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 perceptibleU = 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 + 0.200BnY = 0.299 Rn + 0.587 Gn + 0.114BnZ = 0.066 Gn + 1.116 Bnx = 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