visual displays

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Visual DisplaysVisual Displays

Chapter 2 Burdea

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OutlineOutline

Image Quality IssuesImage Quality Issues PixelsPixels ColorColor Video FormatsVideo Formats Liquid Crystal DisplaysLiquid Crystal Displays CRT DisplaysCRT Displays Projection DisplaysProjection Displays

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Image Quality IssuesImage Quality Issues

Screen resolutionScreen resolution SizeSize ColorColor Blank space Blank space

between the pixelsbetween the pixels

BrightnessBrightness ContrastContrast Refresh rateRefresh rate Sensitivity of Sensitivity of

display to viewing display to viewing angleangle

For each, let’s draw up:•Range of commonly available components•Importance•Cost

•Which would you want most?

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PixelsPixels

Pixel Pixel - The most basic addressable - The most basic addressable image element in a screenimage element in a screen– CRT - Color triad (RGB phosphor dots)CRT - Color triad (RGB phosphor dots)– LCD - Single color elementLCD - Single color element

Screen Resolution Screen Resolution - measure of - measure of number of pixels on a screen (m by n)number of pixels on a screen (m by n)– m - Horizontal screen resolutionm - Horizontal screen resolution– n - Vertical screen resolutionn - Vertical screen resolution

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Other meanings of Other meanings of resolutionresolution

Pitch Pitch - Size of a pixel, distance from - Size of a pixel, distance from center to center of individual pixels.center to center of individual pixels.

Cycles per degree Cycles per degree – How many lines you – How many lines you can see in a degree of FOV.can see in a degree of FOV.

The human eye can resolve 30 cycles per The human eye can resolve 30 cycles per degree (20/20 Snellen acuity).degree (20/20 Snellen acuity).

So how many lines of resolution are So how many lines of resolution are needed for human vision for:needed for human vision for:– monitor at 1 m (17” -> 10”, 22” -> 13”)monitor at 1 m (17” -> 10”, 22” -> 13”)– projector screen at 2 m (4’), 4 m (8’)projector screen at 2 m (4’), 4 m (8’)– REVE at 4m (18’ high)REVE at 4m (18’ high)– How far should you make someone sit in front How far should you make someone sit in front

of a 42” (34” rotated vert) plasma running at of a 42” (34” rotated vert) plasma running at 720p?720p?

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ColorColor

There are no commercially available There are no commercially available small pixel technologies that can small pixel technologies that can individually change color.individually change color.

Color is encoded by placing different-Color is encoded by placing different-colored pixels adjacent to each other.colored pixels adjacent to each other.

Field sequential color uses red, blue Field sequential color uses red, blue and green liquid crystal shutters to and green liquid crystal shutters to change color in front of a change color in front of a monochrome screen.monochrome screen.

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Video FormatsVideo Formats TV StandardsTV Standards

– NTSCNTSC - 720x480, 29.97f/s (60 fields per second), - 720x480, 29.97f/s (60 fields per second), interlacedinterlaced

– PALPAL - 720x576, 25f/s (50 fields/sec) interlaced - 720x576, 25f/s (50 fields/sec) interlaced

VGAVGA - 640x480, 60f/s, noninterlaced - 640x480, 60f/s, noninterlaced SVGASVGA – 800x600, 60f/s noninterlaced – 800x600, 60f/s noninterlaced XGA XGA – 1024x768+, 60+f/s noninterlaced– 1024x768+, 60+f/s noninterlaced

RGBRGB - 3 independent video signals and - 3 independent video signals and synchronization signal, vary in resolution and synchronization signal, vary in resolution and refresh raterefresh rate

Time-multiplexed colorTime-multiplexed color - R,G,B one after another on - R,G,B one after another on a single signal, vary in resolution and refresh ratea single signal, vary in resolution and refresh rate

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InterlacingInterlacing

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Liquid Crystal DisplaysLiquid Crystal Displays

Liquid crystal displays use small flat Liquid crystal displays use small flat chips which change their transparency chips which change their transparency properties when a voltage is applied.properties when a voltage is applied.

