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Visual Displays

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  1. Visual Displays

  2. Image Source (Screen) • Image Quality Issues • Pixels • Color • Video Formats • Liquid Crystal Displays • CRT Displays • Projection Displays ©2000 Larry F. Hodges

  3. Screen resolution Color Blank space between the pixels Intentional image degradation Brightness Contrast Refresh rate Sensitivity of display to viewing angle Image Quality Issues ©2000 Larry F. Hodges

  4. 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 ©2000 Larry F. Hodges

  5. Other meanings of resolution • Pitch - Size of a pixel, distance from center to center of individual pixels. • Cycles per degree - Addressable elements (pixels) divided by twice the FOV measured in degrees. • The human eye can resolve 30 cycles per degree (20/20 Snellen acuity). ©2000 Larry F. Hodges

  6. 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. ©2000 Larry F. Hodges

  7. Video Formats • NTSC - 525x480, 30f/s, interlaced • PAL - 625x480, 25f/s, interlaced • VGA - 640x480, 60f/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 ©2000 Larry F. Hodges

  8. 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. • LCDs elements do not emit light, use backlights behind the LCD matrix ©2000 Larry F. Hodges

  9. LCDs (cont.) ©2000 Larry F. Hodges

  10. 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. ©2000 Larry F. Hodges

  11. 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) ©2000 Larry F. Hodges

  12. 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.) ©2000 Larry F. Hodges

  13. Advantages of LCDs • Flat • Lightweight • Low power consumption ©2000 Larry F. Hodges

  14. Cathode Ray Tubes (CRTs) ©2000 Larry F. Hodges

  15. CRTs (cont.) • Strong electrical fields and high voltage • Very good resolution • Heavy, not flat ©2000 Larry F. Hodges

  16. 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. • Full color obtained by having separate monochromatic projector for each of the R,G,& B color channels ©2000 Larry F. Hodges

  17. 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. • Sensitivity to ambient light. • Delicate optical alignment. ©2000 Larry F. Hodges

  18. Displays in Virtual Reality • Head-Mounted Displays (HMDs) • The display and a position tracker are attached to the user’s head • Head-Tracked Displays (HTDs) • Display is stationary, tracker tracks the user’s head relative to the display. • Example: CAVE, Workbench, Stereo monitor ©2000 Larry F. Hodges

  19. 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. ©2000 Larry F. Hodges

  20. 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 ©2000 Larry F. Hodges