chapter 4 -- graphics hardware1 graphics hardware chapter 4

Chapter 4 -- Graphics Hardware 1

Graphics Hardware

Chapter 4Chapter 4


In this chapter, we describe how the important In this chapter, we describe how the important hardware elements of a computer graphics hardware elements of a computer graphics display system work.display system work.

Hardware technologies: Hardware technologies: printers, pen plotters, laser printers, ink-jet plotters,printers, pen plotters, laser printers, ink-jet plotters,

Display technologiesDisplay technologies monochrome and shadow-mask CRT’s, LCD’s, …monochrome and shadow-mask CRT’s, LCD’s, …

Raster Display SystemsRaster Display Systems Overview, … Overview, …

Input DevicesInput Devices film scanners,… film scanners,…


Here is the typical relation of the devices to one Here is the typical relation of the devices to one another:another:

The key elements are the CPU and the display The key elements are the CPU and the display processor.processor.


1. Hardcopy Technologies

In this section, we discuss various hardcopy In this section, we discuss various hardcopy technologies, then summarize their technologies, then summarize their characteristics.characteristics.

However, several important terms must be However, several important terms must be defined first.defined first.


Dot sizeDot size (also called spot size) is the diameter (also called spot size) is the diameter of a single dot created by a device.of a single dot created by a device.

AddressabilityAddressability is the number of individual dots is the number of individual dots per inch that can be createdper inch that can be created This may differ in the horizontal and vertical This may differ in the horizontal and vertical

directions.directions. Addressability in x is just the reciprocal of the Addressability in x is just the reciprocal of the

distance between the centers of dots at addresses distance between the centers of dots at addresses (x,y) and (x+1,y)(x,y) and (x+1,y)

Addressability in y is defined similarly.Addressability in y is defined similarly.


Interdot distanceInterdot distance is the reciprocal of is the reciprocal of addressability.addressability. It is usually desirable that dot size be somewhat It is usually desirable that dot size be somewhat

greater than the interdot distance, so that smooth greater than the interdot distance, so that smooth shapes can be createdshapes can be created

There are tradeoffs:There are tradeoffs:• Dot size several times the interdot distance allows very Dot size several times the interdot distance allows very

smooth shapes to be printed.smooth shapes to be printed.• whereas a smaller dot size allows finer detail. whereas a smaller dot size allows finer detail.


ResolutionResolution, which is related to the dot size and , which is related to the dot size and can be no greater than addressability, is the can be no greater than addressability, is the number of distinguishable lines per inch that a number of distinguishable lines per inch that a device can create.device can create.

If 40 black lines interleaved with 40 white lines can If 40 black lines interleaved with 40 white lines can be distinguished across one inch, the resolution is 80 be distinguished across one inch, the resolution is 80 lines per inchlines per inch

Resolution also depends on the cross-sectional Resolution also depends on the cross-sectional intensity distribution of a spot. A spot with sharply intensity distribution of a spot. A spot with sharply delineated edges yields higher resolution than does delineated edges yields higher resolution than does one with edges that trail off.one with edges that trail off.


Color Output:Color Output: Many devices to be discussed can create only a few Many devices to be discussed can create only a few

colors at any one point. Additional colors can be colors at any one point. Additional colors can be obtained with dither patterns (discussed in Chapter obtained with dither patterns (discussed in Chapter 11). This results in a decreased spatial resolution 11). This results in a decreased spatial resolution for the resulting image.for the resulting image.

With Color Ribbons a Dot-Matrix printer can With Color Ribbons a Dot-Matrix printer can produce color images in two ways:produce color images in two ways:

• Multiple print heads (each with a different color)Multiple print heads (each with a different color)• Single print head with a multi-colored ribbonSingle print head with a multi-colored ribbon

• More colors than are actually on the ribbon can be created More colors than are actually on the ribbon can be created by overstriking two different colors at the same dot on the by overstriking two different colors at the same dot on the paper. paper.


Hard Copy Devices:Hard Copy Devices: Dot Matrix PrintersDot Matrix Printers

use a print head from 7 to 24 pins (thin, stiff pieces use a print head from 7 to 24 pins (thin, stiff pieces of wire) each of which can be individually fired to of wire) each of which can be individually fired to strike a ribbon against the paper.strike a ribbon against the paper.

