1. Lecture 3: Pointing Devices and Fitts’ Law
Brad Myers
05-440/05-640: Interaction Techniques
Spring, 2016
© Brad Myers

2. Topics Note: “pointer” or “pointing device” (not “mouse”)
Types of pointing devices
Various properties:
Direct vs. indirect
Cursor?
Relative vs. absolute
Rate vs. position controlled (esp. for joysticks)
How many states supported? (hover?, not-pressed?)
Single or multiple touch
Measuring effectiveness
Important measures:
Speed, accuracy, comfort, learnability
Fitts’ law
© Brad Myers

3. Some Pointing Devices (List from HW#1): Others Mouse Laptop touchpad
IBM Pointing Stick on Thinkpad laptops
Touchscreen with fingers (phones, tablets)
Touchscreen with Stylus
Wii controller pointer in the air, pointing at a web page on a "smart TV"
Microsoft Kinect using your hand to point at a web page on a "smart TV"
Large "Smart Board" direct touch wall-size display
Game controller connected to a PC to control the cursor
Contour's "RollerMouse Red plus"
Trackball
Others
Step keys
Light pen
Laser pointer at a screen
Joysticks
Game consoles
© Brad Myers

4. “Cursor”
Same word used for both pointer position and text input position
Are usually different
Text cursor
Will discuss with text entry lecture
Pointer’s cursor often changes shape to show next action
“Cursor” in this lecture
Often no pointer cursor if direct touch
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5. Light pen Starting ~ 1950s Sketchpad, 1963
Camera in pen looks for pixel on screen
Calculates position based on timing
Disadvantages:
Need to hold hand in the air
Low resolution
Photo credit:
© Brad Myers

6. Step Keys Usually to move text cursor
Sometimes can move pointer cursor as well
Microsoft Windows, can move window with ALT-Space, “m”, arrow-keys
Moves pointer cursor as well
© Brad Myers

7. Mouse
Bill English and Doug Engelbart credited with the invention of the mouse (1967)
Comfortable, fast and accurate
Engineered to not move when push a button
Cursor moves straight as hand moves in an arc
Various gain functions
Pixels per cm
© Brad Myers

8. Joysticks Often used in aviation Computer input for games
Most are “rate controlled”
self-centering springs
Movement controls rate of cursor movement
“isometric” – stick doesn’t seem to move
Position controlled
Absolute position of stick controls position
No spring – stick stays where leave it
© Brad Myers

9. IBM Pointing Stick See lecture from 2014 from Ted Selker “Trackpoint”
Rate-controlled isometric joystick
Significant iteration and experimentation
“10 years of human factors work”
Material of the stick matters
Placement of buttons
Transfer function
Training helps performance
© Brad Myers

10. Tablets
Rand Tablet: 1964:
Handwriting and graphics input
CAD/CAM
Move stylus or puck or finger on a surface – not a display screen
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11. Touchscreens Stylus versus finger How hold a tablet?
Tradeoffs:
Naturalness
Accuracy
Amount of content that can fit on the screen
Blocking the view of the content
How hold a tablet?
Palm & hand removal?
© Brad Myers

12. Large Touchscreens Projector or TV Vertical or tabletop
Smartboard
Original Microsoft “surface”
Single or multiple people
Classrooms, museums, small meetings, etc.
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13. Remote interaction with large screens
Examples
Laser pointing at screen
Brad A. Myers, et. al. "Interacting At a Distance: Measuring the Performance of Laser Pointers and Other Devices." CHI'2002. pp  pdf.
Wii controller
Microsoft Kinect – just fingers
Very inaccurate
Tiring
© Brad Myers

14. Issues Direct vs. Indirect Relative vs. absolute
Direct – point at a screen
Light-pen, stylus, finger
Often no visible cursor
More natural
Indirect – movement moves a cursor
Must be learned
Issue – direction of movement (up or down is arbitrary)
Transfer function – can be more accurate and faster
Must have a cursor – Shape can be used as feedback
Software can move position of the cursor
Relative vs. absolute
Relative – only movement used = mouse
Absolute – required for touchscreen
Touchpads can be either
Image credit:
© Brad Myers

15. Issues, 2 How many states supported? Single or multiple touch
Can have “hover” state?
Pointing but not “pressed” or “selected”
Common with mouse, not with touchscreen
Is possible with magnetic stylus on touchscreen
Location but not pressed
How many buttons?
Stylus buttons on side of pen
Other dimensions, like force of press – “3D touch”
Single or multiple touch
Resistive vs. capacitive vs. other sensing
For big displays – which person’s hand?
© Brad Myers

16. Testing How decide whether an input device is “better” than another?
Important measures: speed, accuracy, comfort, learnability
As usual in HCI, need to decide tasks and relative importance of measures
Often (but not always) will have tradeoffs
Focus on speed and accuracy
Since are numeric, can use standard statistical measures
JMP, R, SPSS, Excel, and others to help with calculations
Fitts law
Slides by Jeffrey Rzeszotarski
© Brad Myers

17. Fitts’ Law
1954, Paul Fitts
Figure out how quickly people could move a pointing device to a target and select it
Predictive model
e.g., keystroke-level analysis
To compare pointing devices
Throughput – combines both speed and accuracy

18. Cognitive Processes + Physical Processes=
Pointing Performance

19. Laser Pointer Example
Source: hcibook.com

20. Card, Moran, Newell studies
Card, English, Burr 1978 paper – distance
Time is linear for distance for step keys
Time increases with the log of the distance for continuous devices like the mouse
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21. Card, Moran, Newell studies, 2
Card, English, Burr 1978 paper – width
Positioning time for both the mouse and the joystick decreases with the log of the target size
© Brad Myers

22. T = a + b log2(D/W + 1) T = time to complete a movement
a = fixed cost to start/stop moving & click
b = inherent speed of device
log2(D/W + 1) = “index of difficulty” or ID bits
Higher with distance (D) and lower with width (W)
T = a + b log2(D/W + 1)

23. ID = log2(D/W + 1)
“double the distance, double the width”
equals

24. Card, English, Burr 1978

25. Homework 1 Analysis You can actually do this with your data!
The Fitts’ Tester spreadsheet (on Blackboard) roughly follows Card et al. experiment
Compute IDs from Distance and Width
Chart them and see if they are linear
Use Excel “Fit Trendline” to get coefficients for a and b and compare to other papers

26. Homework 1 Analysis
Keep in mind that the papers you read collected MUCH more data
E.g., 1200 to 1800 trials (four to six hours) before learning curve flattened out – [Card, 1978]
It’s okay to not have any measurable difference as long as you explain why that’s reasonable
Check out the error rates too
The laser pointer paper is good example of how to structure a report

27. Newer Fitts’ Law tests Use circles instead of two rectangles
ISO standard
Doesn’t fit as well on non-square screens
Horizontal and vertical movements may not be equal difficulty
Muscles used
Card, English, Burr 1978 paper showed differences for joystick, etc. but not mouse
Laser pointer study: up to 10x more wiggle vertically
Device properties
Contour's "RollerMouse Red plus"
© Brad Myers

28. Homework 1 Questions? Tom Moran
Bill Curtis, Stu Card, and Allen Newell
Source: SIGCHI Archives