overview of graphic system

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overview of graphic system

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Overview of Graphics Systems

Video Display Devices:

Video Display Devices The primary output device in a graphics system is a video monitor. The operation of most video monitors is based on the standard cathode-ray tube (CRT) design.

Refresh Cathode-Ray Tubes:

Refresh Cathode-Ray Tubes

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Fig 2.2

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Fig 2-2 illustrates the basic operation of, a CRT. A beam of electrons (cathode rays), emitted by an electron gun, passes through focusing and deflection systems that direct the beam toward specified positions on the phosphor coated screen. The phosphor then emits a small spot of light at each position contacted by the electron beam.

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Because the light emitted by the phosphor fades very rapidly, some method is needed for maintaining the screen picture. One way to keep the phosphor glowing is to redraw the picture repeatedly by quickly directing the electron beam back over the same points. This type of display is called a refresh CRT

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Fig 2.3

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The primary components of an electron gun in a CRT are the heated metal cathode and a control grid (Fig. 2-3) Heat is supplied to the cathode by directing a current through a coil of wire, called the filament, inside the cylindrical cathode structure. This causes electrons to be ‘boiled off" the hot cathode surface. In the vacuum inside the CRT envelope, the free, negatively charged electrons are then accelerated towards the phosphor coating by a high positive voltage.

The accelerating anode:

The accelarating voltage can be generated with a positively charged metal coating on the inside of the CRT envelope near the phosphor screen, or an accelerating anode can be used. Sometimes the electron gun is built to contain the accelerating anode and focusing system within the same unit. The accelerating anode

Control Grid:

Control Grid Intensity of the electron beam is controlled by setting voltage levels on the control grid , which is a metal cylinder that fits over the cathode.

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A high negative voltage applied to the control grid will shut off the beam by repelling electrons and stopping them from passing through the small hole at the end of the control grid structure. A smaller negative voltage on the control grid simply decreases the no of electrons passing through.

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Since the amount of light emitted by the phosphor coating depends on the no of electrons striking the screen, we control the brightness of a display by varying the voltage on the control grid.

The focusing system:

The focusing system in a CRT is needed to force the electron beam to converge into a small spot as it strikes the phosphor. Otherwise, the electrons would repel each other, and the beam would spread out as it approaches the screen. Focusing is accomplished with either electric fields or magnetic fields. The focusing system

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Electrostatic focusing is commonly used in TV and computer graphics monitors. With electrostatic focusing, the electron beam passes through a positively charged metal cylinder that forms an electrostatic lens . The action of the electrostatic lens focuses the electron beam at the center of the screen.

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When electrostatic deflection is used, two pairs of parallel plates are mounted inside the CRT envelope. One pair of plates is mounted horizontally, to control the vertical deflection, and the other pair is mounted vertically, to control horizontal deflection

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Cathode-ray tubes are now commonly constructed with magnetic deflection coils mounted on the outside of the CRT envelope. Two pairs of coils are used, with the coils of each pair mounted on opposite sides of the neck of the CRT envelope. Horizontal deflection is accomplished by one pair of coils, and vertical deflection by the other pair. Magnetic lens focusing produces the smallest spot size on the screen and is used in special purpose devices.

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Different kinds of phosphors are available for use in a CRT. Besides color, a major difference between phosphors is their persistence: how long they continue to emit light (that is, have excited electrons returning to the ground state) after the CRT beam is removed. Persistence is defined as the time it takes the emitted light from the screen to decay to one-tenth of its original intensity.

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Lower persistence phosphors require higher refresh rates to maintain a picture on the screen without flicker. A phosphor with low persistence is useful for animation; A high-persistence phosphor is useful for displaying highly complex, static pictures.

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The maximum number of points that can be displayed without overlap on a CRT is referred to as the resolution. A more precise definition of resolution is the number of points per centimeter that can be plotted horizontally and vertically, although it is often simply stated as the total number of points in each direction.

Aspect Ratio.:

Aspect Ratio. Another property of video monitors is aspect ratio. This number gives the ratio of vertical points to horizontal points necessary to produce equal-length lines in both directions on the screen. An aspect ratio of ¾ means that a vertical line plotted with 3 points has the same length as a horizontal line plotted with 4 points.

