digital multimedia, 2nd edition nigel chapman & jenny chapman chapter 6 this presentation ©...

Digital Multimedia, 2nd editionNigel Chapman & Jenny Chapman

Chapter 6

This presentation © 2004, MacAvon Media Productions

Colour

© 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


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


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


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


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


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


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


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


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 = 0.2125R + 0.7145G + 0.0721B

• 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 CB and CR

YUV

176–177


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


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


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


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

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