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

Hence, these printers are raster output devices, Hence, these printers are raster output devices, requiring scan conversion of vector images prior to requiring scan conversion of vector images prior to printing.printing.


Pen Plotters:Pen Plotters: moves the pen over the paper in a random, vector-moves the pen over the paper in a random, vector-

drawing style. drawing style.

In drawing a line, In drawing a line, • the pen is positioned at the start of the line, the pen is positioned at the start of the line, • lowered to the paper, lowered to the paper, • moved in a straight path to the end of the line, moved in a straight path to the end of the line, • raised, and moved to the start of the next line.raised, and moved to the start of the next line.

There are two basic types: There are two basic types: • flatbed plotters and flatbed plotters and • drum plotters.drum plotters.


flatbed plotterflatbed plotter• Pen is moved in x and y on a sheet of paper spread out on Pen is moved in x and y on a sheet of paper spread out on

a table. a table. – The paper is held by static charge, by vacuum, or The paper is held by static charge, by vacuum, or

stretched tightly.stretched tightly.• A carriage moves in one axis over the paper.A carriage moves in one axis over the paper.• The carriage holds a pen mount which can move the The carriage holds a pen mount which can move the

length of the carriage.length of the carriage.• The pen is raised and lowered and the pen mount and The pen is raised and lowered and the pen mount and

carriage move to draw the line.carriage move to draw the line.

• flatbed plotters are available in sizes from 12 by 18 inches flatbed plotters are available in sizes from 12 by 18 inches to 6 by 10 feet and larger.to 6 by 10 feet and larger.


drum plotterdrum plotter• The paper is moved along one axis (as opposed to the The paper is moved along one axis (as opposed to the

carriage moving)carriage moving)• The pen-mount moves across the drumThe pen-mount moves across the drum• Pins on the drum engage pre-punched holes in the paper Pins on the drum engage pre-punched holes in the paper

to prevent slipping.to prevent slipping.

desktop plotter desktop plotter • is a special type of drum plotter, is a special type of drum plotter, • but the paper is moved between pinch rollers instead of but the paper is moved between pinch rollers instead of

using pins and holes.using pins and holes.


Laser PrintersLaser Printers A laser beam is scanned across a positively charged A laser beam is scanned across a positively charged

rotating drum (coated with selenium)rotating drum (coated with selenium) The beam changes the charge, and the area left The beam changes the charge, and the area left

positive will become black.positive will become black. A negatively charged powder adheres to the A negatively charged powder adheres to the

positively charged areas of the drum, and is then positively charged areas of the drum, and is then transferred to the paper.transferred to the paper.


Laser Printers (cont.)Laser Printers (cont.) In color xerography, this process is repeated three In color xerography, this process is repeated three

times times • once for each primary coloronce for each primary color

Laser printers have a microprocessor to do scan Laser printers have a microprocessor to do scan conversion and to control the printer.conversion and to control the printer.

An increasing number also accept the PostScript An increasing number also accept the PostScript document and image description language.document and image description language.

Most laser printers work with 8.5x11 inch paper or Most laser printers work with 8.5x11 inch paper or 8.5x14 inch paper. 8.5x14 inch paper.


Ink-Jet printersInk-Jet printers Spray cyan, magenta, yellow, and sometimes black Spray cyan, magenta, yellow, and sometimes black

ink onto paper.ink onto paper. In most cases they are mounted on a head in a In most cases they are mounted on a head in a

printer-like mechanism.printer-like mechanism. The print head The print head

• moves across the page to draw one scan line, and moves across the page to draw one scan line, and • returns while the paper advances by one inter-scan-line returns while the paper advances by one inter-scan-line

spacing, and spacing, and • draws the next line.draws the next line.

All the colors are deposited simultaneously All the colors are deposited simultaneously • unlike the multi-pass lasers, plotters, and dot matrix unlike the multi-pass lasers, plotters, and dot matrix

printersprinters Very few have variable dot-size capabilities.Very few have variable dot-size capabilities.


Thermal-Transfer printersThermal-Transfer printers another raster hardcopy device.another raster hardcopy device. Use finely spaced (200 per inch) heating nibs to Use finely spaced (200 per inch) heating nibs to

transfer pigments from colored wax paper to plain transfer pigments from colored wax paper to plain paper.paper.

