1.18 Computer Graphics Direct View Storage Device

1.18 Computer Graphics • Random scan systems are designed for line drawing application and cannot display realistic shaded scenes. Since picture definition is stored as a set of line- drawing instructions and not as a set of intensity values for all screen points. • Random scan displays can work at higher resolution than the raster displays. • The images are sharp in random scan displays and have smooth edges unlike the jagged edges and lines in raster displays. Vector Scan Display Raster Scan Display In vector scan display the beam is moved In raster scan display the beam is moved all over between the end points of the graphics the screen one scan line at a time, from top to primitives. bottom and then back to top. Vector display flickers when the number In raster display, the refresh process is independent of primitives in the buffer becomes too of the complexity of the image. large. Scan conversion is not required. Graphics primitives are specified in terms of their endpoints and must be scan converted into their corresponding pixels in the frame buffer. Scan conversion hardware is not As each primitive must be scan converted, real required. time dynamics is far more computational and requires separate scan conversion hardware. Vector display draws continuous and Raster display can display mathematically smooth lines. smooth lines, polygons, and boundaries of curved primitives only by approximating them with pixels on the raster grid. Cost is more. Cost is low. Vector display only draws line Raster display has ability to display areas filled characters. with solid colors or patterns. Direct view StorAge Device Fig. 1.20. Direct View Storage Tube Introduction 1.19 Direct View Storage Tube is different from the usual refresh type display. In DVST there is no refresh buffer, the image is created by drawing vectors or line segments with a relatively slow-moving electron beam. The beam is designed not to draw directly on phosphor but on a fine wire mesh coated with dielectric and mounted just before the screen. A pattern of positive charge is deposited on the grid, and this pattern is transferred to the phosphor coated screen by a continuous flood of electrons emanating from a separate flood gun. Just behind the storage mesh is a second grid, the collector, whose purpose is to smooth out the flow of flood electrons. These electrons pass through the collector at low velocity and are attracted to the positively charged portions of the storage mesh but repelled by the rest. Electrons not repelled by the storage mesh pass right through it and strike the phosphor. To increase the energy of these slow electrons and thus create a bright picture, the screen is maintained at a high positive potential. • Advantage: No refreshing is necessary and the image is absolutely flicker free Supports a very high resolution which is good for displaying complex images. • Disadvantage: Inability to erase parts of an image from the screen. To erase an image from the displayed image, one has to first erase the complete image and then redraw it by omitting the line segment. No color facility is there. liquiD cryStAl DiSplAy This is a non emissive type display which uses optical effects to convert light from some other source into graphics patterns. LCD basically consists of a layer of liquid crystal, sandwiched between two polarizing plates. The polarizers are aligned perpendicular to each other (one vertical and the other horizontal), so that the light incident on the first polarizer will be blocked by the second. Because a polarizer plate only passes photons with their electric fields aligned parallel to the polarizing direction of that plate. The LCD displays are addressed in a matrix fashion. Rows of matrix are defined by a thin layer of horizontal transparent conductors, while columns are defined by another thin layer of vertical transparent conductors; the layers are placed between the LCD layer and the respective polarizer plate. The intersection of the two conductors defines a pixel position. This means that an individual LCD element is required for each display pixel. 1.20 Computer Graphics Fig. 1.21. Liquid crystal display Fig.1.22.Two polarizing filters are arranged alongperpendicular polarizing axes The liquid crystal materials are made up of long rod-shaped crystalline molecules containing cyanobiphenyl units. The individual polar molecules in a nematic LC layer are normally arranged in a spiral fashion such that the direction of polarization of polarized light passing through it is rotated by 90 degrees. Light from an internal source enters the first polarizer and is polarized accordingly. As the light passes through the LC layer it is twisted 90 degrees so that it is allowed to pass through the rear polarizer and then reflect from the reflector behind the Introduction 1.21 rear polarizer. The reflect light when reach the viewers eye traveling in the reverse direction, the LCD appears bright. When an electric current is passed through the LCD layer, the crystalline molecules align themselves parallel to the direction of light and thus have no polarizing effect. The light entering through the front polarizer is not allowed to pass through the rear polarizer due to mismatch of polarization direction. The result is zero reflection of light and the LCD appears black. Fig. 1.23. A schematic matrix electrode configuration of Plasma Display Fig. 1.24. When voltage is applied to the liquid crystal structure, the twisted light passes straight through. 1.22 Computer Graphics In a color LCD there are layers of three liquid crystal panels one on top of another. Each one is filled with a colored liquid crystal. Each one has its own set of horizontal and vertical conductors. Each layer absorbs an adjustable portion of just one color of light passing through it. This is similar to how color images are printed. The principal advantage of the design is that it helps create as many screen pixels as intersections, thus making high-resolution LCD panels. The image painting operation in LCD panels is a different from that of CRT though both are of raster scan type. In a simple LCD panel an entire line of screen pixels are illuminated at one time. Then the next line and so on till the entire screen image is completed. Once set, the screen pixels stay at fixed brightness until they are reset. The time required to set the brightness of a pixel is high compared to that of the CRT. Advantage: • Very compact and light. • Low power consumption. • No geometric distortion. • Little or no flicker depending on backlight technology. • Not affected by screen burn-in. • No high voltage or other hazards present during repair/service. • Can be made in almost any size or shape. • No theoretical resolution limit. Disadvantage: • Limited viewing angle, causing color, saturation, contrast and brightness to vary, even within the intended viewing angle, by variations in posture. • Bleeding and uneven backlighting in some monitors, causing brightness distortion, especially toward the edges. • Smearing and ghosting artifacts caused by slow response times (2-8 ms) and “sample and hold” operation. • Only one native resolution. Displaying resolutions either requires a video scaler, lowering perceptual quality, or display at 1:1 pixel mapping, in which images will be physically too large or won’t fill the whole screen. • Fixed bit depth, many cheaper LCDs are only able to display 262,000 colors. 8-bit S-IPS panels can display 16 million colors and have significantly better black level, but are expensive and have slower response time. • Input lag • Dead pixels may occur either during manufacturing or through use. • In a constant on situation, thermalization may occur, which is when only part of the screen has overheated and therefore looks discolored compared to the rest of the screen. Introduction 1.23 • Not all LCD displays are designed to allow easy replacement of the backlight. • Cannot be used with light guns/pens. plASmA DiSplAy This is a emissive type display that convert electrical energy into light. Here a layer of gas (usually neon) is sandwiched between two glass plates. Thin vertical strips of conductor run across one plate, while horizontal conductors run up and down the other plate. By applying high voltage to a pair of horizontal and vertical conductors, a small section of the gas at the intersection of the conductors breaks down into glowing plasma of electrons and ions. Thus, in the array of gas bulbs, each one can be set to ‘on’ state or ‘off’ state by adjusting voltages in the appropriate pair of conductors. Once set ‘on’ the bulbs remain in that state until explicitly made ‘off’ by momentarily reducing the voltage applied to the pair of conductors. Here no refreshing is necessary. Fig. 1.25 A schematic matrix electrode configuration of Plasma Display Advantages • Picture quality o Capable of producing deeper blacks allowing for superior contrast ratio o Wider viewing angles than those of LCD; images do not suffer from degradation at high angles like LCDs. o Less visible motion blur, thanks in large part to very high refresh rates and a faster response time, contributing to superior performance when displaying content with significant amounts of rapid motion. 1.24 Computer Graphics • Physical o Slim profile o Can be wall mounted o Less bulky than rear-projection televisions Disadvantages • Picture quality o Earlier generation displays were more susceptible to screen burn-in and image retention, although most recent models have a pixel orbiter that moves the

Load more