Download ch-04 - It works

Graphics Hardware
Chapter 4
Chapter 4 -- Graphics Hardware
1

In this chapter, we describe how the important
hardware elements of a computer graphics
display system work.
 Hardware
 printers,
 Display
technologies:
pen plotters, laser printers, ink-jet plotters,
technologies
 monochrome
 Raster
Display Systems
 Overview,
 Input
and shadow-mask CRT’s, LCD’s, …
…
Devices
 film
scanners,…
Chapter 4 -- Graphics Hardware
2
 Here
is the typical relation of the devices to one
another:
 The
key elements are the CPU and the display
processor.
Chapter 4 -- Graphics Hardware
3
1. Hardcopy Technologies

In this section, we discuss various hardcopy
technologies, then summarize their
characteristics.

However, several important terms must be
defined first.
Chapter 4 -- Graphics Hardware
4
 Dot
size (also called spot size) is the diameter
of a single dot created by a device.
 Addressability
is the number of individual dots
per inch that can be created
 This
may differ in the horizontal and vertical
directions.
 Addressability in x is just the reciprocal of the
distance between the centers of dots at addresses
(x,y) and (x+1,y)
 Addressability in y is defined similarly.
Chapter 4 -- Graphics Hardware
5
 Interdot
distance is the reciprocal of
addressability.
 It
is usually desirable that dot size be somewhat
greater than the interdot distance, so that smooth
shapes can be created
 There
are tradeoffs:
• Dot size several times the interdot distance allows very
smooth shapes to be printed.
• whereas a smaller dot size allows finer detail.
Chapter 4 -- Graphics Hardware
6
 Resolution,
which is related to the dot size and
can be no greater than addressability, is the
number of distinguishable lines per inch that a
device can create.
 If
40 black lines interleaved with 40 white lines can
be distinguished across one inch, the resolution is 80
lines per inch
 Resolution
also depends on the cross-sectional
intensity distribution of a spot. A spot with sharply
delineated edges yields higher resolution than does
one with edges that trail off.
Chapter 4 -- Graphics Hardware
7
 Color
Output:
 Many
devices to be discussed can create only a few
colors at any one point. Additional colors can be
obtained with dither patterns (discussed in Chapter
11). This results in a decreased spatial resolution for
the resulting image.
 With Color Ribbons a Dot-Matrix printer can
produce color images in two ways:
• Multiple print heads (each with a different color)
• Single print head with a multi-colored ribbon
• More colors than are actually on the ribbon can be created
by overstriking two different colors at the same dot on the
paper.
Chapter 4 -- Graphics Hardware
8

Hard Copy Devices:
 Dot
Matrix Printers
 use
a print head from 7 to 24 pins (thin, stiff pieces
of wire) each of which can be individually fired to
strike a ribbon against the paper.
 The
print head moves across the paper one step at a
time, the paper is advanced one line, and the print
head makes another pass across the paper.
 Hence,
these printers are raster output devices,
requiring scan conversion of vector images prior to
printing.
Chapter 4 -- Graphics Hardware
9
 Pen
Plotters:
 moves
the pen over the paper in a random, vectordrawing style.
 In
•
•
•
•
drawing a line,
the pen is positioned at the start of the line,
lowered to the paper,
moved in a straight path to the end of the line,
raised, and moved to the start of the next line.
 There
are two basic types:
• flatbed plotters and
• drum plotters.
Chapter 4 -- Graphics Hardware
10
 flatbed
plotter
• Pen is moved in x and y on a sheet of paper spread out on
a table.
– The paper is held by static charge, by vacuum, or
stretched tightly.
• A carriage moves in one axis over the paper.
• The carriage holds a pen mount which can move the
length of the carriage.
• The pen is raised and lowered and the pen mount and
carriage move to draw the line.
• flatbed plotters are available in sizes from 12 by 18 inches
to 6 by 10 feet and larger.
Chapter 4 -- Graphics Hardware
11
 drum
plotter
• The paper is moved along one axis (as opposed to the
carriage moving)
• The pen-mount moves across the drum
• Pins on the drum engage pre-punched holes in the paper to
prevent slipping.
 desktop
plotter
• is a special type of drum plotter,
• but the paper is moved between pinch rollers instead of
using pins and holes.
Chapter 4 -- Graphics Hardware
12
 Laser
Printers
A
laser beam is scanned across a positively charged
rotating drum (coated with selenium)
 The beam changes the charge, and the area left
positive will become black.
 A negatively charged powder adheres to the
positively charged areas of the drum, and is then
transferred to the paper.
Chapter 4 -- Graphics Hardware
13
 Laser
Printers (cont.)
 In
color xerography, this process is repeated three
times
• once for each primary color
 Laser
printers have a microprocessor to do scan
conversion and to control the printer.
 An increasing number also accept the PostScript
document and image description language.
 Most laser printers work with 8.5x11 inch paper or
8.5x14 inch paper.
Chapter 4 -- Graphics Hardware
14
 Ink-Jet
printers
 Spray
cyan, magenta, yellow, and sometimes black
ink onto paper.
 In most cases they are mounted on a head in a
printer-like mechanism.
 The print head
• moves across the page to draw one scan line, and
• returns while the paper advances by one inter-scan-line
spacing, and
• draws the next line.
 All
the colors are deposited simultaneously
• unlike the multi-pass lasers, plotters, and dot matrix
printers
 Very
few have variable dot-size capabilities.
Chapter 4 -- Graphics Hardware
15
 Thermal-Transfer
printers
 another
raster hardcopy device.
 Use finely spaced (200 per inch) heating nibs to
transfer pigments from colored wax paper to plain
paper.
 The wax paper (made up of strips of cyan, magenta,
yellow and black) and the plain paper are drawn
together over the strip of heating nibs
• The nibs are selectively heated to cause the pigment to
transfer.
• Because the nibs heat and cool very rapidly, a single color
image can be created in less than one minute.
 Some
of these printers accept video signals and
digital bitmap input, making them convenient for
creating hardcopy of video images.
Chapter 4 -- Graphics Hardware
16
 Thermal
sublimation dye transfer printers
 work
similarly to the thermal transfer printers.
 Except this process permits 256 intensities of each
color to be transferred
• cyan, magenta,and yellow
 This
creates a high-quality full-color image with a
spatial resolution of 200 dots per inch.
 The process is slower than wax transfer, but the
quality is near-photographic
• therefore it is the clear choice for full-color pre-press
proofs.
Chapter 4 -- Graphics Hardware
17
 Color
Film Recorders:
 There
are two basic techniques:
• The camera records the color image directly from the
color CRT.
– image resolution is limited
• A black and white CRT is photographed through color
filters, and the different color components of the image are
displayed in sequence
– This technique yields very high-quality images.
– Colors are mixed by double exposing parts of the
image through two or more filters (usually with
different intensities)
 The
input can be a raster video signal, a bitmap, or
vector-style instructions.
• Video signal must stay constant during the entire recording
cycle (which can take up to one minute with slow film)
Chapter 4 -- Graphics Hardware
18
2. Display Technologies