The wax paper (made up of strips of cyan, magenta, The wax paper (made up of strips of cyan, magenta, yellow and black) and the plain paper are drawn yellow and black) and the plain paper are drawn together over the strip of heating nibstogether over the strip of heating nibs

• The nibs are selectively heated to cause the pigment to The nibs are selectively heated to cause the pigment to transfer.transfer.

• Because the nibs heat and cool very rapidly, a single color Because the nibs heat and cool very rapidly, a single color image can be created in less than one minute.image can be created in less than one minute.

Some of these printers accept video signals and Some of these printers accept video signals and digital bitmap input, making them convenient for digital bitmap input, making them convenient for creating hardcopy of video images.creating hardcopy of video images.


Thermal sublimation dye transfer printersThermal sublimation dye transfer printers work similarly to the thermal transfer printers.work similarly to the thermal transfer printers. Except this process permits 256 intensities of each Except this process permits 256 intensities of each

color to be transferred color to be transferred • cyan, magenta,and yellowcyan, magenta,and yellow

This creates a high-quality full-color image with a This creates a high-quality full-color image with a spatial resolution of 200 dots per inch.spatial resolution of 200 dots per inch.

The process is slower than wax transfer, but the The process is slower than wax transfer, but the quality is near-photographicquality is near-photographic

• therefore it is the clear choice for full-color pre-press therefore it is the clear choice for full-color pre-press proofs.proofs.


Color Film Recorders:Color Film Recorders: There are two basic techniques:There are two basic techniques:

• The camera records the color image directly from the color The camera records the color image directly from the color CRT.CRT.

– image resolution is limitedimage resolution is limited• A black and white CRT is photographed through color A black and white CRT is photographed through color

filters, and the different color components of the image are filters, and the different color components of the image are displayed in sequencedisplayed in sequence

– This technique yields very high-quality images.This technique yields very high-quality images.– Colors are mixed by double exposing parts of the image Colors are mixed by double exposing parts of the image

through two or more filters (usually with different through two or more filters (usually with different intensities)intensities)

The input can be a raster video signal, a bitmap, or The input can be a raster video signal, a bitmap, or vector-style instructions.vector-style instructions.

• Video signal must stay constant during the entire recording Video signal must stay constant during the entire recording cycle (which can take up to one minute with slow film) cycle (which can take up to one minute with slow film)


2. Display Technologies

Interactive Computer Graphics demands Interactive Computer Graphics demands display devices whose images can be changed display devices whose images can be changed quickly.quickly.

The CRT is by far the most common display The CRT is by far the most common display device, however, solid-state technologies are device, however, solid-state technologies are being developed that may, in the long term, being developed that may, in the long term, substantially reduce the dominance of the substantially reduce the dominance of the CRT.CRT.


Monochrome CRT’sMonochrome CRT’s Essentially the same as those used in black-and-Essentially the same as those used in black-and-

white home television sets.white home television sets.


Color Raster Displays:Color Raster Displays: Shadow Mask CRT.Shadow Mask CRT.


Liquid-Crystal Display (LCD)Liquid-Crystal Display (LCD) It is made up of six layers It is made up of six layers

• The crystal layers polarize the light (or not) and allow it to The crystal layers polarize the light (or not) and allow it to pass through and reach the back reflective layer and come pass through and reach the back reflective layer and come back.back.

• The electric current disables the polarization of some of The electric current disables the polarization of some of the crystals, so it can pass through one layer, but be the crystals, so it can pass through one layer, but be blocked by the next.blocked by the next.


3. Raster-Scan Display Systems

The basic concepts of raster graphics systems The basic concepts of raster graphics systems were presented in Chapter1, and Chapter 2 were presented in Chapter1, and Chapter 2 provided further insight into the types of provided further insight into the types of operations possible with a raster display.operations possible with a raster display.

This section discusses the various elements of This section discusses the various elements of a raster display.a raster display.


4. The Video Controller

The most important task for the video The most important task for the video controller is the constant refresh of the display.controller is the constant refresh of the display.

