CGMB214: Introduction to Computer Graphics

1. CGMB214: Introduction to Computer Graphics Topic 2 Overview of Graphics System

2. Objectives To provide an understanding on the basic feature of graphic hardware components. To provide an understanding on the operations of a video display device.

3. Video Display Devices Cathode Ray Tube Technology

4. Video Display Devices Video Display Devices CRT Technology Flat Panel Displays Refresh Cathode Ray Tubes Raster Scan Display Random Scan Display Colour CRT Monitor Emissive Display Non Emissive Display

5. Refresh Cathode Ray Tubes Focusing System Electron Gun Magnetic Deflection Coils Phosphor-Coated Screen Base Connector Pins

6. Refresh Cathode Ray Tubes: Electron Gun Heating Filament Control Grid Accelerating Anode Focusing Anode Cathode

7. Refresh Cathode Ray Tubes: Electron Gun

8. Refresh Cathode Ray Tubes: Electron Gun Current is directed through filament to supply heat Electron is being emitted from the hot cathode surface Control grid is a metal cylinder that is controlled by voltage. A high negative voltage, will repel electron and stop them from passing A smaller negative decrease the number of electron passing through Control Grid Focusing Anode Accelerating Anode Heating Filament Cathode

9. Refresh Cathode Ray Tubes: Electron Gun Once an electron is being released and the correct amount of voltage is being supplied by the control grid, the electron will move in a straight line through the control grid The focusing anode which is charged with hundreds of positive volts, will force the electron beam to converge to it. However, since there are two anode attracting the electron, the electron will continue moving in a straight line The accelerating anode which is charged with thousand of positive volts will have greater force on attracting the electron beam to converge to it. As focusing anode, the built of the accelerating anode make sure the electron will continue moving in a straight line Control Grid Focusing Anode Accelerating Anode Heating Filament Cathode

10. Refresh Cathode Ray Tubes Magnetic Deflection Coils Phosphor-Coated Screen Connector Pins Base Electron Gun Focusing System Once the electron beam is emitted by the electron, it will pass through focusing and deflection system that direct the beam toward a specific position on the phosphor-coated screen. The phosphor emits a small spot of light at each position contacted by electron beam The brightness of the of the display point is controlled by the voltage provided by the control grid in the electron gun

11. Refresh Cathode Ray Tubes • Resolution • A max number of points that can be displayed without overlap on CRT • Typically 1028X1024 • High resolution means high definition • Persistence • How long the phosphors will emit light after beam goes • Phosphor with low persistence require higher refresh rate to prevent flicker

12. Refresh Cathode Ray Tubes

13. Raster Scan Displays The most common type of graphics monitor that employs CRT The electron beam is swept across the screen one row at a time, from top to bottom Each row is referred as a scan line As the electron beam moves across a scan line, the beam intensity is turned on and off to create a pattern of illuminated spots Screen is divided into smallest element of a rectangular array called pixel or pel(picture element)

14. Raster Scan Displays

15. Raster Scan Displays Screen Display Pixels/pels

16. Raster Scan Displays • Raster system commonly characterized by their resolution, which is the number of pixel positions that can be plotted • Another property of video monitors is aspect ratio: • number of pixel columns divided by the number of scan lines that can be displayed by the system • number of horizontal points to vertical points (or vice versa) necessary to produce equal-length lines in both directions on the screen

17. Raster Scan Displays • Picture definition is stored as set of color values for screen points in memory area called refresh buffer or color buffer or frame buffer (frame refers to total screen area). • Frame buffer with one bit per pixel is called bitmap • Frame buffer with multiple bits per pixel is a pixmap. • Refresh rates are described in units of cycles per second (Hertz) where a cycle corresponds to one frame. • Raster scan displays normally perform refreshing at the rate of 60-80 Hz

18. Raster Scan Displays Zoom in

19. Random Scan Display Electron beam is focussed only where lines need to be drawn Random-scan also known as vector display or stroke-writing display or calligraphic displays Scene definition stored in a display list as a set of line drawing instruction Scene is drawn by cycling through the display list drawing one line at a time

20. Random Scan Display Higher resolution than raster-scan and draw a smoother line because the path beam follows the line path Designed for specialised line-drawing application e.g. CAD

21. Random Scan Displays

22. Random Scan Display Zoom in

23. Colour CRT Monitor • Two ways of displaying colour: • Beam penetration • 2 layers of phosphors (red and green) • Colour depends on the speed of the electron beams • Non-expensive way to get colour • Shadow mask • Used in raster scan • Use 3 phosphor colour dots at each pixel (RGB) • CRT needs to have 3 electron guns

24. Colour CRT Monitor: Beam Penetration fast moderate slow Beam of SLOW electron excited only the outer layer producing red Beam of FAST electron penetrates through the red layer and excites the inner green layer Moderate beam speeds will emits light combination of red and green: orange and yellow Green phosphor layer Red phosphor layer

25. Colour CRT Monitor: Shadow Mask Each pixel position has three phosphor colour dot: Red, Green and Blue (RGB). Each phosphor emits lights according to their colour. Shadow mask CRT needs 3 electron guns, one for each colour dot. The three electron beams are deflect and focused as a group at a shadow mask. Here it activated a dot triangle The phosphor dots in the triangles are arranged so that each electron beam can only activate its corresponding colour dot when it passes the shadow mask Once all the electrons has been arranged in the shadow mask, it will be released on the correct phosphor triangle on the screen Colour variation is produced by varying the intensity level of the electron beams: Eg. To get yellow, blue gun need to be switch off. Electron guns Shadow mask Screen