Explain process of displaying objects in raster display and random display. Draw block diagrams of the architecture of both the display systems.                                                                     

Ans. Raster- Scan Systems  Interactive raster graphics systems typically employ several processing units. In addition to the central processing unit of CPU, a special purpose processor, called the video controller of display controller, is used to control the operation of the display device. Organization of a simple raster system is shown in Fig. 1. Here the frame buffer can be anywhere in the system memory, and the video controller accesses the frame buffer to refresh the screen. In addition to the video controller, more sophisticated raster systems employ other processors as coprocessors and accelerators to implement various graphics operations. 

Video Controller Figure 1 shows a commonly used organization for raster systems. A fixed area of the system memory is reserved for the frame buffer and the video controller is given direct access to the frame buffer memory.  Frame buffer locations and the corresponding screen positions are referenced in Cartesian coordinates. For many graphics monitors the coordinates. For many graphics monitors the coordinate origin is defined at the lower left screen comer. The screen surface is then represented as the first quadrant of a two dimensional system with positive x values increasing to the right and positive y values increasing from bottom to top. (On some personal computers the coordinate origin is referenced at the upper left comer of the screen so the y values are inverted.) Scan lines are then labeled from y at the top of the screen to 0 at the bottom. Along each scan line screen pixel positions are labeled from 0 to x. Graphics patterns are drawn on a random scan system by directing the electron beam along the component lines of the    picture. Lines are defined by the Graphics Monitors values for their coordinate endpoints and these input coordinate values are con and Workstations vertex to x and y deflection voltages. A scene is then drawn one line at a time by positioning the beam to fill in the line between specified endpoints. In Fig. 3, the basic refresh operations of the video controller are diagrammed. Two registers are used to store the coordinates of the screen pixels. Initially the x register is set to 0 and the y register is y to the value stored in the frame buffer for this pixel position is thin retrieved and used to set the intensity of the CRT beam. Then the x register is incremented by 1, and the process repeated for the next pixel on the top scan line. This procedure is line has been processed, the x register is reset to 0 and the y register is decremented by 1. Pixels along this scan line are thin processed in turn, and the    procedure is repeated for each successive scan line. After cycling through all pixels along the bottom scan line (Y = 0), the video controller resets the registers to the first pixel position on the top scan line and the refresh process starts over. 

Graphics Monitors and Workstations Most graphics monitors today operate as raster scan display and here we survey a few of the many graphics hardware configurations available. Graphics systems range h m small general purpose computer systems with graphics capabilities to sophisticated full color systems that are designed specifically for graphics applications. A typical screen resolution for personal computer systems such as the apple Quadra is 640 by 480 although screen resolution and other system capabilities vary depending on the size and cost of the system. Diagonal screen dimensions for general purpose personal computer systems can range from 12 to 21 inches and allowable color selections range from 16 to over 32000 for workstations specifically designed for graphics applications such as the systems shown in typical screen resolution is 1280 by 1024 with a screen diagonal of 16 inches of more Graphics workstation can be configured with 8 to 24 bits per pixel (full-color systems) with higher screen resolutions faster   processors and other options available in high end systems.  

Random Scan Systems  The organization of a simple random scan system is shown in fig. 4. An application program is input and stored in the system memory along with a graphics package. Graphics commands in the application program are translated by the graphics package into a display file stored in the system memory. This display file is then accessed by the display processor to refresh the screen. The display processor cycles through each command in the display file program once during every refresh cycle. Sometimes the display processor in a random scan system is referred to as a display processing unit of a graphics controller. When operated as a random scan display unit a CRT has the electron beam directed only to the parts of the screen where a   picture is to be drawn. Random scan monitors draw a picture one line at a time and for this reason are also referred to as vector displays. The component lines of a picture can be drawn and refreshed by a random scan system in any specified order. A pen plotter operates in a similar way and is an example of a random scan hard copy device. Refresh rate on a random scan system depends on the number of lines to be displayed. Picture definition is now stored as a set of line drawing commands in an area of memory as the refresh display file. Sometimes the refresh display file is called the display list display program of simply the refresh buffer. To display a specified picture the system cycles through the set of commands in the display file drawing each component line in turn. After all line drawing command in the list. Random scan displays are designed to draw all the component lines of a picture 30 to 60 times each second. High quality vector systems are capable of handling approximately 100000 short lines at this refresh rate. When a small set of lines is to be displayed each refresh cycle is delayed to avoid refresh rates greater than 60 frames      per second. Otherwise faster refreshing the set of lines could bum out the phosphor. Graphics patterns are drawn on a random scan system by directing the section 2-4 electron beam along the component lines of the picture. Lines are defined by the Graphics Monitors values for their coordinate endpoints and these input coordinate values are con and Workstations vertex to x and y deflection voltages. A scene is thin drawn one line at a time by positioning the beam to fill in the line between specified endpoints.