Download RGB colour model

Chapter 6
Color
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
colour.
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.
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
brain.
Color & Science
Light is a form of electromagnetic radiation, we
can measure its wavelength.
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http://micro.magnet.fsu.edu/primer/lightandcolor/inde
x.html
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
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Rods: night-vision and cannot distinguish color
Cones: in three sorts which respond to different
wavelengths
Cone Response
Tristimulus Theory
Any colors can be specified by just three
values, giving weights of three
components.
Each type of cone responds to R, G, B
Primary Colors
Additive mixing
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TV, monitors
Subtractive mixing
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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
discussion.
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
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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
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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
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Palette of 256 colors
Color lookup table (CLUT)
Logical colors => physical colors
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24 bits color => indexed color
1/3 of data
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PNG, BMP, TGA, TIFF: palette
GIF: only 256 colors
JPEG: 24 bit color
Missing color: the nearest color
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Visible artifacts
Posterization: gradations replace by sharp boundaries
Web-safe Palette
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216 colors, Displayed on any platform
Posterization
Area of a single color are replaced by a pattern
of dots of several colors
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Pink = red + white
Dithering: high resolution
Halftoning: greyscale images
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N by N pixels = N2+1 grey levels
Other Color Models
CMYK
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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
light
White light
Magenta
Cyan ink
Cyan light
Absorption of
green light
Absorption of red
light
Not possible to manufacture inks which
absorb only light of precisely the
complementary color.
Some unwanted colors are absorbed.
RGB gamut > CMY gamut
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C+M+M != perfect black
With black: CMYK
Hue, saturation, brightness
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As white is mixed in, saturation decreases.
Color wheel
HSV
Color Pickers
Color Differences
Brightness=(R+G+B)/3
Luminance
Y=0.2125R + 0.7154G + 0.0721B
Green contributes far more
Color differences
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U=B-Y and V=R-Y
YUV
YCBCR
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
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CIE L*a*b*: as in CMYK for prepress industry
CIE L*u*v*: as in RGB for monitors and
scanners
Channels and Color Correction
Color images = R + B + C channels
Each channel can be manipulated
separately.
Color correction
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Compensate for the deficiencies of scanners
and other input devices
Time-consuming
Wizards, assistants
Color Correction
Color balance + hue & saturation adjustment
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Color balance
3 sliders: C+R, M+G, Y+B
Replace a specified color with a different
color
CMYK: printing
YCBCR: video
Alpha channel
different from color channels
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Blue screen: alpha = blue channel
Mask out background
JPEG: process each channel separately
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YCBCR: downsample color difference
Consistent Color
Color profiles
White=(255,255,255)
specified in a deviceindependent color space
Gamma
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
EPS, TIFF JFIF, PNG:
color profile
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At least R, G, B, white
point, gamma
Fig. 6.13
ICC
International Color Consortium
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Apple’s ColorSync
Kodak Precision Color Management System
TIFF, EPS: complete ICC profiles
sRGB: standard RGB
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ITU-R BT.709 standard
Gamma=2.2 and white point= 6500 K