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1 Visual Displays Chapter 2 Burdea. 2 Outline Image Quality Issues Image Quality Issues Pixels Pixels Color Color Video Formats Video Formats Liquid Crystal.

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Presentation on theme: "1 Visual Displays Chapter 2 Burdea. 2 Outline Image Quality Issues Image Quality Issues Pixels Pixels Color Color Video Formats Video Formats Liquid Crystal."— Presentation transcript:

1 1 Visual Displays Chapter 2 Burdea

2 2 Outline Image Quality Issues Image Quality Issues Pixels Pixels Color Color Video Formats Video Formats Liquid Crystal Displays Liquid Crystal Displays CRT Displays CRT Displays Projection Displays Projection Displays

3 3 Image Quality Issues Screen resolution Screen resolution Size Size Color Color Blank space between the pixels Blank space between the pixels Brightness Brightness Contrast Contrast Refresh rate Refresh rate Sensitivity of display to viewing angle Sensitivity of display to viewing angle For each, let’s draw up: Range of commonly available components Importance Cost Which would you want most?

4 4 Pixels Pixel - The most basic addressable image element in a screen 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) Screen Resolution - measure of number of pixels on a screen (m by n) –m - Horizontal screen resolution –n - Vertical screen resolution

5 5 Other meanings of resolution Pitch - Size of a pixel, distance from center to center of individual pixels. 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. 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). 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: 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?

6 6 Color There are no commercially available small pixel technologies that can individually change 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. 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. Field sequential color uses red, blue and green liquid crystal shutters to change color in front of a monochrome screen.

7 7 Video Formats TV Standards 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 VGA - 640x480, 60f/s, noninterlaced SVGA – 800x600, 60f/s noninterlaced SVGA – 800x600, 60f/s noninterlaced XGA – 1024x768+, 60+f/s noninterlaced XGA – 1024x768+, 60+f/s noninterlaced RGB - 3 independent video signals and synchronization signal, vary in resolution and refresh rate 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 Time-multiplexed color - R,G,B one after another on a single signal, vary in resolution and refresh rate

8 8 Interlacing

9 9 Liquid Crystal Displays Liquid crystal displays use small flat chips which change their transparency properties when a voltage is applied. 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. 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). Level of voltage controls gray levels (amount of light allowed through). LCDs elements do not emit light, use backlights behind the LCD matrix LCDs elements do not emit light, use backlights behind the LCD matrix

10 10 Liquid Crystal Displays (LCDs) LCDs have cells that either allow light to flow through, or block it. 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 Electricity applied to a cell cause it to untwist and allow light

11 11 LCDs (cont.) Color is obtained by placing filters in front of each LCD element Color is obtained by placing filters in front of each LCD element Usually black space between pixels to separate the filters. 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. 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. Image quality dependent on viewing angle. Black levels not completely black Black levels not completely black

12 12 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?

13 13 LCDs (cont.) Passive LCD screens 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 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

14 14 Advantages of LCDs Flat Flat Lightweight Lightweight Low power consumption Low power consumption

15 15 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.

16 16 CRT Phosphor Screen The screen is coated with phosphor, 3 colors for a color monitor, 1 for monochrome. 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. 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. Intensity is controlled by the amount of time at a specific phosphor location.

17 17 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.

18 18 Beam Movement

19 19 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. Beam Movement

20 20 CRTs (cont.) Strong electrical fields and high voltage Strong electrical fields and high voltage Very good resolution Very good resolution Heavy, not flat Heavy, not flat

21 21 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. Use bright CRT or LCD screens to generate an image which is sent through an optical system to focus on a (usually) large screen.

22 22 Projector Technology see http ://electronics.howstuffworks.com/projection-tv.htm Two Basic Designs 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 In both types of projectors, a lens collects the image from the image-forming element, magnifies the image and focuses it onto a screen

23 23 Basic Projector Designs (Images from Phillips Research) Reflective Projection SystemTransmittive Projection System

24 24 Transmittive Projectors CRT Based One color CRT tube (red, blue, green phosphors) displays an image with one projection lens. 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. 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. 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

25 25 Transmittive Projectors LCD Based 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 Small, lightweight compared to CRT based displays

26 26 Reflective Projectors In reflective projectors, the image is formed on a small, reflective chip. 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. 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: 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 Images from howstuffworks.com

27 27 Advantages/Disadvantages of Projection Display Very large screens can provide large FoV and can be seen by several people simultaneously. 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). Image quality can be fuzzy and somewhat dimmer than conventional displays. (less so these days). Light is measured in lumens (1000, 2000 common) Light is measured in lumens (1000, 2000 common) Sensitivity to ambient light. Sensitivity to ambient light. Delicate optical alignment. Delicate optical alignment.

28 28 Recap Raster Displays Cathode Ray Tubes (CRTs), most “tube” monitors you see. Very common, but big and bulky. 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). Liquid Crystal Displays (LCDs), there are two types transmittive (laptops, those snazzy new flat panel monitors) and reflective (wrist watches).

29 29 Displays in Virtual Reality Head-Mounted Displays (HMDs) 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) Head-Tracked Displays (HTDs) –Display is stationary, tracker tracks the user’s head relative to the display. –Example: CAVE, Workbench, Stereo monitor

30 30 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) An immersive visual display (HMD, large screen projection (CAVE), dome projection) A means of tracking head and/or eye motion A means of tracking head and/or eye motion A source of visual information that is dependent on the user's head/eye motion. A source of visual information that is dependent on the user's head/eye motion.

31 31 Differences HMD/HTD HMD 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 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


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