chapter 1
the human
the human
• Information i/o …
– visual, auditory, haptic, movement
• Information stored in memory
– sensory, short-term, long-term
• Information processed and applied
– reasoning, problem solving, skill, error
• Emotion influences human capabilities
• Each person is different
Vision
Two stages in vision
• physical reception of stimulus
• processing and interpretation of stimulus
The Eye - physical reception
• mechanism for receiving light and
transforming it into electrical energy
• light reflects from objects
• images are focused upside-down on retina
• retina contains rods for low light vision and
cones for colour vision
• ganglion cells (brain!) detect pattern and
movement
Interpreting the signal
• Size and depth
– visual angle indicates how much of view object
occupies
(relates to size and distance from eye)
– visual acuity is ability to perceive detail (limited)
– familiar objects perceived as constant size
(in spite of changes in visual angle when far away)
– cues like overlapping help perception of size and
depth
Interpreting the signal (cont)
• Brightness
–
–
–
–
subjective reaction to levels of light
affected by luminance of object
measured by just noticeable difference
visual acuity increases with luminance as does flicker
• Colour
–
–
–
–
made up of hue, intensity, saturation
cones sensitive to colour wavelengths
blue acuity is lowest
8% males and 1% females colour blind
Interpreting the signal (cont)
• The visual system compensates for:
– movement
– changes in luminance
• Context is used to resolve ambiguity
• Optical illusions sometimes occur due to over
compensation
Optical Illusions
the Ponzo illusion
the Muller Lyer illusion
Reading
• Several stages:
– visual pattern perceived
– decoded using internal representation of language
– interpreted using knowledge of syntax, semantics, pragmatics
•
•
•
•
Reading involves saccades and fixations
Perception occurs during fixations
Word shape is important to recognition
Negative contrast improves reading from computer screen
Hearing
• Provides information about environment:
distances, directions, objects etc.
• Physical apparatus:
– outer ear
– middle ear
– protects inner and amplifies sound
– transmits sound waves as
– inner ear
vibrations to inner ear
– chemical transmitters are released
and cause impulses in auditory nerve
• Sound
– pitch
– loudness
– timbre
– sound frequency
– amplitude
– type or quality
Hearing (cont)
• Humans can hear frequencies from 20Hz to 15kHz
– less accurate distinguishing high frequencies than low.
• Auditory system filters sounds
– can attend to sounds over background noise.
– for example, the cocktail party phenomenon
Touch
• Provides important feedback about environment.
• May be key sense for someone who is visually impaired.
• Stimulus received via receptors in the skin:
– thermoreceptors
– nociceptors
– mechanoreceptors
– heat and cold
– pain
– pressure
(some instant, some continuous)
• Some areas more sensitive than others e.g. fingers.
• Kinethesis - awareness of body position
– affects comfort and performance
Movement
• Time taken to respond to stimulus:
reaction time + movement time
• Movement time dependent on age, fitness etc.
• Reaction time - dependent on stimulus type:
– visual
– auditory
– pain
~ 200ms
~ 150 ms
~ 700ms
• Increasing reaction time decreases accuracy in the unskilled
operator but not in the skilled operator.
Movement (cont)
• Fitts' Law describes the time taken to hit a screen target:
Mt = a + b log2(D/S + 1)
where:
a and b are empirically determined constants
Mt is movement time
D is Distance
S is Size of target
 targets as large as possible
distances as small as possible
Memory
There are three types of memory function:
Sensory memories
Short-term memory or working memory
Long-term memory
Selection of stimuli governed by level of arousal.
sensory memory
• Buffers for stimuli received through senses
– iconic memory: visual stimuli
– echoic memory: aural stimuli
– haptic memory: tactile stimuli
• Examples
– “sparkler” trail
– stereo sound
• Continuously overwritten
Short-term memory (STM)
• Scratch-pad for temporary recall
– rapid access ~ 70ms
– rapid decay ~ 200ms
– limited capacity - 7± 2 chunks
Examples
212348278493202
0121 414 2626
HEC ATR ANU PTH ETR EET
Long-term memory (LTM)
• Repository for all our knowledge
– slow access ~ 1/10 second
– slow decay, if any
– huge or unlimited capacity
• Two types
– episodic
– semantic
– serial memory of events
– structured memory of facts,concepts, skills
semantic LTM derived from episodic LTM
Long-term memory (cont.)
