1. Persistence Refers to Object and Process Characteristics

1. Persistence refers to object and process characteristics that continue to exist even after the process that created it ceases or the machine it is running on is powered off. When an object or state is created and needs to be persistent, it is saved in a non-volatile storage location, like a hard drive, versus a temporary file or volatile random access memory (RAM). In terms of data, persistence means an object should not be erased unless it is really meant to be deleted. This entails proper storage and certain measures that allow the data to persist. In terms of computer threads and processes, a persistent process is one that cannot be killed or shut down. This is usually true for core system processes that are essential to a properly functioning system. For example, even if a Windows operating system (OS) explorer fails or is killed, it simply restarts. A persistent state refers to the retention of that state, even after the process has been killed. In this case, the state is saved in persistent storage before device shutdown and then reloaded when the device turns on, ensuring that the device, workspace or data are in the same state after turning on the device.

2. Computer Graphics has become a common element in today’s modern world. Be it in user interfaces, or data visualization, motion pictures etc, computer graphics plays an important role. The primary output device in a graphics system is a video monitor. Although many technologies exist, but the operation of most video monitors is based on the standard Cathode Ray Tube (CRT) design. Cathode Ray Tubes (CRT) – A cathode ray tube (CRT) is a specialized vacuum tube in which images are produced when an electron beam strikes a phosphorescent surface.It modulates, accelerates, and deflects electron beam(s) onto the screen to create the images. Most desktop computer displays make use of CRT for image displaying purposes.

Construction of a CRT – 1. The primary components are the heated metal cathode and a control grid. 2. The heat is supplied to the cathode (by passing current through the filament). This way the electrons get heated up and start getting ejected out of the cathode filament. 3. This stream of negatively charged electrons is accelerated towards the phosphor screen by supplying a high positive voltage. 4. This acceleration is generally produced by means of an accelerating anode. 5. Next component is the Focusing System, which is used to force the electron beam to converge to small spot on the screen. 6. If there will not be any focusing system, the electrons will be scattered because of their own repulsions and hence we won’t get a sharp image of the object. This focusing can be either by means of electrostatic fields or magnetic fields.

3. Short for Picture Element, a pixel is a single point in a graphic image. Graphics monitors display pictures by dividing the display screen into thousands (or millions) of pixels, arranged in rows and columns. The pixels are so close together that they appear connected. The number of bits used to represent each pixel determines how many colours or shades of grey can be displayed. For example, in 8-bit colour mode, the colour monitor uses 8 bits for each pixel, making it possible to display 2 to the 8th power (256) different colours or shades of grey. On colour monitors, each pixel is actually composed of three dots -- a red, a blue, and a green one. Ideally, the three dots should all converge at the same point, but all monitors have some convergence error that can make colour pixels appear fuzzy. The quality of a display system largely depends on its resolution, how many pixels it can display, and how many bits are used to represent each pixel. VGA systems display 640 by 480, or about 300,000 pixels. In contrast, SVGA systems display 800 by 600, or 480,000 pixels. True Colour systems use 24 bits per pixel, allowing them to display more than 16 million different colours.

4. In Computer Graphics, the relative horizontal and vertical sizes. For example, if a graphic has an aspect ratio of 2:1, it means that the width is twice as large as the height. When resizing graphics, it is important to maintain the aspect ratio to avoid stretching the graphic out of proportion. The term is also used to describe the dimensions of a display resolution. For example, a resolution of 800x600 has an aspect ratio of 4:3.

5. Refers to the sharpness and clarity of an image. The term is most often used to describe monitors, printers, and bit-mapped graphic images. In the case of dot-matrix and laser printers, the resolution indicates the number of dots per inch. For example, a 300-dpi (dots per inch) printer is one that is capable of printing 300 distinct dots in a line 1 inch long. This means it can print 90,000 dots per square inch. For Graphics Monitors, the screen resolution signifies the number of dots (pixels) on the entire screen. For example, a 640-by-480-pixel screen is capable of displaying 640 distinct dots on each of 480 lines, or about 300,000 pixels. This translates into different dpi measurements depending on the size of the screen. For example, a 15-inch VGA monitor (640x480) displays about 50 dots per inch.