Vector & Raster Graphics:

Vector & Raster Graphics There are two kinds of computer graphics – 1)Raster (composed of pixels) and 2)Vector or random (composed of paths). Raster images are more commonly called bitmap images. A bitmap image uses a grid of individual pixels where each pixel can be a different color or shade. Bitmaps are composed of pixels.

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Vector graphics use mathematical relationships between points and the paths connecting them to describe an image. Vector graphics are composed of paths.

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The image below is representative of a bitmap and a vector graphic. Bitmap Image: Vector Graphic:

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vector graphics Generation of images from mathematical descriptions that determine the position, length, and direction in which lines are drawn. Vector graphics is also called stroke or line drawing. Oscilloscopes and some plotters are vector graphics devices.

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Raster graphics Generation of images as a collection of small, independently controlled dots (pixels) arranged in rows and columns. Raster graphics is also referred to as pixel graphics. Almost all current computer output devices, including CRTs, LCDs, LEDs, and plasma screens, use raster graphics.

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Raster graphics cannot draw perfect curved or slopping lines. The appearance of curved or sloping lines improves as the size of the pixels decreases. Software techniques can also be used to improve the visual appearance of pixel based lines.

Raster-scan Displays:

Raster-scan Displays Raster to be a rectangular grid or array of pixel positions : The most common type of graphics monitor employing a CRT is the raster scan display, based on television technology. 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 turned on and off to create a pattern of illuminated spots.

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Picture definition is stored in a memory area called the 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 (scan line) at a time (Fig. 2-7).

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Figure 2.7

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Each screen point is referred to as a pixel or pel ( shortened form of picture element). The capability of a raster-scan system to store intensity information for each screen point makes it well suited for the realistic display of scenes containing subtle(difficult to describe) shading and color patterns. Home television sets and printers are examples of other systems using raster-scan methods.

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Intensity range for pixel positions depends on the capability of the raster system. In a simple black-and-white system, each screen point is either on or off , so only one bit per pixel is needed to control the intensity of screen positions. For a bi-level system, a bit value of 1 indicates that the electron beam is to be turned on at that position, and a value of 0 indicates that the beam intensity is to be off.

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Additional bits are needed when color and intensity variations can be displayed. Up to 24 bits per pixel are included in high-quality systems, which can require severaI megabytes of storage for the frame buffer, depending on the resolution of the system. A system with 24 bits per pixel and a screen resolution of 1024 by 1024 requires 3 megabytes of storage for the frame buffer.

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On a black-and-white system with one bit per pixeI , the frame buffer is commonly called a bitmap. For systems with multiple bits per pixel, the frame buffer is often referred to as a pixmap . Refreshing on raster-scan displays is carried out at the rate of 60 to 80 frames per second, although some systems are designed for higher refresh rates.

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Sometimes, refresh rates are described in units of cycles per second, or Hertz(Hz), where a cycle corresponds to one frame. Using these units, we would describe a refresh rate of 60 frames per second as simply 60 Hz. 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.

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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 (displayed in 1/80th to 1/60th of a second), the electron beam returns (vertical retrace) to the top left comer of the screen to begin the next frame.

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On some raster-scan systems (and in TV sets), each frame is displayed in two passes 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 (Fig. 2-8). 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 all the lines at once from top to bottom.

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Interlacing is primarily used with slower refreshing rates. On an older, 30 frames per- second, noninterlaced display, for instance, some flicker is noticeable. But with interlacing, each of the two passes can be accomplished in 1/60th of a second,which brings the refresh rate nearer to 60 frames per second. This is an effective technique for avoiding flicker, providing that adjacent scan lines contain similar display information.

Random-Scan Displays:

Random-Scan Displays 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 (or stroke-writing or calligraphic diisplays or line drawing displays).

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Characters are also made of sequence of strokes(or short lines). The component lines of a picture can be drawn and refreshed by a random-scan system in any specified order (Fig. 2-9). A pen plotter operates in a similar way and is an example of a random-scan device.

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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 refered to as the refresh display file. Sometimes the refresh display file is called the display list, display program, or simply the refresh Buffer.

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Random-scan systems are designed for linedrawing applications 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, vector displays generally have higher resolution than raster systems. Also, vector displays produce smooth line drawings because the CRT beam directly follows the line path. A raster system, in contrast, produces jagged lines that are plotted as discrete point sets.

