Display devices
⦁    Display devices are also known as output devices.
⦁    Most commonly used output device in a graphics system is a video monitor.

⦁    It is an evacuated glass tube.
⦁    An electron gun at the rear of the tube produce a beam of electrons which is directed towards the screen of the tube by a high voltage typically 15000 to 20000 volts
⦁    Inner side screen is coated with phosphor substance which gives light when it is stroked bye electrons.
⦁    Control grid controls velocity of electrons before they hit the phosphor.
⦁    The control grid voltage determines how many electrons are actually in the electron beam. The negative the control voltage is the fewer the electrons that pass through the grid.
⦁    Thus control grid controls Intensity of the spot where beam strikes the screen.
⦁    The focusing system concentrates the electron beam so it converges to small point when hits the phosphor coating.
⦁    Deflection system directs beam which decides the point where beam strikes the screen.
⦁    Deflection system of the CRT consists of two pairs of parallel plates which are vertical and horizontal deflection plates.
⦁    Voltage applied to vertical and horizontal deflection plates is control vertical and horizontal deflection respectively.
 

There are two techniques used for producing images on the CRT screen:

 Vector scan/Random scan display.
 Raster scan display.

Vector scan/Random scan display

Fig. 1.2 Architecture of a vector display 

⦁    Vector scan display directly traces out only the desired lines on CRT.
⦁    If we want line between point p1 & p2 then we directly drive the beam deflection circuitry which focus beam directly from point p1 to p2.
⦁    If we do not want to display line from p1 to p2 and just move then we can blank the beam as we move it.
⦁    To move the beam across the CRT, the information about both magnitude and direction is required. This information is generated with the help of vector graphics generator.
⦁    Fig. 1.2 shows architecture of vector display. It consists of display controller, CPU, display buffer memory and CRT.
⦁    Display controller is connected as an I/O peripheral to the CPU.
⦁    Display buffer stores computer produced display list or display program.
⦁    The Program contains point & line plotting commands with end point co-ordinates as well as character plotting commands.
⦁    Display controller interprets command and sends digital and point co-ordinates to a vector generator.
⦁    Vector generator then converts the digital co-ordinate value to analog voltages for beam deflection circuits that displace an electron beam which points on the CRT’s screen.
⦁    In this technique beam is deflected from end point to end point hence this techniques is also called random scan.
⦁    We know as beam strikes phosphors coated screen it emits light but that light decays after few milliseconds and therefore it is necessary to repeat through the display list to refresh the screen at least 30 times per second to avoid flicker.
⦁    As display buffer is used to store display list and used to refreshing, it is also called refresh buffer.

Raster scan display
 

⦁    Fig. 1.3 shows the architecture of Raster display. It consists of display controller, CPU, video controller, refresh buffer, keyboard, mouse and CRT.
⦁    The display image is stored in the form of 1’s and 0’s in the refresh buffer.
⦁    The video controller reads this refresh buffer and produces the actual image on screen.
⦁    It will scan one line at a time from top to bottom & then back to the top.

⦁    In this method the horizontal and vertical deflection signals are generated to move the beam all over the screen in a pattern shown in fig. 1.4.
⦁    Here beam is swept back & forth from left to the right.
⦁    When beam is moved from left to right it is ON.
 

⦁    When beam is moved from right to left it is OFF and process of moving beam from right to left after completion of row is known as Horizontal Retrace.
⦁    When beam is reach at the bottom of the screen. It is made OFF and rapidly retraced back to the top left to start again and process of moving back to top is known as Vertical Retrace.
⦁    The screen image is maintained by repeatedly scanning the same image. This process is known as
Refreshing of Screen.
⦁    In raster scan displays a special area of memory is dedicated to graphics only. This memory is called
Frame Buffer.
⦁    Frame buffer holds set of intensity values for all the screen points.
⦁    That intensity is retrieved from frame buffer and display on screen one row at a time.
⦁    Each screen point referred as pixel or Pel (Picture Element).
⦁    Each pixel can be specified by its row and column numbers.
⦁    It can be simply black and white system or color system.
⦁    In simple black and white system each pixel is either ON or OFF, so only one bit per pixel is needed.
⦁    Additional bits are required when color and intensity variations can be displayed up to 24-bits per pixel are included in high quality display systems.
⦁    On a black and white system with one bit per pixel the frame buffer is commonly called a Bitmap. And for systems with multiple bits per pixel, the frame buffer is often referred as a Pixmap.
 

Difference between random scan and raster scan