6.

DDA stands for Digital Differential Analyzer. It is an incremental method of scan conversion of line. In this method calculation is performed at each step but by using results of previous steps.

Suppose at step i, the pixels is (xi,yi)

The line of equation for step i yi=mxi+b...... equation 1

Next value will be yi+1=mxi+1+b...... equation 2

m = yi+1-yi=∆y...... equation 3 yi+1-xi=∆x...... equation 4 yi+1=yi+∆y ∆y=m∆x yi+1=yi+m∆x ∆x=∆y/m xi+1=xi+∆x xi+1=xi+∆y/m

Case1: When |M|<1 then (assume that x1

Case2: When |M|<1 then (assume that y1

xi+1= , y=y+1 Until y → y2

7. Raster Scan Displays are most common type of graphics monitor which employs CRT. It is based on television technology. In raster scan system electron beam sweeps across the screen, from top to bottom covering one row at a time.A pattern of illuminated pattern of spots is created by turning beam intensity on and off as it moves across each row. A memory area called refresh buffer or frame buffer stores picture definition. This memory area holds intensity values for all screen points. Stored intensity values are restored from frame buffer and painted on screen taking one row at a time.Each screen point is referred to as pixels. In raster scan systems refreshing is done at done at a rate of 60-80 frames per second. Refresh rates are also sometimes described in units of cycles per second / Hertz (Hz). At the end of each scan line, electron beam begins to display next scan line after returning to left side of screen. The return to the left of screen after refresh of each scan line is known as horizontal retrace of electron beam. At the end of each frame electron beam returns to top left corner and begins the next frame. Raster-Scan Display Processor: An important function of display process is to digitize a picture definition given in an application program into a set of pixel-intensity values for storage in refresh buffer. This process is referred to as scan conversion. The purpose of display processors is to relieve the CPU from graphics jobs.

Display processors can perform various other tasks like: creating different line styles, displaying color areas, etc. Typically display processors are utilized to interface input devices, such as mouse, joysticks.

8. In Random-Scan Display electron beam is directed only to the ares of screen where a picture has to be drawn. It is also called vector displays, as it draws picture one line at time. It can draw and refresh component lines of a picture in any specified sequence. Pen plotter is an example of random-scan displays. The number of lines regulates refresh rate on random-scan displays. An area of memory called refresh display files stores picture definition as a set of line drawing commands. The system returns back to first line command in the list, after all the drawing commands have been processed. High-quality vector systems can handle around 100, 00 short lines at this refresh rate. Faster refreshing can burn the phosphor. To avoid this every refresh cycle is delayed to prevent refresh rate greater than 60 frames per second. Random-Scan Display Processors: Input in the form of an application program is stored in the system memory along with graphics package. Graphics package translates the graphic commands in application program into a display file stored in 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. Sometimes the display processor in a random-scan is referred as Display Processing Unit / Graphics Controller.

The structure of a simple random-scan is shown below:

9. A perforated metal sheet inside a color monitor. Most color monitor screens use Cathode- ray-tube (CRT) technology in which electrons are fired from an electron gun onto a phosphor coating on the screen's faceplate. The phosphor converts the kinetic energy of the electrons into light and is illuminated in tiny red, green and blue dots, which comprise the image that one sees when looking at a monitor's screen. The phosphors in a group are packed so closely together that the human eye can only perceive them as a single colored pixel. Before the electron beam reaches the phosphor dots it passes through the shadow mask, a perforated metal sheet that ensures that the electron beam hits only the correctly colored phosphor dots and does not illuminate more than one dot. Essentially, the shadow mask "masks" the electron beam, thereby forming a smaller and more rounded point that can hit individual phosphor dots. The shadow mask absorbs electrons that are directed at the wrong color phosphor.

10.

The Beam-Penetration method has been used with random-scan monitors. In this method, the CRT screen is coated with two layers of phosphor, red and green and the displayed color depends on how far the electron beam penetrates the phosphor layers. This method produces four colors only, red, green, orange and yellow. A beam of slow electrons excites the outer red layer only; hence screen shows red color only. A beam of high-speed electrons excites the inner green layer. Thus screen shows a green color.