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Color Models and Graphics Displays

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Color and Graphics Displays
Jian Huang
CS594
Physics
• It’s all electromagnetic (EM) radiation
– Different colors correspond to radiation of
different wavelengths
– Intensity of each wavelength specified by
amplitude
• Frequency = 2 pi/wavelength
• We perceive EM radiation with in the 400700 nm range, the tiny piece of spectrum
between infra-red and ultraviolet
Visible Light
Color and Wavelength
Most light we see is not just a single wavelength, but a
combination of many wavelengths like below. This profile
is often referred to as a spectrum, or spectral power
distribution.
3-Component Color
• The de facto representation of color on
screen display is RGB. (additive color)
• Some printers use CMY(K), (subtractive
color)
• Why?
– The color spectrum can be represented by 3
basis functions?
The Eye
Color is Human Sensation
• Cone and rod receptors in the retina
• Rod receptor is mostly for luminance
perception
• 3 different types of cone receptors in the
fovea of retina, responsible for color
representation. Each type is sensitive to
different wavelengths
Cone Receptors
• There are three types of cones, referred
to as S, M, and L. They are roughly
equivalent to blue, green, and red
sensors, respectively.
• Their peak sensitivities are located at
approximately 430nm, 560nm, and
610nm for the "average" observer.
Limitation of Knowledge
• We don’t know the precise light sensitivity
on each person’s retina.
So, what is the standard color?
• The basis of comparison is not math!!
• The basis of comparison is human color
matching experiments
• 100% mathematically correct light object
interaction need to be evaluated at more
than 3 points in the spectrum
Main Color Spaces
•
•
•
•
CIE XYZ, xyY
RGB, CMYK
HSV (Munsell, HSL, IHS)
Lab, UVW, YUV, YCrCb, Luv,
Differences in Color Spaces
• What is the use? For display, editing,
computation, compression, …?
• Several key (very often conflicting) features
may be sought after:
– Additive (RGB) or subtractive (CMYK)
– Separation of luminance and chromaticity
– Equal distance between colors are equally
perceivable
CIE Standard
• CIE: International Commission on
Illumination (Comission Internationale de
l’Eclairage).
• Human perception based standard (1931),
established with color matching experiment
• Standard observer: a composite of a group
of 15 to 20 people
CIE Experiment
CIE Experiment Result
• Three pure light
source: R = 700
nm, G = 546 nm,
B = 436 nm.
CIE Color Space
• 3 hypothetical light
sources, X, Y, and
Z, which yield
positive matching
curves
• Y: roughly
corresponds to
luminous efficiency
characteristic of
human eye
CIE Color Space
CIE xyY Space
• Irregular 3D volume shape is
difficult to understand
• Chromaticity diagram (the same
color of the varying intensity, Y,
should all end up at the same
point)
Color Gamut
• The range of color
representation of a
display device
RGB (monitors)
• The de facto standard
The RGB Cube
• RGB color space is perceptually
non-linear
• RGB space is a subset of the
colors human can perceive
• Con: what is �bloody red’ in
RGB?
CMY(K): printing
• Cyan, Magenta, Yellow (Black) – CMY(K)
• A subtractive color model
dye color
cyan
absorbs
red
reflects
blue and green
magenta
green
blue and red
yellow
blue
red and green
black
all
none
RGB and CMY
• Converting between RGB and CMY
RGB and CMY
HSV
• This color model is based on polar coordinates,
not Cartesian coordinates.
• HSV is a non-linearly transformed (skewed)
version of RGB cube
– Hue: quantity that distinguishes color family, say red
from yellow, green from blue
– Saturation (Chroma): color intensity (strong to weak).
Intensity of distinctive hue, or degree of color sensation
from that of white or grey
– Value (luminance): light color or dark color
HSV Hexcone
• Intuitive interface to color
Lab: photoshop
• Photoshop uses this model to get
more control over color
• It’s named CIE Lab model (refined
from the original CIE model
• Liminance: L
• Chrominance: a – ranges from green
to red and b ranges from blue to
yellow
Luv and UVW
• A color model for which, a unit change in luminance and
chrominance are uniformly perceptible
U = 13 W* (u - uo ); V = 13 W* (v - vo); W = 25 ( 100 Y ) 1/3 - 17
where Y , u and v can be calculated from :
X = O.607 Rn + 0.174 Gn + 0.200Bn
Y = 0.299 Rn + 0.587 Gn + 0.114Bn
Z = 0.066 Gn + 1.116 Bn
x=X/(X+Y+Z)
y=Y/(X+Y+Z)
z=Z/(X+Y+Z)
u = 4x / ( -2x + 12y + 3 )
v = 6y / ( -2x + 12y + 3 )
• Luv is derived from UVW and Lab, with all components
guaranteed to be positive
Yuv and YCrCb: digital video
• Initially, for PAL analog video, it is now also used in CCIR
601 standard for digital video
• Y (luminance) is the CIE Y primary.