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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 linedrawing commands have been processed, the system cycles back to the first line command in the list. Random-scan displays are designed to draw all the component lines of a picture 30 to 60 times each second. Highquality vector systems are capable of handling approximately 100,000 "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 of the set of lines could burn out the phosphor.

Color CRT Monitors:

Color CRT Monitors A CRT monitor displays color pictures by using a combination of phosphors that emit different-colored light. By combining the emitted light from the different phosphors, a range of colors can be generated.

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The two basic techniques for producing color displays with a CRT are the beam-penetration method and the shadow-mask method.

The beam-penetration method:

The beam-penetration method for displaying color pictures has been used with random-scan monitors. Two layers of phosphor, usually red and green, are coated onto the inside of the CRT screen, and the displayed color depends on how far the electron beam penetrates into the phosphor layers. The beam-penetration method

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A beam of slow electrons excites only the outer red layer. A beam of very fast electrons penetrates through the red layer and excites the inner green layer. At intermediate beam speeds, combinations of red and green light are emitted to show two additional colors, orange and yellow. .

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The speed of the electrons, and hence the screen color at any point, is controlled by the beam-acceleration voltage. Beam penetration has been an inexpensive way to produce color in random-scan monitors, but only four colors are possible, and the quality of pictures is not as good as with other methods

Shadow-mask methods:

They are commonly used in raster scan systems (including color TV) because they produce a much wider range of colors than the beam- penetration method. A shadow-mask CRT has three phosphor color dots at each pixel position. One phosphor dot emits a red light, another emits a green light, and the third emits a blue light. Shadow-mask methods

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This type of CRT has three electron guns, one for each color dot, and a shadow-mask grid just behind the phosphor-coated screen. Figure below illustrates the delta delta shadow-mask method, commonly used in color CRT systems

Displaying COLOR Images on a CRT:

53 Displaying COLOR Images on a CRT Triad Arrangement : three phosphors, one for each color (red, green, and blue), for each pixel A shadow mask is often used to ensure that the electron beams from the guns fall on the correct phosphors

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The three electron beams are deflected and focused as a group onto the shadow mask, which contains a series of holes aligned with the phosphor-dot patterns. When the three beams pass through a hole in the shadow mask, they activate a dot triangle , which appears as a small color spot on the screen. The phosphor dots in the triangles are arranged so that each electron beam can activate only its corresponding color dot when it passes through the shadow mask.

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Color CRTs in graphics systems are designed as RGB monitors. These monitors use shadow-mask methods and take the intensity level for each electron gun (red, green, and blue) directly from the computer system without any intermediate processing. High-quality raster-graphics systems have 24 bits per pixel in the frame buffer, allowing 256 voltage settings for each electron gun and nearly 17 million color choices for each pixel. An RGB color system with 24 bits of storage per pixel is generally referred to as a full-color system or a true-color system.

Direct-View Storage Tubes(DVST):

Direct-View Storage Tubes(DVST) It is a CRT with a long persistence phosphor. Provides flicker free display. An alternative method for maintaining a screen image is to store the picture information inside the CRT instead of refreshing the screen. A direct-view storage tube (DVST) stores the picture information as a charge distribution just behind the phosphor-coated screen.(ie. On storage grid)

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Two electron guns are used in a DVST. One, the primary gun(storage grid) , is used to store the picture pattern; the second, the flood gun , maintains the picture display.

DVST:

DVST

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A DVST monitor has both disadvantages and advantages compared to the refresh CRT. Because no refreshing is needed, very complex pictures can be displayed at very high resolutions without flicker.

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Disadvantages of DVST systems are that they ordinarily do not display color and that selected parts of a picture cannot be erased. To eliminate a picture section, the entire screen must be erased and the modified picture is to be redrawn. The erasing and redrawing process can take several seconds for a complex picture. Limited interactive support. No animation is possible. For these reasons, storage displays have been largely replaced by raster systems.

Flat-Panel Displays:

Flat-Panel Displays The term flat-panel display refers to a class of video devices that have reduced volume, weight, and power requirements compared to a CRT. A significant feature of flat-panel displays is that they are thinner than CRTs, and we can hang them on walls or wear them on our wrists.

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Since we can even write on some flat-panel displays, they will soon be available as pocket notepads. Current uses for flat-panel displays include small TV monitors, calculators, pocket video games, laptop computers etc.

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we can separate flat-panel displays into two categories: emissive displays and Non emissive displays .