• Semantic memory structure
– provides access to information
– represents relationships between bits of information
– supports inference
• Model: semantic network
– inheritance – child nodes inherit properties of parent nodes
– relationships between bits of information explicit
– supports inference through inheritance
LTM - semantic network
Models of LTM - Frames
• Information organized in data structures
• Slots in structure instantiated with values for instance of data
• Type–subtype relationships
DOG
Fixed
legs: 4
Default
diet: carniverous
sound: bark
Variable
size:
colour
COLLIE
Fixed
breed of: DOG
type: sheepdog
Default
size: 65 cm
Variable
colour
Models of LTM - Scripts
Model of stereotypical information required to interpret situation
Script has elements that can be instantiated with values for context
Script for a visit to the vet
Entry conditions: dog ill
vet open
owner has money
Result:
dog better
owner poorer
vet richer
Props:
examination table
medicine
instruments
Roles:
vet examines
diagnoses
treats
owner brings dog in
pays
takes dog out
Scenes:
arriving at reception
waiting in room
examination
paying
Tracks:
dog needs medicine
dog needs operation
Models of LTM - Production rules
Representation of procedural knowledge.
Condition/action rules
if condition is matched
then use rule to determine action.
IF dog is wagging tail
THEN pat dog
IF dog is growling
THEN run away
LTM - Storage of information
• rehearsal
– information moves from STM to LTM
• total time hypothesis
– amount retained proportional to rehearsal time
• distribution of practice effect
– optimized by spreading learning over time
• structure, meaning and familiarity
– information easier to remember
LTM - Forgetting
decay
– information is lost gradually but very slowly
interference
– new information replaces old: retroactive interference
– old may interfere with new: proactive inhibition
so may not forget at all memory is selective …
… affected by emotion – can subconsciously `choose' to forget
LTM - retrieval
recall
– information reproduced from memory can be assisted by cues, e.g.
categories, imagery
recognition
– information gives knowledge that it has been seen before
– less complex than recall - information is cue
Thinking
Reasoning
deduction, induction, abduction
Problem solving
Deductive Reasoning
• Deduction:
–
derive logically necessary conclusion from given premises.
e.g. If it is Friday then she will go to work
It is Friday
Therefore she will go to work.
• Logical conclusion not necessarily true:
e.g.
If it is raining then the ground is dry
It is raining
Therefore the ground is dry
Deduction (cont.)
• When truth and logical validity clash …
e.g. Some people are babies
Some babies cry
Inference - Some people cry
Correct?
• People bring world knowledge to bear
Inductive Reasoning
• Induction:
– generalize from cases seen to cases unseen
e.g.
all elephants we have seen have trunks
therefore all elephants have trunks.
• Unreliable:
– can only prove false not true
… but useful!
• Humans not good at using negative evidence
e.g. Wason's cards.
Wason's cards
7 E 4 K
If a card has a vowel on one side it has an even number on the other
Is this true?
How many cards do you need to turn over to find out?
…. and which cards?
Abductive reasoning
• reasoning from event to cause
e.g.
Sam drives fast when drunk.
If I see Sam driving fast, assume drunk.
• Unreliable:
– can lead to false explanations
Problem solving
• Process of finding solution to unfamiliar task using knowledge.
• Several theories.
• Gestalt
–
–
–
–
problem solving both productive and reproductive
productive draws on insight and restructuring of problem
attractive but not enough evidence to explain `insight' etc.
move away from behaviourism and led towards information processing
theories
Problem solving (cont.)
Problem space theory
– problem space comprises problem states
– problem solving involves generating states using legal operators
– heuristics may be employed to select operators
e.g. means-ends analysis
– operates within human information processing system
e.g. STM limits etc.
– largely applied to problem solving in well-defined areas
e.g. puzzles rather than knowledge intensive areas
Problem solving (cont.)
• Analogy
– analogical mapping:
• novel problems in new domain?