There are two fundamental types of refresh: There are two fundamental types of refresh: interlaced (used in TV’s)interlaced (used in TV’s) non-interlaced. (most video controllers)non-interlaced. (most video controllers)


Output:Output: RGB RGB

separate cables carry red, green, and blue signals to separate cables carry red, green, and blue signals to the three electron guns of the shadow-mask CRT. the three electron guns of the shadow-mask CRT. another cable carries the sync for vertical and another cable carries the sync for vertical and horizontal retraces.horizontal retraces.

MonochromeMonochrome NTSC NTSC

National Television System Committee -- North National Television System Committee -- North American commercial televisionAmerican commercial television

525 scan lines (only 480 are visible)525 scan lines (only 480 are visible) Bandwidth limits the quality to an effective Bandwidth limits the quality to an effective

resolution of about 350x350. resolution of about 350x350.


4.1 Video Mixing

Another useful video-controller function is Another useful video-controller function is video mixing.video mixing.

Two imagesTwo images one defined in the frame buffer.one defined in the frame buffer. another defined by video signal coming from another defined by video signal coming from

some other source (camera, …)some other source (camera, …) merged into a composite image.merged into a composite image.


Examples of this merging are seen regularly on Examples of this merging are seen regularly on television news, sports, and weather showstelevision news, sports, and weather shows


5. Input Devices for Operator Interaction There are five basic logical devices:There are five basic logical devices:

the locator - indicates position or orientation.the locator - indicates position or orientation. the pick - to select a displayed entitythe pick - to select a displayed entity the valuator - to input a single real number.the valuator - to input a single real number. the keyboard - to input a character stringthe keyboard - to input a character string the choice - to select from a set of possible the choice - to select from a set of possible

actions or choices.actions or choices.


5.1 Locator Devices

Data TabletData Tablet MouseMouse TrackballTrackball JoystickJoystick Touch PanelTouch Panel


5.2 Keyboard Devices

The alphanumeric keyboard is the prototypical The alphanumeric keyboard is the prototypical text input device. text input device. The important functional characteristic of this The important functional characteristic of this

device is that it creates a code uniquely device is that it creates a code uniquely corresponding to a pressed key.corresponding to a pressed key.

Cording (pressing several keys at once) is Cording (pressing several keys at once) is generally not possible -- unless the additional generally not possible -- unless the additional keys were shift, control, or other special keys)keys were shift, control, or other special keys)


5.3 Valuator Devices

Most valuator devices that provide scalar values Most valuator devices that provide scalar values are based on potentiometersare based on potentiometers like the volume and tone controls on a stereo set.like the volume and tone controls on a stereo set. They are usually rotary (dials) with either a They are usually rotary (dials) with either a

limited range, or a free-turning (unbounded limited range, or a free-turning (unbounded range)range)

linear ones are used infrequently in graphics linear ones are used infrequently in graphics systemssystems


5.4 Choice Devices

Function keys are the most common choice Function keys are the most common choice device.device.

Other choice devices are the buttons on many Other choice devices are the buttons on many tablet pucks, or mice.tablet pucks, or mice.

Choice devices are used to enter commands or Choice devices are used to enter commands or menu options in a graphics programmenu options in a graphics program


6. Image Scanners

Although data tablets can be used to digitize Although data tablets can be used to digitize existing line drawing manually, existing line drawing manually, this is a slow tedious process.this is a slow tedious process. unsuitable for mare than a few simple unsuitable for mare than a few simple

drawings.drawings. it does not work at all for half-tone imagesit does not work at all for half-tone images

Image scanners provide an efficient solutionImage scanners provide an efficient solution


There are may types:There are may types: A Television Camera with a digital frame A Television Camera with a digital frame

grabbergrabber moderate resolution (1000x1000)moderate resolution (1000x1000)

Slow Scan CCD television camerasSlow Scan CCD television cameras 2000x2000 image in about 30 seconds2000x2000 image in about 30 seconds

Scan headScan head grid of light sensing cellsgrid of light sensing cells mounted on the print head of a printer mounted on the print head of a printer

• resolution of 80 units per inchresolution of 80 units per inch


Photo ScannerPhoto Scanner highest resolution ones use laser light sources and highest resolution ones use laser light sources and

have resolutions greater than 2000 units per inch.have resolutions greater than 2000 units per inch.

chapter 4 -- graphics hardware1 graphics hardware chapter 4

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