Transcript Color Models and Graphics Displays

Computer Graphics
CC416
Lecture 03: Color Display & Color Models
Dr. Manal Helal – Fall 2014
The Physics of Color
• 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.
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
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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
3D LMS Colour Space
The normalized spectral sensitivity of human cone cells of short (S,
420–440 nm)-, middle (M, 530–540 nm)- and long (L, 560–580 nm)wavelength types, using three pure light source: R = 700 nm, G = 546
nm, B = 436 nm.
Cλ = r(λ) + g(λ) + b(λ)
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
• the XZ plane will
contain all possible
chromaticities at that
luminance.
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
COLOR MODELS FOR RASTER GRAPHICS
• The purpose of a color model is to allow convenient
specification of colors within some color gamut.
• Three hardware-oriented color models are RGB
(CRT monitors), YIQ (broadcast TV color system),
and CMY (color-printing devices)
• To be related directly to intuitive color notions of
hue, saturation, and brightness, another class of
models (HSV, HSL, HVC, etc) are developed with
ease of use as a goal
Copyright @ 2001 by Jim X. Chen:
[email protected]
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
Additive vs. Subtractive Color
Models
• Since additive color models display color as a result
of light being transmitted (added) the total absence of
light would be perceived as black. Subtractive color
models display color as a result of light being
absorbed (subtracted) by the printing inks. As more
ink is added, less and less light is reflected. Where
there is a total absence of ink the resulting light being
reflected (from a white surface) would be perceived
as white.
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Additive vs. Subtractive Color
Models
25
The YIQ color model (US color TV broadcasting)
•
A recording of RGB for transmission efficiency and for
downward compatibility with black and white television.
• Here Y=luminance, the same as the CIE Y primary.
Only the Y component of a color TV signal is shown on
black-and-white TVs.
• I stands for in-phase, while Q stands for quadrature,
referring to the components used in quadrature
amplitude modulation. Some forms of NTSC now use the
YUV color space, which is also used by other systems
such as PAL.
• More bits of bandwidth are used to represent Y than to
represent I and Q, because our eye is more sensitive to
changes in luminance
The YIQ color space at Y=0.5
From RGB to YIQ
From YIQ to RGB
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
RGB to HSV Conversion
• Preliminaries:
R' = R/255
Cmax = max(R', G', B’)
Δ = Cmax – Cmin
• Hue calculation:
• Saturation calculation:
• Value calculation:
V = Cmax
G' = G/255
B' = B/255
Cmin = min(R', G', B')
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 CRT
• Requires precision
geometry
• Patterned phosphors
on CRT face
• Aligned metal shadow
mask
• Three electron guns
• Less bright than
monochrome CRTs
LCD Displays
• Liquid Crystal Display
• Organic molecules that
remain in crystalline structure
without external force, but realigns 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