Graphics Harrdware Chapter 4

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Graphics Hardware

How imp elements of a computer graphics display system work

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Hardcopy Technologies

• Important terms– Dot size(also called spot size) : diameter of a

single dot on device’s output– Addressability : Number of individual dots per

inch(may differ in horizontal and vertical dir)– Interdot distance : distance between centers of

dots at addresses x,y and x+1,y– Resolution : number of distinguishable lines per

inch a device can create. It also depends upon the intensity distribution of a spot

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• Important terms– Dot Matrix printers use a print head of 7 to 24 pins

each which can be individually fired to strike a ribbon against paper.

– Print head moves across the paper one step at a time , the paper is advanced one line and the print head makes another pass.

– Colored ribbons can be used to produce color hardcopy

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• Important terms– Random and raster plotters– Pen plotters move a pen over a piece of paper in

random, vector drawing style– They include a microprocessor that accepts commands

eg “draw line”, “draw circle”, “draw text”, “move”, “set line style”, “select pen”

– Flatbed plotter moves a pen in x and y dir on a sheet of paper spread out on a table

– Drum plotters move the paper along one axis and the pen along the other axis

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• Laser Printer– Laser printers scan a laser beam across a positively

charged rotating drum. The areas hit by laser beam lose their charge and the positive charge remains only where the copy is to be black. A negatively charged powder toner adheres to the positive areas of the drum and is then transferred to blank paper to form a copy.

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• Inkjet Printer– Inkjet printers spray cyan , magenta , yellow and

sometimes black ink onto paper.– Inkjet is mounted on a head in a printer like

mechanism. The print head moves across the page to draw one scan line, returns while paper advances by one line.

– Sometimes paper is wrapped around a drum, the drum then rotates rapidly while the print head slowly moves along the drum.

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Display Technologies

• The CRT is by far the most common display device and will remain so for many years.

• The electron gun emits a stream of electrons that is accelerated towards phosphor coated screen by a high positive voltage.

• Focusing system forces the electrons into a narrow beam • Beam is directed toward a particular point on the screen

by the magnetic field produced by the deflection coils.• When the electrons hit the screen the phosphor emits

visible light.

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• Refresh rate of a CRT is the number of times per second the image is redrawn. It is typically 60/sec for raster displays.

• Flicker develops as the refresh rate decreases.• The refresh rate above which a picture stops

flickering and fuses into a steady image is called CFF(critical fusion frequency)

• The longer the persistence the lower the CFF• CFF increases with image intensity

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• The horizontal scan rate is the number of scan lines/sec a CRT is able to display.

• The resolution of a monochromatic CRT defined just as resolution for hardcopy devices.

• The larger the spot size the lower the resolution.

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• All early color televisions and the majority of computer monitors, past and present, use shadow mask technology.

• A shadow mask is a metal plate punched with tiny holes that separate the colored phosphors in the layer behind the front glass of the screen.

• Three electron guns at the back of the screen sweep across the mask, with the beams only reaching the screen if they pass through the holes.

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• As the guns are physically separated at the back of the tube, their beams approach the mask from three slightly different angles, so after passing through the holes they hit slightly different locations on the screen.

• The screen is patterned with dots of colored phosphor positioned so that each can only be hit by one of the beams coming from the three electron guns.

• The red, green, and blue phosphors for each pixel are generally arranged in a triangular shape (sometimes called a "triad").

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• Direct View Storage Tube (DVST) behaves like a CRT with highly persistent phosphor.

• Pictures drawn on there will be seen for several minutes (40-50 minutes) before fading.

• It is similar to CRT as far as the electronic gun and phosphor-coated mechanisms are concerned.

• But instead of the electron beam directly writing the pictures on the phosphor coated CRT screen, the writing is done with the help of a fine-mesh wire grid.

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DVST CRT

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• A Liquid crystal display is made up of 6 layers.• Front layer is vertical polarizer plate.• Next is a layer with thin grid wires electrodeposited on

the surface adjoining the crystals• Next is a thin (.0005 inch) liquid crystal layer• Then a layer with horizontal grid wires .• Next a horizontal polarizer• Finally a reflector • Advantages of LCDs are low cost , low weight, small size

and low power consumption.

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• The Plasma panel is an array of tiny neon bulbs.• Each bulb can be put into an on/off state until

explicitly changed.• This means that plasma panels need not be

refreshed.• Plasma panel has the advantages of being flat,

transparent and rugged and no need of refresh buffer.

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Raster Scan Display System

• Most raster displays have some specialized hardware who does scan conversion and performs raster conversion of moving, copying and modifying pixel.

• We call this HW a graphics display processor. We will discuss :

• Simple raster display system• Raster display system with peripheral display processor• Additional display processor functionality• Raster display system with integrated display processor

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Simple raster system display system

• Simple raster display System• Raster Display System with Peripheral Display

Processor

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Graphics Harrdware Chapter 4