LCD elements are arranged in an LCD elements are arranged in an nn x x mm array call the array call the LCD matrix.LCD matrix.

Level of voltage controls gray levels Level of voltage controls gray levels (amount of light allowed through).(amount of light allowed through).

LCDs elements do not emit light, use LCDs elements do not emit light, use backlights behind the LCD matrixbacklights behind the LCD matrix

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Liquid Crystal Displays Liquid Crystal Displays (LCDs)(LCDs)

LCDs have cells that either allow light LCDs have cells that either allow light to flow through, or block it.to flow through, or block it.

Electricity applied to a cell cause it to Electricity applied to a cell cause it to untwist and allow lightuntwist and allow light

http://electronics.howstuffworks.com/lcd2.htm

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LCDs LCDs (cont.)(cont.)

Color is obtained by placing filters in front Color is obtained by placing filters in front of each LCD elementof each LCD element

Usually black space between pixels to Usually black space between pixels to separate the filters.separate the filters.

Because of the physical nature of the LCD Because of the physical nature of the LCD matrix, it is difficult to make the individual matrix, it is difficult to make the individual LCD pixels very small.LCD pixels very small.

Image quality dependent on viewing angle.Image quality dependent on viewing angle. Black levels not completely blackBlack levels not completely black

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LCD resolution is often quoted as LCD resolution is often quoted as number of color elements not number of color elements not number of RGB triads.number of RGB triads.

R G B R BG R G

B R G B R G B R

R G B R BG R G

Example: 320 horizontal by 240 vertical elements Example: 320 horizontal by 240 vertical elements = 76,800 elements= 76,800 elements

Equivalent to 76,800/3 = 25,500 RGB pixelsEquivalent to 76,800/3 = 25,500 RGB pixels

"Pixel Resolution" is 185 by 139 "Pixel Resolution" is 185 by 139 (320/1.73, (320/1.73, 240/1.73)240/1.73)

How many pixel transistors for a 1024x768 display?How many pixel transistors for a 1024x768 display?

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Passive LCD screensPassive LCD screens– Cycle through each Cycle through each

element of the LCD element of the LCD matrix applying the matrix applying the voltage required for voltage required for that element. that element.

– Once aligned with Once aligned with the electric field the the electric field the molecules in the LCD molecules in the LCD will hold their will hold their alignment for a short alignment for a short time time

Active LCD screensActive LCD screens– Each element Each element

contains a small contains a small transistor that transistor that maintains the maintains the voltage until the voltage until the next refresh cycle.next refresh cycle.

– Higher contrast and Higher contrast and much faster much faster response than response than passive LCD passive LCD

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Advantages of LCDsAdvantages of LCDs

Flat Flat LightweightLightweight Low power consumptionLow power consumption

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Cathode Ray Tubes (CRTs)Cathode Ray Tubes (CRTs)

Heating element on the yolk.

Phosphor coated screen

Electrons are boiled off the filament and drawn to the focusing system.

The electrons are focused into a beam and “shot” down the cylinder.

The deflection plates “aim” the electrons to a specific position on the screen.

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CRT Phosphor ScreenCRT Phosphor Screen

The screen is coated with The screen is coated with phosphor, 3 colors for a phosphor, 3 colors for a color monitor, 1 for color monitor, 1 for monochrome.monochrome.

For a color monitor, three For a color monitor, three guns light up red, green, or guns light up red, green, or blue phosphors.blue phosphors.

Intensity is controlled by Intensity is controlled by the amount of time at a the amount of time at a specific phosphorspecific phosphor location.location.

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Color Color CRTCRT

G R B G

B G R B

G R B G

FLUORESCENCEFLUORESCENCE - Light emitted while the phosphor is being - Light emitted while the phosphor is being struck by electrons.struck by electrons.

