Color 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

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Color and graphics displays l.jpg

Color and Graphics Displays

Jian Huang


Physics l.jpg

  • 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

Color and wavelength l.jpg
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 l.jpg
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?

Color is human sensation l.jpg
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 l.jpg
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 don’t know the precise light sensitivity on each person’s 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


  • 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 l’Eclairage).

  • 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 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 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

Rgb and cmy l.jpg

  • Converting between RGB and CMY

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  • 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

  • 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 l.jpg
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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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)

  • >1 RGB, scale down (in what space?)

  • Not a trivial question (sometimes known as tone mapping)

Tone mapping l.jpg
Tone mapping

  • Real scene: large range of luminance (from 10 -6 to 10 6 cd/m2 )

  • Limitation of the display1-100 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)

Gamma correction35 l.jpg
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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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 l.jpg
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

Raster display l.jpg
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

  • 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

  • Advantages

    • Allows solids to be displayed

    • Leverages low- cost CRT H/W

    • Whole Screen is constantly updated

Lcd displays l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
Raster Displays

  • Display synchronized with CRT sweep

  • Special memory for screen update

  • Pixels are the discrete elements displayed

  • Generally, updates are visible

Double buffer l.jpg
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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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