• use knowledge of similar problem from similar domain
– analogical mapping difficult if domains are semantically different
• Skill acquisition
– skilled activity characterized by chunking
• lot of information is chunked to optimize STM
– conceptual rather than superficial grouping of problems
– information is structured more effectively
Errors and mental models
Types of error
• slips
–
–
–
right intention, but failed to do it right
causes: poor physical skill,inattention etc.
change to aspect of skilled behaviour can cause slip
• mistakes
–
–
wrong intention
cause: incorrect understanding
humans create mental models to explain behaviour.
if wrong (different from actual system) errors can occur
Emotion
• Various theories of how emotion works
– James-Lange: emotion is our interpretation of a physiological
response to a stimuli
– Cannon: emotion is a psychological response to a stimuli
– Schacter-Singer: emotion is the result of our evaluation of our
physiological responses, in the light of the whole situation we are
in
• Emotion clearly involves both cognitive and physical
responses to stimuli
Emotion (cont.)
The biological response to physical stimuli is called affect
Affect influences how we respond to situations
 positive  creative problem solving
 negative  narrow thinking
“Negative affect can make it harder to do even
easy tasks; positive affect can make it easier to
do difficult tasks”
(Donald Norman)
Emotion (cont.)
• Implications for interface design
– stress will increase the difficulty of problem
solving
– relaxed users will be more forgiving of
shortcomings in design
– aesthetically pleasing and rewarding interfaces
will increase positive affect
Individual differences
• long term
– sex, physical and intellectual abilities
• short term
– effect of stress or fatigue
• changing
– age
Ask yourself:
will design decision exclude section of user population?
Psychology and the Design of
Interactive System
• Some direct applications
– e.g.
blue acuity is poor
 blue should not be used for important detail
• However, correct application generally requires understanding of context
in psychology, and an understanding of particular experimental conditions
• A lot of knowledge has been distilled in
– guidelines (chap 7)
– cognitive models (chap 12)
– experimental and analytic evaluation techniques (chap 9)
chapter 2
the computer
The Computer
a computer system is made up of various elements
each of these elements affects the interaction
– input devices – text entry and pointing
– output devices – screen (small&large), digital paper
– virtual reality – special interaction and display devices
– physical interaction – e.g. sound, haptic, bio-sensing
– paper – as output (print) and input (scan)
– memory – RAM & permanent media, capacity & access
– processing – speed of processing, networks
Interacting with computers
to understand human–computer interaction
… need to understand computers!
what goes in and out
devices, paper,
sensors, etc.
what can it do?
memory, processing,
networks
A ‘typical’ computer system
?
•
•
•
screen, or monitor, on which there are windows
keyboard
mouse/trackpad
window 1
•
variations
– desktop
– laptop
– PDA
window 2
12-37pm
the devices dictate the styles of interaction that the system supports
If we use different devices, then the interface will support a different style of
interaction
How many …
• computers in your house?
– hands up, …
… none, 1, 2 , 3, more!!
• computers in your pockets?
are you thinking …
… PC, laptop, PDA ??
How many computers …
in your house?
– PC
– TV, VCR, DVD, HiFi,
cable/satellite TV
– microwave, cooker,
washing machine
– central heating
– security system
can you think of more?
in your pockets?
– PDA
– phone, camera
– smart card, card with
magnetic strip?
– electronic car key
– USB memory
try your pockets and
bags
Interactivity?
Long ago in a galaxy far away … batch processing
– punched card stacks or large data files prepared
– long wait ….
– line printer output
… and if it is not right …
Now most computing is interactive
– rapid feedback
– the user in control (most of the time)
– doing rather than thinking …
Is faster always better?
Richer interaction
sensors
and devices
everywhere
text entry devices
keyboards (QWERTY et al.)
chord keyboards, phone pads
handwriting, speech
Keyboards
• Most common text input device
• Allows rapid entry of text by experienced users
• Keypress closes connection, causing a character code to be
sent
• Usually connected by cable, but can be wireless
layout – QWERTY
• Standardised layout
but …
– non-alphanumeric keys are placed differently
– accented symbols needed for different scripts
– minor differences between UK and USA keyboards
• QWERTY arrangement not optimal for typing
– layout to prevent typewriters jamming!
• Alternative designs allow faster typing but large social base of QWERTY
typists produces reluctance to change.
QWERTY (ctd)
2
1
Q
4
3
W
E
S
A
Z
5
R
D
X
6
C
T
F
Y
G
V
8
7
U
H
B
SPACE
N
9
I
J
M
0
O
K
P
L
,
.
alternative keyboard layouts
Alphabetic
– keys arranged in alphabetic order
– not faster for trained typists
– not faster for beginners either!
