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 DisplayIn vector scan display the beam is moved between the end points of the graphics primitives.

In raster scan display the beam is moved all over the screen one scan line at a time, from top to bottom and then back to top.

Vector display flickers when the number of primitives in the buffer becomes too large.

In raster display, the refresh process is independent of the complexity of the image.

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 required.

As each primitive must be scan converted, real time dynamics is far more computational and requires separate scan conversion hardware.

Vector display draws continuous and smooth lines.

Raster display can display mathematically smooth lines, polygons, and boundaries of curved primitives only by approximating them with pix-els on the raster grid.

Cost is more. Cost is low.Vector display only draws line characters.

Raster display has ability to display areas filled 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 DiSplAyThis 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 along perpendicular

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 DiSplAyThis 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 entire picture slower than is noticeable to the human eye, which reduces the effect of burn-in but does not prevent it.

o Earlier generation displays (2006 and prior) had phosphors that lost luminosity over time, resulting in gradual decline of absolute image brightness

o Earlier generation (circa 2001 and earlier) models were susceptible to “large area flicker”

o Heavier screen-door effect when compared to LCD or OLED based TVs

• Physical

o Generally do not come in smaller sizes than 37 inches

o Heavier than LCD due to the requirement of a glass screen to hold the gases

• Other

o Use more electricity, on average, than an LCD TV

o Do not work as well at high altitudes due to pressure differential between the gases inside the screen and the air pressure at altitude. It may cause a buzzing noise. Manufacturers rate their screens to indicate the altitude parameters.

o For those who wish to listen to AM radio, or are Amateur Radio operators (Hams) or Shortwave Listeners (SWL), the Radio Frequency Interference (RFI) from these devices can be irritating or disabling.

o Due to the strong infrared emissions inherent with the technology, standard IR repeater systems cannot be used in the viewing room. A more expensive “plasma compatible” sensor must be used.

Introduction 1.25

exerciSe

multiple choice questions

1. What will be the resolution (in ppf.) of 3 × 3 inch image that has 768 × 768 pixels?

(a) 256 (b) 254

(c) 255 (d) None of these

2. What will be the CMY co-ordinates of a color at (0-2, 1, 0-5) in the RGB space?

(a) (0-4, 0-2, 0-3) (b) (0-8, 0, 0-3)

(c) (0-8, 0, 0-5) (d) (0-4, 0, 0-5)

3. If 3-byte pixel values are used in a 24-bit look up table representation, how many bytes does the table occupy?

Ans. 224 × 24/8 bytes

4. What is the name of the path the electron beam takes when returning to the left side of the CRT screen?

(a) Vertical retrace (b) Vertical entrance

(c) Horizontal retrace (d) Horizontal comeback

5. Refreshing on raster scan displays is carried out at the rate of

(a) 60 to 80 frames per sec (b) 40 to 60 frames per sec

(c) 30 to 60 frames per sec (d) None of these.

6. The maximum number of points that can be displayed without overlap on a CRT is referred to as

(a) Resolution (b) Attenuation

(c) Persistence (d) None of these

Answers1. (a) 2. (c) 3. 224 × 24/8 bytes 4. (c) 5. (a) 6. (a)

Short Answer type

1. What is persistence? Why is the electron beam allowed to over scan?

Ans. When the electron beam strike the phosphor dots they emit light. The incident is known as phosphorescence. The duration of phosphorescence exhibited by a phosphor is known as persistence.

1.26 Computer Graphics

When a dot of phosphor material is struck by the electron beam, it glows for a fraction of second and then fades. As the brightness of the dots begins to reduce, the screen image become unstable and gradually fades out. In order to maintain a stable image, the electron beam must sweep the entire screen or over scan and the rerun to redraw it a number of times per second.

2. Differentiate between Raster scan and Random scan displays.

3. What do you mean by raster scan and random scan system?

4. What is aspect ratio? Give examples

5. Explain briefly the RGB and CMY color model.

6. Give examples of some Graphics devices.

Ans. 1. Display system where the graphics are rendered in the screen of the computer e.g. CRT (raster/random type), flat-panel displays, plasma panels etc.

2. Hardware components with which the user interacts to generate necessary instructions for the creation of the required graphics e.g. printer, plotter, keyboard, mouse, joysticks, scanner etc.

7. What do you mean by resolution of an image?

8. Compute the size of a 640 × 480 image at 240 pixels per inch.

9. Find the CMY coordinates of a color at (0.2, 1 , 0.5) in the RGB space.

10. Explain shadow mask methods for color monitor.

Ans. In case of color monitor there are three electron guns for each primary color RED, GREEN and BLUE. The phosphor coating inside the display screen also consists of the primary colors. These phosphor dots are capable of red, green and blue light respectively. To ensure that the electron beam strike the correct phosphor dot a shadow mask is used just before the screen. Shadow mask is thin metal sheet with a regular array of holes. The tiny holes on the shadow mask constrain each electron beam to hit its corresponding phosphor dots.

11. What Is Flicker? How can this be reduced?

Ans. When the refreshing rate falls below certain limit then our visible system is unable to integrate the light impulses from the phosphor dots into a steady picture, the fact is known as flicker.

In order to avoid flicker, the screen image must be redrawn sufficiently quickly that the eye cannot tell that refresh is going on. If the refresh rate is sufficiently high then it is possible to reduce flicker. Some monitor use the technique called interlacing to double the refresh rate.