Y = 0.299R + 0.587G + 0.114B
• Chrominance is defined as the difference between a color and
a reference white at the same luminance. It can be represented
by U and V -- the color differences.
U = B – Y; V = R - Y
• YCrCb is a scaled and shifted version of YUV and used in
JPEG and MPEG (all components are positive)
Cb = (B - Y) / 1.772 + 0.5; Cr = (R - Y) / 1.402 + 0.5
Examples (RGB, HSV, Luv)
Color Matching on Monitors
• Use CIE XYZ space as the standard
• Use a simple linear conversion
• Color matching on printer is more difficult,
approximation is needed (CMYK)
Gamut Mapping
• Negative RGB: add white (maintains hue,
de-saturate)
• >1 RGB, scale down (in what space?)
• Not a trivial question (sometimes known as
tone mapping)
Tone mapping
• Real scene: large range of luminance
(from 10 -6 to 10 6 cd/m2 )
• Limitation of the display 1-100 cd/m2
• cd : candela, unit for measuring
intensity of flux of light
Gamma Correction
• The phosphor
dots are not a
linear system
(voltage vs.
intensity)
Gamma correction
• Without gamma correction, how will
(0,255,127) look like?
• Normally gamma is within 1.7 and 2.8
• Who is responsible for Gamma correction?
• SGI does it for you
• PC/Mac etc, you should do it yourself
No gamma correction
Gamma corrected to 1.7
Residual Gamma or System
Gamma
• Systems such as SGI monitor has a gamma
of 2.4, but they only gamma correct for 1.7.
• The residue gamma is 2.4/1.7 = 1.4, why?
• Depends on how you see it? Bright screen,
dark room causes changes in your eye
transfer function too.
• What about web pages? Which screen do
you intend for?
CRT Display
•
•
•
•
Cathode Ray Tubes (CRTs)
Most common display device
Evacuated glass bottle
Electrons attracted to focusing anode
cylinder
• Vertical and Horizontal deflection plates
• Beam strikes phosphor coating on front of
tube
Vector Display
• Oscilloscopes were some of the 1st
computer displays, used by both
analog and digital computers
• Computation results used to drive the
vertical and horizontal axis (x,y),
intensity could also be controlled (z)
• Used mostly for line drawings, called
vector, calligraphic display
• Display list had to be constantly
updated
Raster Display
• TV boom made it
cheap
• Entire screen painted
30 times/ sec
• Screen is traversed 60
times/ sec
• Even/ Odd lines on
alternate scans,
�interlace’.
Color CRT
• Requires precision
geometry
• Patterned phosphors on
CRT face
• Aligned metal shadow
mask
• Three electron guns
• Less bright than
monochrome CRTs
Pro/Con for Raster CRT Display
• Advantages
– Allows solids to be
displayed
– Leverages low- cost
CRT H/W
– Whole Screen is
constantly updated
•Disadvantages
•Requires screen- sized memory array
(frame buffer)
•Discrete spatial sampling (pixels)
•Moire patterns: when shadow- mask
and dot- pitch frequencies mismatch
•Convergence (varying angles of
approach distance of e-beam across CRT
face)
•Limit on practical size (< 40 inches)
•Spurious X- ray radiation
•Occupies a large volume
LCD Displays
• Liquid Crystal Display
• Organic molecules that remain
in crystalline structure without
external force, but re-aligns
themselves like liquid under
external force
• So LCDs realigns themselves to
EM field and changes their own
polarizations
Passive LCD
• LCD slowly transit between states.
• In scanned displays, with a large number of pixels,
the percentage of the time that LCDs are excited is
very small.
• Crystals spend most of their time in intermediate
states, being neither "On" or "Off".
• These displays are not very sharp and are prone to
ghosting.
Active Matrix LCD
• E field is retained by a capacitor so that
the crystal remains in a constant state.
• Transistor switches are used to transfer
charge into the capacitors during
scanning.
• The capacitors can hold the charge for
significantly longer than the refresh
period
• Crisp display with no shadows.
• More expensive to produce.
Plasma Display
• Basically fluorescent tubes
• High- voltage discharge excites gas mixture (He,
Xe), upon relaxation UV light is emitted, UV light
excites phosphors
•Large view angle
•Large format display
•Less efficient than CRT, more power
•Large pixels: 1mm (0.2 mm for CRT)
•Phosphors depletion
Raster Displays
•
•
•
•
Display synchronized with CRT sweep
Special memory for screen update
Pixels are the discrete elements displayed
Generally, updates are visible
Double Buffer
•
•
•
•
Adds a second frame buffer
Swaps during vertical blanking
Updates are invisible
Costly
Memory Rasterizer
• Maintains a copy of the screen (or some part of it)
in memory
• Relies on a fast copy
• Updates are nearly invisible
True Color and Indexed Color FB
High Color FB
• Popular PC/( SVGA) standard (popular with
Gamers)
• Each pixel can be one of 2^ 15 colors
• Can exhibit worse quantization (banding) effects
than indexed- color
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