Dvorak
–
–
–
–
–
common letters under dominant fingers
biased towards right hand
common combinations of letters alternate between hands
10-15% improvement in speed and reduction in fatigue
But - large social base of QWERTY typists produce market pressures not to
change
special keyboards
• designs to reduce fatigue for RSI
• for one handed use
e.g. the Maltron left-handed keyboard
Chord keyboards
only a few keys - four or 5
letters typed as combination of keypresses
compact size
– ideal for portable applications
short learning time
– keypresses reflect letter shape
fast
– once you have trained
BUT - social resistance, plus fatigue after extended use
NEW – niche market for some wearables
phone pad and T9 entry
• use numeric keys with
multiple presses
2–abc
3-def
4-ghi
5-jkl
6-mno
7-pqrs
8-tuv
9-wxyz
hello = 4433555[pause]555666
surprisingly fast!
• T9 predictive entry
–
–
–
–
type as if single key for each letter
use dictionary to ‘guess’ the right word
hello = 43556 …
but 26 -> menu ‘am’ or ‘an’
Handwriting recognition
• Text can be input into the computer, using a pen and a
digesting tablet
– natural interaction
• Technical problems:
– capturing all useful information - stroke path, pressure, etc. in a
natural manner
– segmenting joined up writing into individual letters
– interpreting individual letters
– coping with different styles of handwriting
• Used in PDAs, and tablet computers …
… leave the keyboard on the desk!
Speech recognition
• Improving rapidly
• Most successful when:
– single user – initial training and learns peculiarities
– limited vocabulary systems
• Problems with
–
–
–
–
external noise interfering
imprecision of pronunciation
large vocabularies
different speakers
Numeric keypads
• for entering numbers quickly:
– calculator, PC keyboard
• for telephones
not the same!!
ATM like phone
1
2
3
7
8
9
4
5
6
4
5
6
7
8
9
1
2
3
*
0
#
0
.
=
telephone
calculator
positioning, pointing and drawing
mouse, touchpad
trackballs, joysticks etc.
touch screens, tablets
eyegaze, cursors
the Mouse
• Handheld pointing device
– very common
– easy to use
• Two characteristics
– planar movement
– buttons
(usually from 1 to 3 buttons on top, used for making a
selection, indicating an option, or to initiate drawing etc.)
the mouse (ctd)
Mouse located on desktop
– requires physical space
– no arm fatigue
Relative movement only is detectable.
Movement of mouse moves screen cursor
Screen cursor oriented in (x, y) plane,
mouse movement in (x, z) plane …
… an indirect manipulation device.
– device itself doesn’t obscure screen, is accurate and fast.
– hand-eye coordination problems for novice users
How does it work?
Two methods for detecting motion
• Mechanical
– Ball on underside of mouse turns as mouse is moved
– Rotates orthogonal potentiometers
– Can be used on almost any flat surface
• Optical
–
–
–
–
light emitting diode on underside of mouse
may use special grid-like pad or just on desk
less susceptible to dust and dirt
detects fluctuating alterations in reflected light intensity to calculate relative
motion in (x, z) plane
Even by foot …
• some experiments with the footmouse
– controlling mouse movement with feet …
– not very common :-)
• but foot controls are common elsewhere:
– car pedals
– sewing machine speed control
– organ and piano pedals
Touchpad
• small touch sensitive tablets
• ‘stroke’ to move mouse pointer
• used mainly in laptop computers
• good ‘acceleration’ settings important
– fast stroke
• lots of pixels per inch moved
• initial movement to the target
– slow stroke
• less pixels per inch
• for accurate positioning
Trackball and thumbwheels
Trackball
– ball is rotated inside static housing
• like an upside down mouse!
–
–
–
–
–
relative motion moves cursor
indirect device, fairly accurate
separate buttons for picking
very fast for gaming
used in some portable and notebook computers.
Thumbwheels …
– for accurate CAD – two dials for X-Y cursor position
– for fast scrolling – single dial on mouse
Joystick and keyboard nipple
Joystick
– indirect
pressure of stick = velocity of movement
– buttons for selection
on top or on front like a trigger
– often used for computer games
aircraft controls and 3D navigation
Keyboard nipple
– for laptop computers
– miniature joystick in the middle of the keyboard
Touch-sensitive screen
•
Detect the presence of finger or stylus on the screen.