PHOSPHORESCENCEPHOSPHORESCENCE - Light given off once the electron beam - Light given off once the electron beam is removed.is removed.

PERSISTENCEPERSISTENCE - Is the time from the removal of excitation to - Is the time from the removal of excitation to the moment when phosphorescence has decayed to 10% of the moment when phosphorescence has decayed to 10% of the initial light output.the initial light output.

•Red, Green and Blue electron guns.

•Screen coated with phosphor triads.

•Each triad is composed of a red, blue and green phosphor dot.

•Typically 2.3 to 2.5 triads per pixel.

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Beam MovementBeam Movement

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scan line scan line - one row on the screen- one row on the screen

interlace vs. non-interlace interlace vs. non-interlace - Each frame is - Each frame is either drawn entirely, or as two either drawn entirely, or as two consecutively drawn fields that alternate consecutively drawn fields that alternate horizontal scan lines. horizontal scan lines.

vertical sync vertical sync ((vertical retracevertical retrace) - the motion ) - the motion of the beam moving from the bottom of the of the beam moving from the bottom of the image to the top, after it has drawn a frame.image to the top, after it has drawn a frame.

refresh rate refresh rate - how many frames are drawn - how many frames are drawn per second. Eye can see 24 frames per per second. Eye can see 24 frames per second. TV is 30 Hz, monitors are at least second. TV is 30 Hz, monitors are at least 60 Hz.60 Hz.

Beam Movement

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CRTs CRTs (cont.)(cont.)

Strong electrical fields and high Strong electrical fields and high voltagevoltage

Very good resolution Very good resolution Heavy, not flatHeavy, not flat

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Projection DisplaysProjection Displays

Use bright CRT or Use bright CRT or LCD screens to LCD screens to generate an image generate an image which is sent which is sent through an optical through an optical system to focus on system to focus on a (usually) large a (usually) large screen.screen.

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Projector TechnologyProjector Technologysee httpsee http://electronics.howstuffworks.com/projection-tv.htm://electronics.howstuffworks.com/projection-tv.htm

Two Basic DesignsTwo Basic Designs– Transmittive projectorsTransmittive projectors - Shine light through the - Shine light through the

image-forming element (CRT tube, LCD panel) image-forming element (CRT tube, LCD panel) – Reflective projectorsReflective projectors - Bounce light off the image- - Bounce light off the image-

forming element forming element

In both types of projectors, a lens collects the In both types of projectors, a lens collects the image from the image-forming element, image from the image-forming element, magnifies the image and focuses it onto a magnifies the image and focuses it onto a screen screen

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Basic Projector DesignsBasic Projector Designs(Images from Phillips Research)(Images from Phillips Research)

Reflective Projection System Transmittive Projection System

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Transmittive ProjectorsTransmittive Projectors

CRT BasedCRT Based One color CRT tube (red, blue, One color CRT tube (red, blue,

green phosphors) displays an green phosphors) displays an image with one projection lens. image with one projection lens.

One black-and-white CRT with a One black-and-white CRT with a rapidly rotating color filter wheel rapidly rotating color filter wheel (red, green, blue filters) is placed (red, green, blue filters) is placed between the CRT tube and the between the CRT tube and the projection lens. projection lens.

Three CRT tubes (red, green, Three CRT tubes (red, green, blue) with three lenses project the blue) with three lenses project the images. The lenses are aligned images. The lenses are aligned so that a single color image so that a single color image appears on the screen.appears on the screen.

Old CRT-based projectors are usually heavy and large

compared to other technologies

New ones are tiny

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Transmittive ProjectorsTransmittive Projectors

LCD Based LCD Based – Use a bright light to illuminate an LCD panel, Use a bright light to illuminate an LCD panel,

and a lens projects the image formed by the and a lens projects the image formed by the LCD onto a screen. LCD onto a screen.

Small, lightweight compared to CRT based Small, lightweight compared to CRT based displaysdisplays

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Reflective Reflective ProjectorsProjectors

In reflective projectors, the image is In reflective projectors, the image is formed on a small, reflective chip.formed on a small, reflective chip.

