Color and Graphics Displays Jian Huang CS 594

Color and Graphics Displays Jian Huang CS 594

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 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.

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 430 nm, 560 nm, and 610 nm 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, xy. Y RGB, CMYK HSV (Munsell, HSL, IHS) Lab, UVW, YUV, YCr. Cb, 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 xy. Y Space • Irregular 3 D 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 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

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. 200 Bn Y = 0. 299 Rn + 0. 587 Gn + 0. 114 Bn Z = 0. 066 Gn + 1. 116 Bn x=X/(X+Y+Z) y=Y/(X+Y+Z) z=Z/(X+Y+Z) u = 4 x / ( -2 x + 12 y + 3 ) v = 6 y / ( -2 x + 12 y + 3 ) • Luv is derived from UVW and Lab, with all components guaranteed to be positive

Yuv and YCr. Cb: 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. 299 R + 0. 587 G + 0. 114 B • 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 • YCr. Cb 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/m 2 ) • Limitation of the display 1 -100 cd/m 2 • 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 1 st 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: 1 mm (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