– works by interrupting matrix of light beams, capacitance changes or ultrasonic
reflections
– direct pointing device
•
Advantages:
– fast, and requires no specialised pointer
– good for menu selection
– suitable for use in hostile environment: clean and safe from damage.
•
Disadvantages:
– finger can mark screen
– imprecise (finger is a fairly blunt instrument!)
• difficult to select small regions or perform accurate drawing
– lifting arm can be tiring
Stylus and light pen
Stylus
– small pen-like pointer to draw directly on screen
– may use touch sensitive surface or magnetic detection
– used in PDA, tablets PCs and drawing tables
Light Pen
– now rarely used
– uses light from screen to detect location
BOTH …
– very direct and obvious to use
– but can obscure screen
Digitizing tablet
• Mouse like-device with cross hairs
• used on special surface
- rather like stylus
• very accurate
- used for digitizing maps
Eyegaze
• control interface by eye gaze direction
– e.g. look at a menu item to select it
• uses laser beam reflected off retina
– … a very low power laser!
•
•
•
•
mainly used for evaluation (ch x)
potential for hands-free control
high accuracy requires headset
cheaper and lower accuracy devices available
sit under the screen like a small webcam
Cursor keys
•
•
•
•
Four keys (up, down, left, right) on keyboard.
Very, very cheap, but slow.
Useful for not much more than basic motion for text-editing tasks.
No standardised layout, but inverted “T”, most common
Discrete positioning controls
• in phones, TV controls etc.
– cursor pads or mini-joysticks
– discrete left-right, up-down
– mainly for menu selection
display devices
bitmap screens (CRT & LCD)
large & situated displays
digital paper
bitmap displays
• screen is vast number of coloured dots
resolution and colour depth
• Resolution … used (inconsistently) for
– number of pixels on screen (width x height)
• e.g. SVGA 1024 x 768, PDA perhaps 240x400
– density of pixels (in pixels or dots per inch - dpi)
• typically between 72 and 96 dpi
• Aspect ratio
– ration between width and height
– 4:3 for most screens, 16:9 for wide-screen TV
• Colour depth:
–
–
–
–
how many different colours for each pixel?
black/white or greys only
256 from a pallete
8 bits each for red/green/blue = millions of colours
anti-aliasing
Jaggies

diagonal lines that have discontinuities in due to horizontal raster scan
process.
Anti-aliasing


softens edges by using shades of line colour
also used for text
Cathode ray tube
• Stream of electrons emitted from electron gun, focused and directed by
magnetic fields, hit phosphor-coated screen which glows
• used in TVs and computer monitors
electron beam
electron gun
focussing and
deflection
phosphorcoated screen
Health hazards of CRT !
•
•
•
•
X-rays: largely absorbed by screen (but not at rear!)
UV- and IR-radiation from phosphors: insignificant levels
Radio frequency emissions, plus ultrasound (~16kHz)
Electrostatic field - leaks out through tube to user. Intensity dependant on
distance and humidity. Can cause rashes.
• Electromagnetic fields (50Hz-0.5MHz). Create induction currents in
conductive materials, including the human body. Two types of effects
attributed to this: visual system - high incidence of cataracts in VDU
operators, and concern over reproductive disorders (miscarriages and
birth defects).
Health hints …
•
•
•
•
•
do not sit too close to the screen
do not use very small fonts
do not look at the screen for long periods without a break
do not place the screen directly in front of a bright window
work in well-lit surroundings
 Take extra care if pregnant.
but also posture, ergonomics, stress
Liquid crystal displays
•
Smaller, lighter, and … no radiation problems.
•
Found on PDAs, portables and notebooks,
… and increasingly on desktop and even for home TV
•
also used in dedicted displays:
digital watches, mobile phones, HiFi controls
•
How it works …
–
–
–
–
Top plate transparent and polarised, bottom plate reflecting.
Light passes through top plate and crystal, and reflects back to eye.