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The emissive displays (or emitters) are devices that convert electrical energy into light. Non emissive displays (or non emitters) use optical effects to convert sunlight or light from some other source into graphics patterns. The most important example of a non emissive flat-panel display is a liquid-crystal device.

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Plasma panels, also called gas-discharge displays, are constructed by filling the region between two glass plates with a mixture of gases that usually includes neon. Fig 2-11 in page 66

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A third type of emissive device is the light-emitting diode (LED). A matrix of diodes is arranged to form the pixel positions in the display, and picture definition is stored in a refresh buffer. As in scan-line refreshing of a CRT, information is read from the refresh buffer and converted to voltage levels that are applied to the diodes to produce the light patterns in the display.

Liquid-crystal display(LCD):

Liquid-crystal display(LCD) The term liquid-crystal refers to the fact that these compounds have a crystalline arrangement of molecules, yet they flow like a liquid. Flat-panel displays commonly uses nematic (thread like) li q uid-crystal compounds that tend to keep the long axes of the rod-shaped molecules aligned. A flat-panel display can then be constructed with a nematic liquid crystal, as demonstrated in Fig. 2-16.

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Two glass plates, each containing a light polarizer at right angles to the-other plate, sandwich the liquid-crystal material. Rows of horizontal transparent conductors are built into one glass plate, and columns of vertical conductors are put into the other plate. The intersection of two conductors defines a pixel position. Normally, the molecules are aligned as shown in the "on state" of Fig. 2-16.

Liquid Crystal Devices (LCDs):

Liquid Crystal Devices (LCDs)

Three-Dimensional Viewing Devices :

Three-Dimensional Viewing Devices Graphics monitors for the display of three-dimensional scenes have been devised using a technique that reflects a CRT image from a vibrating, flexible mirror. The operation of such a system is demonstrated in Fig. 2-17.

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As the varifocal mirror vibrates, it changes focal length. These vibrations are synchronized with the display of an object on a CRT so that each point on the object is reflected from the mirror into a spatial position corresponding to the distance of that point from a specified viewing position. This allows us to walk around an object or scene and view it from different sides.

RASTER-SCAN SYSTEMS :

RASTER-SCAN SYSTEMS SYSTEM BUS CPU SYSTEM MEMORY Video Controller MONITOR I/0 DEVICES

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Interactive raster graphics systems typically employ several processing units. In addition to the central processing unit, or CPU, a special-purpose processor, called the video controller or display controller , is used to control the operation of the display device. Organization of a simple raster system is shown in Fig. (2-25). above.

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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.

Fig 2.26:

Fig 2.26 Frame Buffer SYSTEM BUS CPU SYSTEM MEMORY Video Controller MONITOR I/0 DEVICES

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Figure 2-26 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 coordinate origin is defined at the lower left screen corner (Fig. 2-27).

2.27:

2.27 y X

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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. Fig 2.27.1) 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 max .

Fig 2.27.1:

Fig 2.27.1 y X

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RASTER - SCAN GENERATOR X REGISTER Y REGISTER MEMORY ADDRESS PIXEL REGISTER FRAME BUFFER HORIZONTAL AND VERTICAL DEFLECTION VOLTAGES INTENSITY BASIC VIDEO-CONTROLLER REFRESH OPERATIONS Fig 2.28

BASIC REFESH OPERATIONS OF VIDEO CONTROLLER:

BASIC REFESH OPERATIONS OF VIDEO CONTROLLER In Fig. 2-28, the basic refresh operations of the video controller are diagrammed. Two registers are used to store the coordinates of the screen pixels. Initially X-register set to zero and y-register set to y-max. Value related to above X & Y retrieved from frame buffer to set the intensity of the CRT. X-register is incremented by 1 and the process is repeated for the next pixel on the scan line. This procedure is repeated for each pixel along the scan line

BASIC REFESH OPERATIONS OF VIDEO CONTROLLER:

BASIC REFESH OPERATIONS OF VIDEO CONTROLLER X-register is reset to 0 and Y-register is decremented by 1 after processing the last pixel on the top scan line. Pixels along the scan line are processed as given above and the procedure is continued for all successive scan lines. Cycling through all pixels along scan line Y=0(bottom line),the video controller resets the register to the first pixel position on the scan line and the refresh process starts over.

Raster-Scan system with a Display Processor:

Raster-Scan system with a Display Processor

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Display processor manipulate commands, refreshing, scan conversion. Otherwise cpu is busy with other works.

