Color and Graphics Displays

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Color and Graphics Displays

• Jian Huang
• CS594

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Physics

• Its 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 400-700 nm
  range, the tiny piece of spectrum between
  infra-red and ultraviolet

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Visible Light
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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.
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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?

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The Eye
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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

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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.

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Limitation of Knowledge

• We dont know the precise light sensitivity on
  each persons retina.

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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

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Main Color Spaces

• CIE XYZ, xyY
• RGB, CMYK
• HSV (Munsell, HSL, IHS)
• Lab, UVW, YUV, YCrCb, Luv,

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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

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CIE Standard

• CIE International Commission on Illumination
  (Comission Internationale de lEclairage).
• Human perception based standard (1931),
  established with color matching experiment
• Standard observer a composite of a group of 15
  to 20 people

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CIE Experiment
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CIE Experiment Result

• Three pure light source R 700 nm, G 546 nm,
  B 436 nm.

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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

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CIE Color Space
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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)

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Color Gamut

• The range of color representation of a display
  device

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RGB (monitors)

• The de facto standard

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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?

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CMY(K) printing

• Cyan, Magenta, Yellow (Black) CMY(K)
• A subtractive color model

dye color absorbs reflects cyan
red blue and green magenta green blue and
red yellow blue red and green black all none

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RGB and CMY

• Converting between RGB and CMY

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RGB and CMY
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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

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HSV Hexcone

• Intuitive interface to color

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Lab photoshop

• Photoshop uses this model to get more control
  over color
• Its 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

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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

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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

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Examples (RGB, HSV, Luv)
 







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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)

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Gamut Mapping

• Negative RGB add white (maintains hue,
  de-saturate)
• gt1 RGB, scale down (in what space?)
• Not a trivial question (sometimes known as tone
  mapping)

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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

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Gamma Correction

• The phosphor dots are not a linear system
  (voltage vs. intensity)

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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

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No gamma correction
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Gamma corrected to 1.7
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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?

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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

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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

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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.

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Color CRT

• Requires precision geometry
• Patterned phosphors on CRT face
• Aligned metal shadow mask
• Three electron guns
• Less bright than monochrome CRTs

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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 (lt 40 inches)
• Spurious X- ray radiation
• Occupies a large volume

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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

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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.

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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.

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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

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Raster Displays

• Display synchronized with CRT sweep
• Special memory for screen update
• Pixels are the discrete elements displayed
• Generally, updates are visible

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Double Buffer

• Adds a second frame buffer
• Swaps during vertical blanking
• Updates are invisible
• Costly

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Memory Rasterizer

• Maintains a copy of the screen (or some part of
  it) in memory
• Relies on a fast copy
• Updates are nearly invisible

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True Color and Indexed Color FB
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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