Voltage applied to crystal changes polarisation and hence colour
N.B. light reflected not emitted => less eye strain
special displays
Random Scan (Directed-beam refresh, vector display)
–
–
–
–
draw the lines to be displayed directly
no jaggies
lines need to be constantly redrawn
rarely used except in special instruments
Direct view storage tube (DVST)
– Similar to random scan but persistent => no flicker
– Can be incrementally updated but not selectively erased
– Used in analogue storage oscilloscopes
large displays
• used for meetings, lectures, etc.
• technology
plasma
video walls
projected
– usually wide screen
– lots of small screens together
– RGB lights or LCD projector
– hand/body obscures screen
– may be solved by 2 projectors + clever software
back-projected
– frosted glass + projector behind
situated displays
• displays in ‘public’ places
– large or small
– very public or for small group
• display only
– for information relevant to location
• or interactive
– use stylus, touch sensitive screem
• in all cases … the location matters
– meaning of information or interaction is related to the location
Hermes a situated display
• small displays beside office doors
• handwritten notes left using stylus
•small
office
owner reads notes using web interface
displays
beside
office doors
handwritten
notes left
using stylus
office owner
reads notes
using web interface
Digital paper
• what?
– thin flexible sheets
– updated electronically
– but retain display
• how?
– small spheres turned
– or channels with coloured liquid
and contrasting spheres
– rapidly developing area
appearance
cross
section
virtual reality and 3D interaction
positioning in 3D space
moving and grasping
seeing 3D (helmets and caves)
positioning in 3D space
• cockpit and virtual controls
– steering wheels, knobs and dials … just like real!
• the 3D mouse
– six-degrees of movement: x, y, z + roll, pitch, yaw
• data glove
– fibre optics used to detect finger position
• VR helmets
– detect head motion and possibly eye gaze
• whole body tracking
– accelerometers strapped to limbs or reflective dots and video
processing
pitch, yaw and roll
yaw
pitch
roll
3D displays
• desktop VR
– ordinary screen, mouse or keyboard control
– perspective and motion give 3D effect
• seeing in 3D
– use stereoscopic vision
– VR helmets
– screen plus shuttered specs, etc.
VR headsets
• small TV screen for each eye
• slightly different angles
• 3D effect
VR motion sickness
• time delay
– move head … lag … display moves
– conflict: head movement vs. eyes
• depth perception
– headset gives different stereo distance
– but all focused in same plane
– conflict: eye angle vs. focus
• conflicting cues => sickness
– helps motivate improvements in technology
simulators and VR caves
•
•
•
•
•
scenes projected on walls
realistic environment
hydraulic rams!
real controls
other people
physical controls, sensors etc.
special displays and gauges
sound, touch, feel, smell
physical controls
environmental and bio-sensing
dedicated displays
• analogue representations:
– dials, gauges, lights, etc.
• digital displays:
– small LCD screens, LED lights, etc.
• head-up displays
– found in aircraft cockpits
– show most important controls
… depending on context
Sounds
• beeps, bongs, clonks, whistles and whirrs
• used for error indications
• confirmation of actions e.g. keyclick
Touch, feel, smell
• touch and feeling important
– in games … vibration, force feedback
– in simulation … feel of surgical instruments
– called haptic devices
• texture, smell, taste
– current technology very limited
BMW iDrive
•
•
•
•
for controlling menus
feel small ‘bumps’ for each item
makes it easier to select options by feel
uses haptic technology from Immersion Corp.
physical controls
• specialist controls needed …
– industrial controls, consumer products, etc.
easy-clean
smooth buttons
multi-function
control
large buttons
clear dials
tiny buttons
Environment and bio-sensing
• sensors all around us
– car courtesy light – small switch on door
– ultrasound detectors – security, washbasins
– RFID security tags in shops
– temperature, weight, location
• … and even our own bodies …
– iris scanners, body temperature, heart rate,
galvanic skin response, blink rate
paper: printing and scanning
print technology
fonts, page description,
WYSIWYG
scanning, OCR
Printing
• image made from small dots
– allows any character set or graphic to be printed,
• critical features:
– resolution
• size and spacing of the dots
• measured in dots per inch (dpi)
– speed
• usually measured in pages per minute
– cost!!
Types of dot-based printers
• dot-matrix printers
– use inked ribbon (like a typewriter
– line of pins that can strike the ribbon, dotting the paper.
– typical resolution 80-120 dpi
• ink-jet and bubble-jet printers
– tiny blobs of ink sent from print head to paper
– typically 300 dpi or better .
