CGMB214: Introduction to Computer Graphics
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Transcript of CGMB214: Introduction to Computer Graphics
TOPIC 2OVERVIEW OF GRAPHICS SYSTEM
CGMB214: Introduction to Computer Graphics
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.
CATHODE RAY TUBE TECHNOLOGY
Video Display Devices
Video Display Devices
Video Display Devices
CRT Technology Flat Panel Displays
Refresh Cathod
e Ray Tubes
Raster Scan
Display
Random Scan Display
Colour CRT
Monitor Non Emissive Display
Emissive Display
Refresh Cathode Ray Tubes
Connector Pins
Base
Electron Gun
Focusing System
Magnetic Deflection
Coils
Phosphor-Coated Screen
Refresh Cathode Ray Tubes: Electron Gun
Heating Filament
Cathode
Control Grid
Focusing Anode
Accelerating Anode
Refresh Cathode Ray Tubes: Electron Gun
Refresh Cathode Ray Tubes: Electron Gun
Heating Filament Cathode
Control Grid
Focusing Anode
Accelerating Anode
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
Refresh Cathode Ray Tubes: Electron Gun
Heating Filament Cathode
Control Grid
Focusing Anode
Accelerating Anode
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
Refresh Cathode Ray Tubes
Connector Pins Base Electron
GunFocusing System
Magnetic Deflection
Coils
Phosphor-Coated Screen
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 beamThe brightness of the of the display point is controlled by the
voltage provided by the control grid in the electron gun
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
Refresh Cathode Ray Tubes
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 lineAs 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)
Raster Scan Displays
Raster Scan Displays
Screen Display
Pixels/pels
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
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 bitmapFrame 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
Raster Scan Displays
Zoom in
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
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
Random Scan Displays
Random Scan Display
Zoom in
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
Colour CRT Monitor: Beam Penetration
slow
Beam of SLOW electron excited only the outer layer producing red
moderatefast
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
Red phosphor layer
Green phosphor layer
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.
Electron guns
Shadow mask
Screen
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.