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Figure 2-29 shows one way to set up the organization of a raster system containing a separate display processor, sometimes referred to as a graphics controller or a display coprocessor. The purpose of the display processor is to free the CPU from the graphics chores. In addition to the system memory, a separate display processor memory area can also be provided.

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A major task of the display processor is digitizing a picture definition given in an application program into a set of pixel-intensity values for storage in the frame buffer. This digitization process is caIled scan conversion . Graphics commands specifying straight lines and other geometric objects are scan converted.

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A character is defined as a set of discrete intensity points. Scan converting a straight-line segment, for rectangular grid of pixels, for example, means that we have to locate the pixel positions closest to the line path positions and store the intensity for each position in the frame buffer.

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In an effort to reduce memory requirements in raster systems, methods have been devised for organizing the frame buffer as a linked list and encoding the intensity information.

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One way to do this is to store each scan line as a set of integer pairs. One number of each pair indicates an intensity value, and the second number specifies the number of adjacent pixels on the scan line that are to have that intensity. This technique, called run-length encoding , can result in a considerable saving in storage space if a picture is to be constructed mostly with long runs of a single color each. A similar approach can be taken when pixel intensities change linearly. Another approach is to encode the raster as a set of rectangular areas (cell encoding).

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For example, consider a screen containing plain black text on a solid white background. There will be many long runs of white pixels in the blank space, and many short runs of black pixels within the text.

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Let us take a hypothetical single scan line, with B representing a black pixel and W representing white: WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWBWWWWWWWWWWWWWW If we apply the run-length encoding (RLE) data compression algorithm to the above hypothetical scan line, we get the following: 12W1B12W3B24W1B14W Interpret this as twelve W's, one B, twelve W's, three B's, etc.

RANDOM-SCAN SYSTEMS :

RANDOM-SCAN SYSTEMS SYSTEM BUS CPU SYSTEM MEMORY DISPLAY PROCESSOR MONITOR I/0 DEVICES

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Random scan display system draws a set of lines in any order. That’s why the name random.

Random scan Displays:

Random scan Displays Random scan displays, often termed vector displays (or stroke-writing or calligraphic diisplays or line drawing displays). Here the electron gun of a CRT illuminates points and/or straight lines in any order . The display processor repeatedly reads a variable 'display file' defining a sequence of X,Y coordinate pairs and brightness or colour values, and converts these to voltages controlling the electron gun.

Random scan Displays:

Random scan Displays Random Scan Display

ARCHITECTURE OF A SIMPLE RANDOM-SCAN SYSTEM:

ARCHITECTURE OF A SIMPLE RANDOM-SCAN SYSTEM SYSTEM BUS CPU SYSTEM MEMORY DISPLAY PROCESSOR MONITOR I/0 DEVICES

Random scan Displays:

Random scan Displays Vector graphics terminals are computer graphics displays that draw lines instead of bitmapped images. A vector graphics display is basically a computer controlled oscilloscope. It draws lines and curves by moving the electron beam exactly where the line is supposed to go, instead of breaking it into dots and scanning the electron beam back and forth, like a television set does. That is, to draw a line from point a to point b, it moves the electron beam to point a, turns it on, moves it directly to point b, and turns it off.

Raster scan Displays :

Raster scan Displays Also known as bit-mapped or raster displays . Their whole display area is updated many times in a second from image data held in raster memory. a raster is a series of adjacent parallel 'lines' which together form an image on a display screen . In early analogue television sets each such line is scanned continuously, not broken up into distinct units. In computer or digital displays these lines are composed of independently colored pixels (picture elements).

INPUT DEVICES:

INPUT DEVICES Keyboards Mouse Trackball and Spaceball Joysticks Data Glove Digitizers Image Scanners Touch Panels

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Light Pens Voice Systems

Ergo keyboard:

Ergo keyboard

hiwale:

hiwale

joystick:

joystick

keyboard:

keyboard

keyguard:

keyguard

Magnetic stripe reader:

Magnetic stripe reader

Maxim-sml:

Maxim-sml

mouse:

mouse

ocr:

ocr

scanner:

scanner

spaceball:

spaceball

Spaceball:

Spaceball

tablet:

tablet

Touch screen:

Touch screen

Track ball:

Track ball

HARD COPY DEVICES:

