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2.  The primary output device in a graphics system is a video monitor. These monitors are based on Cathode Ray Tube (CRT) design.  CRT is a vacuum tube/electron tube.  It has an electron gun at one end that emits beam of electrons (cathode rays).  This beam is directed towards the front of the tube that is the screen. 2

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4.  The inner side of the screen is coated with phosphor substance which gives off light when it is stroked by electrons.  It is possible to control the point at which the electron beam strikes the screen by deflecting the beam in the CRT. 4

5.  CRT consists of following components: 1. Electron Gun:  The components of electron gun are: - heating filament - cathode - control grid 5

6.  When the current is passed through the filament, it supplies the heat to the cathode and causes the electrons to boil off from the hot cathode surface.  Control grid is a metal cylinder that fits over the cathode.  It is used to control the intensity of electron beam by setting voltages. 6

7.  If high negative voltage is applied to the control grid the electron beam shuts off because electrons get repelled from the negative voltage of grid and cannot pass from the small hole at the end of the control grid.  Smaller negative voltage on the grid decreases the number of electrons passing through.  Brightness of the display depends on the number of electrons striking the screen. Therefore control grid controls the brightness of the display. 7

8. 2. Focussing System (Anode): it is the part of CRT that is used to force the electron beam to converge to small spots as it strikes the phosphor. Otherwise the electrons will repel each other and the beam would spread out as it reaches the screen. Anode is positively charged which attracts the negatively charged electrons and causes them to converge towards center of the screen. 8

9. 3. Accelerating system: it also uses an anode which accelerates the negatively charged electrons towards the phosphor coated screen. 4. Horizontal Deflection plates: it is a pair of plates placed vertically inside the CRT to control the horizontal deflection. 5. Vertical Deflection plates: it is a pair of plates placed horizontally to control the vertical deflection. Amount of deflection depends on the amount of voltage applied to the plates. 9

10.  It is the most common method of displaying images on the CRT screen.  The electron beam sweeps across the screen, one row at a time from top to bottom.  The image is displayed by how each pixel is intensified.  The image to be displayed is stored in the frame buffer in the form of intensity values for each pixel. 10

11. 11  These intensity values are then retrieved from the refresh buffer and painted on the screen one row at a time.  In simple black and white systems, only one bit per pixel is needed to control the intensity of screen points.  1 indicates electron beam is turned on  0 indicates electron beam is turned off

12. 12  For color systems there are 24 bits per pixel.  Therefore, color systems have larger sized frame buffer (3 MB).  For black and white system frame buffer is called bitmap and for color systems it is called pixmap.  Horizontal retrace: After sweeping across each row (scan line), the electron beam returns to the left of the screen to start displaying the next scan line or row. This is called horizontal retrace.  Vertical retrace: the electron beam returns to the top left corner of screen to begin the next frame when it completes one frame. This is called vertical retrace.

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14. 14  Refresh rate: it means how many times the screen is redrawn or refreshed in one second. Refresh rate in the raster scan systems is between 60 to 80 Hz per second.  Interlaced Scanning: in this alternate lines are scanned instead of all lines. Scanning is done in two passes. Advantage: We can see the entire screen displayed in half time.

15.  Raster scan systems consist of a number of processing units: - CPU - System Memory - Frame Buffer - Video Controller - Monitor - System Bus - I/O Devices 15

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17.  Raster Scan systems have a special-purpose processor called video/display controller. It is used to control the operations of a display device.  It can directly access the frame buffer.  The function of the video controller is to read the frame buffer and produce the actual image on the screen. 17

18.  Frame Buffer locations and corresponding screen positions are referenced in Cartesian Coordinates. - The origin is defined at the top left screen corner. - Positive x values increase towards the right and positive y values increase towards the top. - Scan lines are labeled from ymax at the top of the screen to 0 at the bottom. - In each line, the screen pixels are labeled from 0 to xmax. - For storing x and y values, two registers are used, x register and y register. 18

19.  When we start scanning from the top left corner of the screen, x register is set to 0 and y register is set to ymax.  As the electron beam moves towards the next pixel, x register is incremented by 1, that is x=x+1.  This process is repeated till x register stores xmax.  After that x register is reset to 0 and y register is decremented by 1, that is y=y-1. This starts the process for next scan line.  All lines are then scanned in the same manner till the electron beam reaches lower right corner, that is y=0 and x=xmax.  Video controller then resets the registers to first pixel position on the top scan line and starts again. 19

20.  CRT monitors are being replaced by flat-panel devices.  Flat panel devices have: - less weight than CRT’s - less power requirements than CRT’s - are thinner than CRT’s  Flat panel devices are used in TV monitors, laptops, calculators, video games, etc. 20

21.  Plasma Panel displays  Thin-film electroluminescent displays  Light emitting diode (LED)  Liquid Crystal Device (LCD) 21

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23.  Also called gas discharge displays.  It consist of 2 glass plates.  The area between plates is filled with a mixture of gases including neon.  Vertical conducting ribbons are placed on one plate and horizontal ribbons are placed on the other plate.  Voltages applied to a pair of vertical and horizontal conductors cause the gas to breakdown into glowing plasma of electrons and ions.  Refresh rate of these displays is 60 times/ second 23

24.  Similar to plasma panel.  Difference is the area between glass plates is filled with phosphor instead of gas.  Require more power than plasma panel.  Do not display good color. 24

25. Diodes  LED displays consist of a number of diodes arranged in the form of pixel positions in the display.  When the voltage is applied to the diodes they produce light patterns.  Pixel values are read from the frame buffer. 25

26.  Liquid crystal is a substance in a state between solid and liquid but which has properties of both.  They have crystalline arrangement but they flow like a liquid.  This property is used in LCD’s. 26

27.  It has two glass plates.  One plate has horizontal conductors and other plate has vertical conductors.  The intersection of two conductors forms a pixel position.  In between the plates is a liquid crystal that is twisted.  Light passing through the material is twisted so that it will pass through the other plate.  Light is then reflected back to the users.  To turn off the pixel, voltage is applied to the two conductors that untwists the crystals so that light will not be twisted and will not be passed through the plates. 27

28. 28  In these systems, the electron beam does not sweep each and every row, however, it is directed only to the parts of the screen where a picture is to be drawn.  Also known as vector displays.  Draws one line at a time.  Parts of the picture can be displayed in any order.

29. 29  In these systems, the refresh buffer does not store the intensity values of the picture, but a set of line drawing commands.  Refresh buffer is also called display file.  The system cycles through the set of commands in the display file to display a picture.  Refresh rate in random scan systems is 30 to 40 times per second.

30. 30 Raster Scan  It is a point plotting device.  It sweeps in a particular order.  It is well suited for realistic display of scenes containing shading and color patterns.  It is used for photos. That is why Photoshop is a raster editing program. Random Scan  It is a line drawing device.  It draws the image in any order.  It is used for line drawing applications and cannot display realistic shaded scenes.  It is used for text, logos, letterheads.

31. 31 1. Resolution: Resolution of a display device is defined as the number of pixels on the entire screen of the device. For example, 1024 X 768 resolution means, The screen has 1024 pixels in each line and The screen has 768 horizontal lines. 2. Persistence: It is the property of phosphor that is how long they continue to emit light. Persistence is defined as the time it takes the emitted light to decay to one-tenth of its original intensity. Computer monitors have persistence of 10-60 microseconds. Lower persistence phosphor requires higher refresh rates to maintain a picture on the screen without flicker.