• laser printer
– like photocopier: dots of electrostatic charge deposited on drum, which picks
up toner (black powder form of ink) rolled onto paper which is then fixed with
heat
– typically 600 dpi or better.
Printing in the workplace
• shop tills
– dot matrix
– same print head used for several paper rolls
– may also print cheques
• thermal printers
–
–
–
–
special heat-sensitive paper
paper heated by pins makes a dot
poor quality, but simple & low maintenance
used in some fax machines
Fonts
• Font – the particular style of text
Courier font
Helvetica font
Palatino font
Times Roman font
• §´  (special symbol)
• Size of a font measured in points (1 pt about 1/72”)
(vaguely) related to its height
This is ten point Helvetica
This is twelve point
This is fourteen point
This is eighteen point
and this is twenty-four point
Fonts (ctd)
Pitch
– fixed-pitch – every character has the same width
e.g. Courier
– variable-pitched – some characters wider
e.g. Times Roman – compare the ‘i’ and the “m”
Serif or Sans-serif
– sans-serif – square-ended strokes
e.g. Helvetica
– serif – with splayed ends (such as)
e.g. Times Roman or Palatino
Readability of text
• lowercase
– easy to read shape of words
• UPPERCASE
– better for individual letters and non-words
e.g. flight numbers: BA793 vs. ba793
• serif fonts
– helps your eye on long lines of printed text
– but sans serif often better on screen
Page Description Languages
• Pages very complex
– different fonts, bitmaps, lines, digitised photos, etc.
• Can convert it all into a bitmap and send to the printer
… but often huge !
• Alternatively Use a page description language
– sends a description of the page can be sent,
– instructions for curves, lines, text in different styles, etc.
– like a programming language for printing!
• PostScript is the most common
Screen and page
• WYSIWYG
– what you see is what you get
– aim of word processing, etc.
• but …
– screen: 72 dpi, landscape image
– print: 600+ dpi, portrait
• can try to make them similar
but never quite the same
• so … need different designs, graphics etc, for screen and print
Scanners
• Take paper and convert it into a bitmap
• Two sorts of scanner
– flat-bed: paper placed on a glass plate, whole page converted into bitmap
– hand-held: scanner passed over paper, digitising strip typically 3-4” wide
• Shines light at paper and note intensity of reflection
– colour or greyscale
• Typical resolutions from 600–2400 dpi
Scanners (ctd)
Used in
– desktop publishing for incorporating
photographs and other images
– document storage and retrieval systems, doing
away with paper storage
+ special scanners for slides and photographic
negatives
Optical character recognition
• OCR converts bitmap back into text
• different fonts
– create problems for simple “template matching”
algorithms
– more complex systems segment text, decompose
it into lines and arcs, and decipher characters that
way
• page format
– columns, pictures, headers and footers
Paper-based interaction
• paper usually regarded as output only
• can be input too – OCR, scanning, etc.
• Xerox PaperWorks
– glyphs – small patterns of /\\//\\\
• used to identify forms etc.
• used with scanner and fax to control applications
• more recently
– papers micro printed - like wattermarks
• identify which sheet and where you are
– special ‘pen’ can read locations
• know where they are writing
memory
short term and long term
speed, capacity, compression
formats, access
Short-term Memory - RAM
• Random access memory (RAM)
–
–
–
–
on silicon chips
100 nano-second access time
usually volatile (lose information if power turned off)
data transferred at around 100 Mbytes/sec
• Some non-volatile RAM used to store basic set-up information
• Typical desktop computers:
64 to 256 Mbytes RAM
Long-term Memory - disks
• magnetic disks
– floppy disks store around 1.4 Mbytes
– hard disks typically 40 Gbytes to 100s of Gbytes
access time ~10ms, transfer rate 100kbytes/s
• optical disks
– use lasers to read and sometimes write
– more robust that magnetic media
– CD-ROM
- same technology as home audio, ~ 600 Gbytes
– DVD - for AV applications, or very large files
Blurring boundaries
• PDAs
– often use RAM for their main memory
• Flash-Memory
– used in PDAs, cameras etc.
– silicon based but persistent
– plug-in USB devices for data transfer
speed and capacity
• what do the numbers mean?
