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Visual Displays Chapter 2 Burdea
Outline • Image Quality Issues • Pixels • Color • Video Formats • Liquid Crystal Displays • CRT Displays • Projection Displays
Screen resolution Size Color Blank space between the pixels Brightness Contrast Refresh rate Sensitivity of display to viewing angle Image Quality Issues • For each, let’s draw up: • Range of commonly available components • Importance • Cost • Which would you want most?
Pixels • Pixel - The most basic addressable image element in a screen • CRT - Color triad (RGB phosphor dots) • LCD - Single color element • Screen Resolution - measure of number of pixels on a screen (m by n) • m - Horizontal screen resolution • n - Vertical screen resolution
Other meanings of resolution • Pitch - Size of a pixel, distance from center to center of individual pixels. • Cycles per degree – How many lines you can see in a degree of FOV. • The human eye can resolve 30 cycles per degree (20/20 Snellen acuity). • So how many lines of resolution are needed for human vision for: • monitor at 1 m (17” -> 10”, 22” -> 13”) • projector screen at 2 m (4’), 4 m (8’) • REVE at 4m (18’ high) • How far should you make someone sit in front of a 42” (34” rotated vert) plasma running at 720p?
Color • There are no commercially available small pixel technologies that can individually change color. • Color is encoded by placing different-colored pixels adjacent to each other. • Field sequential color uses red, blue and green liquid crystal shutters to change color in front of a monochrome screen.
Video Formats • TV Standards • NTSC - 720x480, 29.97f/s (60 fields per second), interlaced • PAL - 720x576, 25f/s (50 fields/sec) interlaced • VGA - 640x480, 60f/s, noninterlaced • SVGA – 800x600, 60f/s noninterlaced • XGA – 1024x768+, 60+f/s noninterlaced • RGB - 3 independent video signals and synchronization signal, vary in resolution and refresh rate • Time-multiplexed color - R,G,B one after another on a single signal, vary in resolution and refresh rate
Liquid Crystal Displays • Liquid crystal displays use small flat chips which change their transparency properties when a voltage is applied. • LCD elements are arranged in an n x m array call the LCD matrix. • Level of voltage controls gray levels (amount of light allowed through). • LCDs elements do not emit light, use backlights behind the LCD matrix
Liquid Crystal Displays (LCDs) • LCDs have cells that either allow light to flow through, or block it. • Electricity applied to a cell cause it to untwist and allow light http://electronics.howstuffworks.com/lcd2.htm
LCDs (cont.) • Color is obtained by placing filters in front of each LCD element • Usually black space between pixels to separate the filters. • Because of the physical nature of the LCD matrix, it is difficult to make the individual LCD pixels very small. • Image quality dependent on viewing angle. • Black levels not completely black
LCDs (cont.) LCD resolution is often quoted as number of color elements not number of RGB triads. Example: 320 horizontal by 240 vertical elements = 76,800 elements Equivalent to 76,800/3 = 25,500 RGB pixels "Pixel Resolution" is 185 by 139 (320/1.73, 240/1.73) How many pixel transistors for a 1024x768 display?
Passive LCD screens Cycle through each element of the LCD matrix applying the voltage required for that element. Once aligned with the electric field the molecules in the LCD will hold their alignment for a short time Active LCD screens Each element contains a small transistor that maintains the voltage until the next refresh cycle. Higher contrast and much faster response than passive LCD LCDs (cont.)
Advantages of LCDs • Flat • Lightweight • Low power consumption
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.
CRT Phosphor Screen • The screen is coated with phosphor, 3 colors for a color monitor, 1 for monochrome. • For a color monitor, three guns light up red, green, or blue phosphors. • Intensity is controlled by the amount of time at a specific phosphor location.
Color CRT FLUORESCENCE - Light emitted while the phosphor is being struck by electrons. PHOSPHORESCENCE - Light given off once the electron beam is removed. PERSISTENCE - Is the time from the removal of excitation to the moment when phosphorescence has decayed to 10% of 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.
Beam Movement scan line - one row on the screen interlace vs. non-interlace - Each frame is either drawn entirely, or as two consecutively drawn fields that alternate horizontal scan lines. vertical sync (vertical retrace) - the motion of the beam moving from the bottom of the image to the top, after it has drawn a frame. refresh rate - how many frames are drawn per second. Eye can see 24 frames per second. TV is 30 Hz, monitors are at least 60 Hz.
CRTs (cont.) • Strong electrical fields and high voltage • Very good resolution • Heavy, not flat
Projection Displays • Use bright CRT or LCD screens to generate an image which is sent through an optical system to focus on a (usually) large screen.
Projector Technologysee http://electronics.howstuffworks.com/projection-tv.htm • Two Basic Designs • Transmittive projectors - Shine light through the image-forming element (CRT tube, LCD panel) • Reflective projectors - Bounce light off the image-forming element • In both types of projectors, a lens collects the image from the image-forming element, magnifies the image and focuses it onto a screen
Basic Projector Designs(Images from Phillips Research) Reflective Projection System Transmittive Projection System
Transmittive Projectors CRT Based • One color CRT tube (red, blue, green phosphors) displays an image with one projection lens. • One black-and-white CRT with a rapidly rotating color filter wheel (red, green, blue filters) is placed between the CRT tube and the projection lens. • Three CRT tubes (red, green, blue) with three lenses project the images. The lenses are aligned so that a single color image appears on the screen. Old CRT-based projectors are usually heavy and large compared to other technologies New ones are tiny
Transmittive Projectors • LCD Based • Use a bright light to illuminate an LCD panel, and a lens projects the image formed by the LCD onto a screen. • Small, lightweight compared to CRT based displays
Reflective Projectors • In reflective projectors, the image is formed on a small, reflective chip. • When light shines on the chip, the image is reflected off it and through a projection lens to the screen. • Recent innovations in reflective technology have been in the the following areas: • Microelectromechanical systems (MEMS) • Digital micromirror device (DMD, DLP) • Grating light valve (GLV) • Liquid crystal on silicon (LCOS) • Images from howstuffworks.com
Advantages/Disadvantagesof Projection Display • Very large screens can provide large FoV and can be seen by several people simultaneously. • Image quality can be fuzzy and somewhat dimmer than conventional displays. (less so these days). • Light is measured in lumens (1000, 2000 common) • Sensitivity to ambient light. • Delicate optical alignment.
Recap Raster Displays • Cathode Ray Tubes (CRTs), most “tube” monitors you see. Very common, but big and bulky. • Liquid Crystal Displays (LCDs), there are two types transmittive (laptops, those snazzy new flat panel monitors) and reflective (wrist watches).
Displays in Virtual Reality • Head-Mounted Displays (HMDs) • The display and a position tracker are attached to the user’s head • Most use Active Maxtrix LCD (ala laptops) • Head-Tracked Displays (HTDs) • Display is stationary, tracker tracks the user’s head relative to the display. • Example: CAVE, Workbench, Stereo monitor
Visually Coupled Systems A system that integrates the natural visual and motor skills of an operator into the system he is controlling. Basic Components • An immersive visual display (HMD, large screen projection (CAVE), dome projection) • A means of tracking head and/or eye motion • A source of visual information that is dependent on the user's head/eye motion.
HMD Eyes are fixed distance and location from the display screen(s) Line-of-sight of the user is perpendicular to the display screen(s) or at a fixed, known angle to the display screen(s). Only virtual images in world HTD Distance to display screen(s) varies Line-of-sight to display screen(s) almost never is perpendicular Usually much wider FoV than HMD Combines virtual and real imagery Differences HMD/HTD