Cannot mix all visible colours from R, G and B Restricted to RGB gamut Basis of colour reproduction in monitors Represent any colour with three numbers (r,g,b) Use as pixel's stored value in bitmapped image RGB Colour 158–160

© 2004, MacAvon Media Productions 6 Can choose number of bits for each of r, g and b More bits per component means more colours can be distinguished, but image files will be larger 8 bits (1 byte) per component: 24-bit colour, millions of colours If r = g = b, colour is a shade of grey, so greyscale can be represented by a single value 8 bits permits 256 greys Colour Depth 161

© 2004, MacAvon Media Productions 6 8-bit colour only permits 256 colours Instead of storing (r, g, b) for each pixel, store an index into a palette or colour lookup table (CLUT) Index can be small, usually a single byte Palette stores up to 256 (for 1-byte index) 24- bit values To determine colour of a pixel, look at the stored value, use it to look up full 24-bit (r, g, b) value in palette Indexed Colour 163–167

© 2004, MacAvon Media Productions 6 Ideally choose 256 most important colours in an image to store in its palette When 24-bit image is reduced to indexed colour, some colours may be missing form the palette Replace missing colour by nearest, may lead to posterization Dither – use pattern of dots and optical mixing Web-safe palette – 216 colours guaranteed to reproduce accurately on all platforms and browsers Colour Palettes 167–169

© 2004, MacAvon Media Productions 6 Real inks do not correspond to ideal subtractive primaries Combining three inks for black is undesirable Printers use four process colours, cyan, magenta, yellow and black CMYK gamut is not the same as RGB Implications for using images prepared for print (CMYK) on the Web (RGB) CMYK Colour 172–173

© 2004, MacAvon Media Productions 6 Alternative way of specifing colour Hue (roughly, dominant wavelength) Saturation (purity) Value (brightness) Model HSV as a cylinder: H angle, S distance from axis, V distance along axis Basis of popular style of colour picker HSV 173–176

© 2004, MacAvon Media Productions 6 Can be useful to separate brightness and colour information, especially for video Luminance Y not simply related to R, G and B because eye is more sensitive to some colours Y = R G B Can store Y plus two colour difference values B-Y and R-Y Strictly use non-linearly scaled Y' and weighted colour differences U and V or C B and C R YUV 176–177

© 2004, MacAvon Media Productions 6 Instead of considering 24-bit image as array of 3-byte pixels, consider it as 3 arrays of 1-byte pixels i.e. 3 greyscale images, k/a channels, one for each of R, G and B Can adjust levels &c of each channel independently to make colour corrections Actually quite tricky, so Photoshop also provides specialized colour correction tools Channels 179–183

© 2004, MacAvon Media Productions 6 Different monitors, scanners, &c have different colour characteristics – different colours correspond to same numerical value Vary with model, age, temperature,… Captured by (ICC) colour profile, which includes values for: R, G and B chromaticities White point (may be given as colour temperature) Gamma Colour Profiles 183–184

© 2004, MacAvon Media Productions 6 Software maps between colour spaces of different devices e.g. scanner and monitor, designers's monitor and customer's monitor Use colour profiles of devices to perform the mapping Store input device profile in image file Only works if profiles are accurate, which relies on frequent calibration Colour Management 184–186

© 2004, MacAvon Media Productions 6 'Standard' RGB colour space Chromaticities, white point and gamma supposed to be typical of monitors W3C standard Convert image colours to sRGB Should still display well on most monitors without using colour management software Alternatively can colour manage without storing input device profile sRGB 187