• some sizes (all uncompressed) …
– this book, text only ~ 320,000 words, 2Mb
– the Bible ~ 4.5 Mbytes
– scanned page ~ 128 Mbytes
• (11x8 inches, 1200 dpi, 8bit greyscale)
– digital photo ~ 10 Mbytes
• (2–4 mega pixels, 24 bit colour)
– video ~ 10 Mbytes per second
• (512x512, 12 bit colour, 25 frames per sec)
virtual memory
• Problem:
– running lots of programs + each program large
– not enough RAM
• Solution - Virtual memory :
– store some programs temporarily on disk
– makes RAM appear bigger
• But … swopping
– program on disk needs to run again
– copied from disk to RAM
– slows t h i n g s
d o w n
Compression
• reduce amount of storage required
• lossless
– recover exact text or image – e.g. GIF, ZIP
– look for commonalities:
• text: AAAAAAAAAABBBBBCCCCCCCC
10A5B8C
• video: compare successive frames and store change
• lossy
– recover something like original – e.g. JPEG, MP3
– exploit perception
• JPEG: lose rapid changes and some colour
• MP3: reduce accuracy of drowned out notes
Storage formats - text
• ASCII - 7-bit binary code for to each letter and character
• UTF-8 - 8-bit encoding of 16 bit character set
• RTF (rich text format)
- text plus formatting and layout information
• SGML (standardized generalised markup language)
- documents regarded as structured objects
• XML (extended markup language)
- simpler version of SGML for web applications
Storage formats - media
• Images:
– many storage formats :
(PostScript, GIFF, JPEG, TIFF, PICT, etc.)
– plus different compression techniques
(to reduce their storage requirements)
• Audio/Video
– again lots of formats :
(QuickTime, MPEG, WAV, etc.)
– compression even more important
– also ‘streaming’ formats for network delivery
methods of access
• large information store
– long time to search => use index
– what you index -> what you can access
• simple index needs exact match
• forgiving systems:
– Xerox “do what I mean” (DWIM)
– SOUNDEX – McCloud ~ MacCleod
• access without structure …
– free text indexing (all the words in a document)
– needs lots of space!!
processing and networks
finite speed (but also Moore’s law)
limits of interaction
networked computing
Finite processing speed
• Designers tend to assume fast processors, and make interfaces more and
more complicated
• But problems occur, because processing cannot keep up with all the tasks
it needs to do
– cursor overshooting because system has buffered keypresses
– icon wars - user clicks on icon, nothing happens, clicks on another, then
system responds and windows fly everywhere
• Also problems if system is too fast - e.g. help screens may scroll through
text much too rapidly to be read
Moore’s law
• computers get faster and faster!
• 1965 …
– Gordon Moore, co-founder of Intel, noticed a pattern
– processor speed doubles every 18 months
– PC … 1987: 1.5 Mhz, 2002: 1.5 GHz
• similar pattern for memory
– but doubles every 12 months!!
– hard disk … 1991: 20Mbyte : 2002: 30 Gbyte
• baby born today
– record all sound and vision
– by 70 all life’s memories stored in a grain of dust!
the myth of the infinitely
fast machine
• implicit assumption … no delays
an infinitely fast machine
• what is good design for real machines?
• good example … the telephone :
–
–
–
–
type keys too fast
hear tones as numbers sent down the line
actually an accident of implementation
emulate in deisgn
Limitations on interactive
performance
Computation bound
– Computation takes ages, causing frustration for the user
Storage channel bound
– Bottleneck in transference of data from disk to memory
Graphics bound
– Common bottleneck: updating displays requires a lot of effort - sometimes
helped by adding a graphics co-processor optimised to take on the burden
Network capacity
– Many computers networked - shared resources and files, access to printers
etc. - but interactive performance can be reduced by slow network speed
Networked computing
Networks allow access to …
– large memory and processing
– other people (groupware, email)
– shared resources – esp. the web
Issues
– network delays – slow feedback
– conflicts - many people update data
– unpredictability
The internet
• history …
– 1969: DARPANET US DoD, 4 sites
– 1971: 23; 1984: 1000; 1989: 10000
• common language (protocols):
– TCP – Transmission Control protocol
• lower level, packets (like letters) between machines
– IP – Internet Protocol
• reliable channel (like phone call) between programs on machines
– email, HTTP, all build on top of these