28-07-2012, 12:12 PM
CAD Display Devices
CAD Display Devices.ppt (Size: 923.5 KB / Downloads: 66)
Examples of Computer Graphics Devices
CRT, EGA/CGA/VGA/SVGA monitors,
plotters, data matrix, laser printers, Films,
flat panel devices, Video digitizers, scanners,
LCD panels, keyboard, joystick, mouse,
touch screen, track ball, etc.
The most commonly used display device is the
CRT monitor
CRT Display
Types of CRT display devices
DVST (Direct View Storage Tube)
Calligraphic or Random Scan display system
Refresh and raster scan display system
Operation of CRT
The operation of CRT is based on the concept of energizing an electron beam that strikes the phosphor coated screen at a very high speed.
The energy transfer from the electrons to the phosphor due to the impact causes it to illuminate and glow.
The negatively charged cathode is heated until electrons generated in a diverging clouds (as electrons repel each other) and then they are focused into a beam via the focusing unit.
Phosphorus-coated Screen
The CRT surface is coated with special crystals called phosphors.
Phosphors glow when they are hit by a high-energy electron beam.
The glow given off by the phosphor during exposure to the electron beam is known as fluorescence.
The continuing glow given off after the beam is removed is known as phosphorescence. Its duration is known as the phosphor’s persistence.
DVST - Direct View Storage Tube
Modifying any part of the image requires redrawing the entire modified image
Change in the image requires to generate a new charge distribution in the DVST
Erasing takes about 0.5 seconds. All lines and characters must be erased
Slow process of drawing – typically a few seconds are necessary for a complex picture
No animation possible with DVST
Calligraphic or Random Scan display system
Also called Vector, Stroke, Line drawing displays
Characters are also made of sequences of strokes (or short lines)
Vectored – electron beam is deflected from endpoint to end-point
Random scan - Order of deflection is dictated by the arbitrary order of the display commands
Phosphor has short persistence – decays in 10-100 ms
Refresh and raster scan display system
Used in television screens
Unlike DVST and random-scan which were linedrawing devices, refresh CRT is apoint-plotting device
Raster displays store the display primitives (lines, characters, shaded and patterned areas) in a refresh buffer
Refresh buffer (also called frame buffer) stores the drawing primitives in terms of points and pixels components