When light shines on the chip, the When light shines on the chip, the image is reflected off it and through image is reflected off it and through a projection lens to the screen. a projection lens to the screen.

Recent innovations in reflective Recent innovations in reflective technology have been in the the technology have been in the the following areas:following areas:– Microelectromechanical systemsMicroelectromechanical systems

((MEMSMEMS) ) Digital micromirror device (DMD, DLP) Digital micromirror device (DMD, DLP) Grating light valve (GLV) Grating light valve (GLV)

– Liquid crystal on siliconLiquid crystal on silicon ( (LCOSLCOS) ) Images from howstuffworks.comImages from howstuffworks.com

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Advantages/DisadvantagesAdvantages/Disadvantagesof Projection Displayof Projection Display

Very large screens can provide large FoV Very large screens can provide large FoV and can be seen by several people and can be seen by several people simultaneously.simultaneously.

Image quality can be fuzzy and somewhat Image quality can be fuzzy and somewhat dimmer than conventional displays. (less so dimmer than conventional displays. (less so these days).these days).

Light is measured in lumens (1000, 2000 Light is measured in lumens (1000, 2000 common)common)

Sensitivity to ambient light.Sensitivity to ambient light. Delicate optical alignment.Delicate optical alignment.

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Recap Raster DisplaysRecap Raster Displays

Cathode Ray Tubes (CRTs), most Cathode Ray Tubes (CRTs), most “tube” monitors you see. Very “tube” monitors you see. Very common, but big and bulky.common, but big and bulky.

Liquid Crystal Displays (LCDs), there Liquid Crystal Displays (LCDs), there are two types transmittive (laptops, are two types transmittive (laptops, those snazzy new flat panel those snazzy new flat panel monitors) and reflective (wrist monitors) and reflective (wrist watches).watches).

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Displays in Virtual RealityDisplays in Virtual Reality Head-Mounted Displays (HMDs)Head-Mounted Displays (HMDs)

– The display and a position tracker The display and a position tracker are attached to the user’s head are attached to the user’s head

– Most use Active Maxtrix LCD (ala Most use Active Maxtrix LCD (ala laptops)laptops)

Head-Tracked Displays (HTDs)Head-Tracked Displays (HTDs)– Display is stationary, tracker tracks Display is stationary, tracker tracks

the user’s head relative to the the user’s head relative to the display.display.

– Example: CAVE, Workbench, Stereo Example: CAVE, Workbench, Stereo monitormonitor

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Visually Coupled SystemsVisually Coupled Systems

A system that integrates the natural visual and A system that integrates the natural visual and motor skills of an operator into the system he motor skills of an operator into the system he is controllingis controlling..

Basic ComponentsBasic Components An immersive visual display (HMD, large An immersive visual display (HMD, large

screen projection (CAVE), dome projection)screen projection (CAVE), dome projection) A means of tracking head and/or eye motionA means of tracking head and/or eye motion A source of visual information that is A source of visual information that is

dependent dependent on the user's head/eye on the user's head/eye motion.motion.

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Differences HMD/HTDDifferences HMD/HTD

HMDHMD– Eyes are fixed Eyes are fixed

distance and location distance and location from the display from the display screen(s)screen(s)

– Line-of-sight of the Line-of-sight of the user is perpendicular user is perpendicular to the display to the display screen(s) or at a screen(s) or at a fixed, known angle to fixed, known angle to the display screen(s).the display screen(s).

– Only virtual images Only virtual images in worldin world

HTDHTD– Distance to display Distance to display

screen(s) variesscreen(s) varies– Line-of-sight to Line-of-sight to

display screen(s) display screen(s) almost never is almost never is perpendicularperpendicular

– Usually much wider Usually much wider FoV than HMDFoV than HMD

– Combines virtual Combines virtual and real imageryand real imagery

visual displays

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