HARD COPY DEVICES Printers & Plotters dot-matrix printer laser printer ink-jet pen plotter

Output Devices:

122 Cathode Ray Tubes Flat panel devices, e.g. LCDs Stereoscopic and Virtual Reality systems Output Devices

Graphics Software:

123 Graphics Software Two categories of Graphics software Special-purpose application packages General programming packages

General programming packages :

General programming packages Examples: GL, OpenGL, VRML(Virtual Reality Modeling Language), Java 3D A general graphics programming package provides an extensive set of graphics functions that can be used in a high-level programming language, such as C or FORTRAN. Basic functions in a general package include those for generating picture components (straight lines, polygons, circles, and other figures), setting color and intensity values, selecting views, and applying transformations.

Special purpose applications packages:

Special purpose applications packages Examples CAD Painting programs Various medical and business systems

Coordinate Representations:

Coordinate Representations With few exceptions, general graphics packages are designed to be used with Cartesian coordinate specifications. If coordinate values for a picture are specified in some other reference frame (spherical, hyberbolic, etc.), they must be converted to Cartesian coordinates before they can be input to the graphics package. Special-purpose packages may allow use of other coordinate frames that are appropriate to the application.

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In general; several different Cartesian reference frames are used to construct and display a scene. We can construct the shape of individual objects, such as trees or furniture, in a scene within separate coordinate reference frames called modeling coordinates , or sometimes local coordinates or master coordinates. Once individual object shapes have been specified, we can place the objects into appropriate positions within the scene using a reference frame called world coordinates.

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Finally, the world-coordinate description of the scene is transferred to one or more output-device reference frames for display. These display coordinate systems are referred to as device coordinates or screen coordinates in the case of a video monitor.

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A graphics system first converts world- coordinate positions to normalized device coordinates, in the range from 0 to 1, before final conversion to specific device coordinates. (x mc , y mc ) (x wc , y wc ) (x nc , y nc ) (x dc , y dc )

Graphics Functions :

Graphics Functions A general-purpose graphics package provides users with a variety of functions for creating and manipulating pictures. These routines can be categorized according to whether they deal with output, input, attributes, transformations, viewing, or general control.

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The basic building blocks for pictures are referred to as output primitives. They include character strings and geometric entities, such as points, straight lines, curved Lines, filled areas (polygons, circles, etc.), and shapes defined with arrays of color points. Routines for generating output primitives provide the basic tools for constructing pictures.

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Attributes are the properties of the output primitives; that is, an attribute describes how a particular primitive is to be displayed. They include intensity and color specifications, line styles, text styles, and area-filling patterns.

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We can change the size, position, or orientation of an object within a scene using geometric transformations. Similar modeling transformations are used to construct a scene using object descriptions given in modeling coordinates..

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Given the primitive and attribute definition of a picture in world coordinates, a graphics package projects a selected view of the picture on an output device. Viewing transformations are used to specify the view that is to be printed and the portion of the output display area that is to be used

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Pictures can be subdivided into component parts, called structures or segments or objects , depending on the software package in use. Each structure defines one logical unit of the picture.

Software Standards:

Software Standards The primary goal of standardized graphics software is portability. When packages are designed with standard graphics functions, software can be moved easily from one hardware system to another and used in different implementations and applications. Without standards, programs designed for one hardware system often cannot be transferred to another system without extensive rewriting of the programs.

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International and national standards planning organizations in many countries have cooperated in an effort to develop a generally accepted standard for computer graphics. After considerable effort, this work on standards led to the development of the Graphical Kernel System (GKS).

GKS Graphical Kernel System :

GKS Graphical Kernel System This system was adopted as the first graphics software standard by the International S tandards Organization(ISO) and by various national standards organizations, including the American National Standards Institute (ANSI). Although GKS was originally designed as a two-dimensional graphics package, a three-dimensional GKS extension was subsequently developed.

PHIGS (Programmer's Hierarchical Interactive Graphics standard) :

PHIGS (Programmer's Hierarchical Interactive Graphics standard) The second software standard to be developed and approved by the standards orgainzations was PHIGS . which is an extension of GKS increased capabilities for object modeling color specifications, surface rendering, and picture manipulations are provided in PHIGS. Subsequently, an extension of PHIGS, called PHIGS+, was developed to provide three-dimensional surface-shading capabilities not available in PHIGS.

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