1.4 Computer Graphics Scan Conversion

The image space in computer graphics is, generally speaking, not continuous. It consists of a set of discrete pixels, i.e. picture elements that are arranged in a row- and-column fashion. Hence, a horizontal or vertical line segment becomes a group of adjacent pixels in a row or column, respectively, and a slanted line segment becomes something that resembles a staircase. The area of the computer graphics that is responsible for converting a continuous figure, such as a line segment, into its discrete approximation is called scan conversion.

Aliasing Effect

The distortion introduced by the conversion from continuous space to discrete space is referred to as aliasing effect. Anti-aliasing is the process of minimizing the negative impact of the aliasing effect.

Dot Pitch

Dot pitch is a specification for a computer display, computer printer, image scanner, or other pixel based device. It is the distance between adjacent sets (triads) of red, green and blue dots. This is also same as the shortest distance between any two dots of the same color i.e. from green to green, or red to red like that.

Display Device

The most prominent part in a personal computer is the display system that is responsible for graphic display. The display system may be attached with a PC to display character, picture and video output. The most common types of display system employ: • Raster scan method • Random scan method. The display systems are often referred to as Video Display Unit (VDU). However, every display system has three basic parts- the display adapter that creates and holds image information, the monitor which displays the image information and the cable that carries the image data between the display adapter and the monitor.

Raster Scan Display

This type of display basically employs a Cathode Ray Tube (CRT). The CRT works just like the picture tube of a television set. In a raster scan system, the electron beam is swept across the screen, one row at a time from top to bottom. As the electron beam moves across each row, the beam intensity is turn on and off to create a pattern Introduction 1.5 of illuminated spots. Picture definition is stored in a memory area called a refresh buffer or frame buffer. This memory area holds the set of intensity values for all the screen points. Stored intensity values are then retrieved from the refresh buffer and painted on the screen one row at a time. Each screen point is referred to as a pixel. In CRT the viewing surface is coated with a layer of arrayed phosphor dots. At the back of the CRT is a set of the electron guns which produce controlled stream of electrons. The phosphor material emits light when stuck by these high energy electrons. The frequency and intensity of the light emitted depends on the type of the phosphor materials used and energy of the electrons. In order to maintain a stable image, the electron beam must sweep the entire surface of the screen and then return to redraw it a number of times per second. This process is called refreshing the screen. The refresh rate is the number of times per second the screen is refreshed. Refreshing on raster scan display is carried out at the rate of 60 to 80 frames per second. Sometimes, refresher rates are described in units of cycles per second or Hertz where a cycle corresponds to one frame. If the electron beam takes too long to return and redraw a pixel, the pixel will began to fade; it will return to full brightness only when redrawn. Over the full surface of the screen, this becomes visible as a flicker in the image, which can be distracting and hard on the eyes. At the end of each scan line, the electron beam returns to the left side of the screen to begin displaying the next scan line. The return to the left of the screen, after refreshing each scan line, is called the horizontal retrace of the electron beam. And at the end of each frame the electron beam returns to the top left corner of the screen to begin the next frame is known as vertical retrace.

Fig. 1.1. Horizontal Retrace and Vertical Retrace. On some raster-scan system each frame is displayed in two pass using an interlaced refresh procedure. In the first pass the beam sweeps across every other scan line from top to bottom. Then after the vertical retrace the beam sweeps out the remaining scan lines. Interlacing of the scan lines in this way allows us to see the entire screen displayed in one-half the time it would have taken to sweep across 1.6 Computer Graphics all the lines at once from top to bottom. Interlacing is primarily used with slower refreshing rates. In the figure, the odd-numbered lines represent scanning one half of the screen and the even-numbered lines represent scanning of the other half

Fig. 1.2. Interlaced refresh procedure Cathode Ray Tube

A CRT is similar to a big glass bottle. It contains three electron guns that emits beam of electrons, deflection apparatus (magnetic or electrostatic) which deflects these beams both up and down and sidewise, and a phosphor-coated screen over which these beams impinge. The vacuum is necessary to let those electron beams travel across the tube without running into air molecules that could absorb or scatter them. The primary component in an electron gun is a negatively charged cathode encapsulated by a metal cylinder known as the control grid. A heating element inside the cathode causes the cathode to be heated up as the current is passed and as a result electrons emit from the cathode surface. These electrons are accelerated towards the CRT screen by a high positive voltage applied near the screen or by an accelerating anode. If allowed to continue uninterrupted, the naturally diverging electrons would simply flood the entire screen. The cloud of electrons is forced to converge to a small spot on the CRT screen by a focusing system using an electrostatic or magnetic field.

Fig. 1.3. Cathode Ray Tube