color - PowerPoint

							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 400-
  700 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   absorbs   reflects
    cyan        red       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               •Disadvantages
   – Allows solids to be      •Requires screen- sized memory array
     displayed                (frame buffer)
   – Leverages low- cost
     CRT H/W                  •Discrete spatial sampling (pixels)
   – Whole Screen is          •Moire patterns: when shadow- mask
     constantly updated       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