Interactive Computer Graphics demands
display devices whose images can be changed
quickly.
The CRT is by far the most common display
device, however, solid-state technologies are
being developed that may, in the long term,
substantially reduce the dominance of the CRT.
Chapter 4 -- Graphics Hardware
19

Monochrome CRT’s
 Essentially
the same as those used in black-andwhite home television sets.
Chapter 4 -- Graphics Hardware
20

Color Raster Displays:
 Shadow
Mask CRT.
Chapter 4 -- Graphics Hardware
21
 Liquid-Crystal
 It
Display (LCD)
is made up of six layers
• The crystal layers polarize the light (or not) and allow it to
pass through and reach the back reflective layer and come
back.
• The electric current disables the polarization of some of
the crystals, so it can pass through one layer, but be
blocked by the next.
Chapter 4 -- Graphics Hardware
22
3. Raster-Scan Display Systems

The basic concepts of raster graphics systems
were presented in Chapter1, and Chapter 2
provided further insight into the types of
operations possible with a raster display.

This section discusses the various elements of
a raster display.
Chapter 4 -- Graphics Hardware
23
4. The Video Controller

The most important task for the video
controller is the constant refresh of the display.

There are two fundamental types of refresh:
 interlaced
(used in TV’s)
 non-interlaced. (most video controllers)
Chapter 4 -- Graphics Hardware
24

Output:
 RGB
 separate
cables carry red, green, and blue signals to
the three electron guns of the shadow-mask CRT.
another cable carries the sync for vertical and
horizontal retraces.
 Monochrome
 NTSC
 National
Television System Committee -- North
American commercial television
 525 scan lines (only 480 are visible)
 Bandwidth limits the quality to an effective
resolution of about 350x350.
Chapter 4 -- Graphics Hardware
25
4.1 Video Mixing


Another useful video-controller function is
video mixing.
Two images
 one
defined in the frame buffer.
 another defined by video signal coming from
some other source (camera, …)
 merged into a composite image.
Chapter 4 -- Graphics Hardware
26

Examples of this merging are seen regularly on
television news, sports, and weather shows
Chapter 4 -- Graphics Hardware
27
5. Input Devices for Operator
Interaction

There are five basic logical devices:
 the
locator - indicates position or orientation.
 the pick - to select a displayed entity
 the valuator - to input a single real number.
 the keyboard - to input a character string
 the choice - to select from a set of possible
actions or choices.
Chapter 4 -- Graphics Hardware
28
5.1 Locator Devices





Data Tablet
Mouse
Trackball
Joystick
Touch Panel
Chapter 4 -- Graphics Hardware
29
5.2 Keyboard Devices

The alphanumeric keyboard is the prototypical
text input device.
 The
important functional characteristic of this
device is that it creates a code uniquely
corresponding to a pressed key.
 Cording (pressing several keys at once) is
generally not possible -- unless the additional
keys were shift, control, or other special keys)
Chapter 4 -- Graphics Hardware
30
5.3 Valuator Devices

Most valuator devices that provide scalar
values are based on potentiometers
 like
the volume and tone controls on a stereo
set.
 They are usually rotary (dials) with either a
limited range, or a free-turning (unbounded
range)
 linear ones are used infrequently in graphics
systems
Chapter 4 -- Graphics Hardware
31
5.4 Choice Devices

Function keys are the most common choice
device.

Other choice devices are the buttons on many
tablet pucks, or mice.

Choice devices are used to enter commands or
menu options in a graphics program
Chapter 4 -- Graphics Hardware
32
6. Image Scanners

Although data tablets can be used to digitize
existing line drawing manually,
 this
is a slow tedious process.
 unsuitable for mare than a few simple
drawings.
 it does not work at all for half-tone images

Image scanners provide an efficient solution
Chapter 4 -- Graphics Hardware
33

There are may types:
 A Television
Camera with a digital frame
grabber
 moderate
 Slow
resolution (1000x1000)
Scan CCD television cameras
 2000x2000
 Scan
image in about 30 seconds
head
 grid
of light sensing cells
 mounted on the print head of a printer
• resolution of 80 units per inch
Chapter 4 -- Graphics Hardware
34
 Photo
Scanner
 highest
resolution ones use laser light sources and
have resolutions greater than 2000 units per inch.
Chapter 4 -- Graphics Hardware
35