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Chapter 6
Multimedia Systems
Key Points
Colour is a subjective sensation produced
in the brain.
The measurable physical quantity that
best models colour is a spectral power
distribution (SPD).
In the RGB colour model, a colour is
represented by three values, giving the
proportions of red (R), green (G) and blue
(B) light which make up light of the desired
Key Points
The RGB colour gamut is a proper
subset of the visible colours.
The colour depth of an image is number
of bits used to hold a colour value.
Indexed colour: The stored 8-bit value is
used as an offset into a colour lookup
table (CLUT) or palette containing up to
256 24-bit colour values.
Key Points
Dithering helps avoid the posterization that
can occur when indexed colour is used, by using
patterns of dots to simulate a wider range of
colours using optical mixing.
The CMYK model is important for printing, as it
models the way colour is produced by mixing ink
or paint.
Other colour models include HSV, YUV and its
variants, and the CIE device-independent
spaces Lab and Luv.
Key Points
A 24-bit image can be thought of as being
made up of three 8-bit channels, one
each for red, green and blue in RGB
Colour correction etc. can be performed by
using techniques from Chapter 5 on
individual channels.
Colour management software uses
device profiles to help maintain
consistent colour.
Color & Science
Color is a subjective sensation produced in the
Color & Science
Light is a form of electromagnetic radiation, we
can measure its wavelength.
Microwave Ovens
The wavelength is tuned to frequencies
that are readily absorbed by water
molecules in food causing them to absorb
energy and release heat as they vibrate.
Containers holding the food in microwave
ovens usually contain very little water and
tend to remain cool.
Color & Science
Visible light (可見光): 400nm~700nm
Color & Science
Spectral power distribution (SPD)
Color & Science
Receptor Cells
Rods: night-vision and cannot distinguish color
Cones: in three sorts which respond to different
Cone Response
Tristimulus Theory
Any colors can be specified by just three
values, giving weights of three
Each type of cone responds to R, G, B
Primary Colors
Additive mixing
TV, monitors
Subtractive mixing
Paint absorbs light
Color Temperature
The color temperature model is based on the
relationship between the temperature of a
theoretical standardized material, called a black
body radiator, and the energy distribution of its
emitted light as the radiator is brought to
increasingly higher temperatures, measured in
Kelvin (K).
Although this radiator does not exist, many
metals behave very similar to a black body so
we can take a metal pot as an example for this
Color Temperature
RGB Colors
RGB color model
Not possible to represent all visible color as a
combination of fixed R, G, B.
Other Color Spaces
Color Depth
Color depth
24, 30, 36, 48 bits
Grey scale image: R=G=B
PNG format: up to 48 bits
Scanner: 48 bits
Accurate approximations when reduced to a lower resolution
for display
Millions of colors, true color: 24 bits
Thousands of colors, hi-color: 16 bits
256 colors: 8 bits
Grey-level: 256 = 8 bits
Indexed Color
Direct color
Indexed color
Palette of 256 colors
Color lookup table (CLUT)
Logical colors => physical colors
24 bits color => indexed color
1/3 of data
PNG, BMP, TGA, TIFF: palette
GIF: only 256 colors
JPEG: 24 bit color
Missing color: the nearest color
Visible artifacts
Posterization: gradations replace by sharp boundaries
Web-safe Palette
216 colors, Displayed on any platform
Area of a single color are replaced by a pattern
of dots of several colors
Pink = red + white
Dithering: high resolution
Halftoning: greyscale images
N by N pixels = N2+1 grey levels
Other Color Models
C = G+B = W-R
M = R+B = W-G
Y = R+G = W-B
Complementary color
Art and design
The dyes do not supply color to light reflected
off surface, but to light that penetrates through
them and gets reflected or scattered back from
beneath it.
Mixing Colored Inks
White light
Cyan ink
Cyan light
Absorption of red
White light
Cyan ink
Cyan light
Absorption of
green light
Absorption of red
Not possible to manufacture inks which
absorb only light of precisely the
complementary color.
Some unwanted colors are absorbed.
RGB gamut > CMY gamut
C+M+M != perfect black
With black: CMYK
Hue, saturation, brightness
As white is mixed in, saturation decreases.
Color wheel
Color Pickers
Color Differences
Y=0.2125R + 0.7154G + 0.0721B
Green contributes far more
Color differences
U=B-Y and V=R-Y
Device-independent Color Spaces
RGB, CMYK: device-dependent
CIE XYZ: device-independent
Not perceptually uniform
the same change in one of vales produced
the same change in appearance
R: (1 to 11) != (101 to 111)
Perceptually uniform
CIE L*a*b*: as in CMYK for prepress industry
CIE L*u*v*: as in RGB for monitors and
Channels and Color Correction
Color images = R + B + C channels
Each channel can be manipulated
Color correction
Compensate for the deficiencies of scanners
and other input devices
Wizards, assistants
Color Correction
Color balance + hue & saturation adjustment
Color balance
3 sliders: C+R, M+G, Y+B
Replace a specified color with a different
CMYK: printing
YCBCR: video
Alpha channel
different from color channels
Blue screen: alpha = blue channel
Mask out background
JPEG: process each channel separately
YCBCR: downsample color difference
Consistent Color
Color profiles
specified in a deviceindependent color space
Gamma Correction
Sample Input
Gamma Corrected Input
Monitor Output
Graph of Input
Graph of Correction L' = L ^ (1/2.5)
Graph of Output
Color Profile
color profile
At least R, G, B, white
point, gamma
Fig. 6.13
International Color Consortium
Apple’s ColorSync
Kodak Precision Color Management System
TIFF, EPS: complete ICC profiles
sRGB: standard RGB
ITU-R BT.709 standard
Gamma=2.2 and white point= 6500 K