Fitts' Law and Interactor Design

name: inverse
layout: true
class: center, middle, inverse
---
# Properties of People II: Motor Control

Lauren Bricker

CSE 340 Spring 23
---
layout: false

# Agenda

- Administrivia
- Menus video checkpoint due (Tues 2-May)
- Menus code part 1-4 due (Fri 5-May)
 - Code is due **Saturday**
 - Resubmissions
 - Layout will be accepted until 05/14
 - Accessibility will be accepted until 05/21
- Today
 
 - Quick check on [Ed](https://edstem.org/us/courses/38124/lessons/59423/slides/331522) about Menus codebase
 - Fitts' Law and implications for interface design

---
# Let's try an experiment
.left-column-half[![:img Image of an interface with a red dot and many grey dots on the left
and a dialog on right to control randomization and highlight distance
and width in a Fitts' law experiment., 80%, width](img/pm/fitts.png)
]
.right-column-half[
-  [Fitts' Law Experiment](http://simonwallner.at/ext/fitts/) (simonwallner.at/ext/fitts) Scroll
   down to just below 'A word of warning'
- Click the red circles as fast as possible
]
---

[//]: # (Outline Slide)
# Today's goals

- Experience Fitts' Law
- Discuss its implications for design
- Introduce Guiard's model of Bi-Manual Control
- Discuss its implications for design

---
.left-column-half[
# Throughput

![:img Picture of graph of fitts law data collected when I did the
experiment. Y axis is time and x is index of difficulty (ID). It's fairly
noisy because fitts law is designed to measure multiple
people. However it does fit a line., 80%, width](img/pm/timeoverid.png)

- Note: We will use D for Dist and W for Width throughout the rest of this lecture
]
.jax[$$MT = a + b*log_2({Dist \over Size} + 1)$$]

.right-column-half[
where
]

---
count: false
.left-column-half[
# Throughput

- Note: We will use D for Dist and W for Width throughout the rest of this lecture
]

.jax[$$MT = a + b*log_2({Dist \over Size} + 1)$$]

.right-column-half[
where
- *MT* is movement time (in seconds)
]

---
count: false

.left-column-half[
# Throughput

- Note: We will use D for Dist and W for Width throughout the rest of this lecture
]

.jax[$$MT = a + b*log_2({Dist \over Size} + 1)$$]

.right-column-half[

where
- *MT* is movement time (in seconds)
- ID is the *Index of Difficulty* of a movement (in bits)
.jax[$$log_2({Dist \over Size} + 1)$$]
]

---
count: false

.left-column-half[
# Throughput

- Note: We will use D for Dist and W for Width throughout the rest of this lecture
]

.jax[$$MT = a + b*log_2({Dist \over Size} + 1)$$]

.right-column-half[

where
- *MT* is movement time (in seconds)
- ID is the *Index of Difficulty* of a movement (in bits)
.jax[$$log_2({Dist \over Size} + 1)$$]
- *ID/MT* is the *Throughput* of a device (in bits/second)
]

---
count: false

.left-column-half[
# Throughput

- Note: We will use D for Dist and W for Width throughout the rest of this lecture
]
.jax[$$MT = a + b*log_2({Dist \over Size} + 1)$$]

.right-column-half[
where
- *MT* is movement time (in seconds)
- ID is the *Index of Difficulty* of a movement (in bits)
.jax[$$log_2({Dist \over Size} + 1)$$]
- *ID/MT* is the *Throughput* of a device (in bits/second)
- *a* and *b* are empirically derived constants

]

???
This is just a line

Fitts’ law tells us about difficulty for pointing and selection tasks
- Time to move the hand depends only on relative precision required
- MT increases as __distance__ from target increases
- MT decreases as __size__ of target increases
- Diagram this

---
# Original Fitts' experiment

.left-column-half[
![:img Mock up of the original task in Fitt's experiment, 80%, width](https://upload.wikimedia.org/wikipedia/commons/1/16/Fitts_Task_English.jpg)
.font-small[[Wikpedia](https://en.wikipedia.org/wiki/Fitts%27s_law)]
]
.right-column-half[
Original experiment from Paul Morris Fitts involved tapping on plates as quickly as possible.
]

---

# Compare Device Performance & Human Performance

.left-column-half[
![:img Graph of Fitts law lines for various body parts (Neck Arm Wrist
Finger) with time on the Y axis and bits on the X axis, 100%, width](img/pm/body-fitts.png)

]

.right-column-half[
.font-small[
Device/Part | Study | Throughput (bits/s) | Relative to Optimal
-------|-------|------------ | -----------
Finger | Langolf (1976) | 38
Wrist | Langolf (1976) | 23
EyeTracker| Ware & Mikaelian (1987) | 13.7 |
Hand | Fitts (1954) | 10.6
Arm | Langolf (1976) | 9.5
Xbox 360 | Zaranek (2014) | 5 | **.47** (arm)
Mouse | Mackenzie (2001) | 4.9 | .46 (hand)
Neck | Pfaff (1985) | 4.2 |
Trackball | Mackenzie (2001)| 3.0 | .28 (hand)
Touchpad | Mackenzie (2001) | 2.9 | .27 (hand)
Head | Hansen (2018) | 2.5 | **.51** (neck)
Gaze | Hansen (2018) | 2.1 | **.15** (eyetracker)
Joystick | Mackenzie (2001)| 1.8 | .17 (hand)
Playstation Move | Zaranek (2014) | 1.5 | .16 (arm)
Kinect | Zaranek (2014) | 1 | .1 (arm)
]
]
.footnote[Card, S. K., Mackinlay, J. D., & Robertson, G. G. (1991). 
A morphological analysis of the design space of input devices. ACM
Transactions on Information Systems (TOIS), 9(2), 99-122.
]
???
Useful to know if you're designing something like VR

Doesn't have much impact on interface design

Doesn't capture everything; e.g. error rate is higher for eye tracker

---
# Warnings

Applies to *expert, errorless* use

![:img bar chart of Average WPM for error rates of 1% to 20% in a
study that falsely inserted typing errors is much higher for 1% than
20%,40%, width](img/pm/errors.png)

.footnote[Error rate was manipulated by faking errors.
Ahmed Sabbir Arif and Wolfgang Stuerzlinger. CHI 2010. Predicting the cost of error correction in character-based text entry technologies. In Proceedings of the SIGCHI Conference on Human Factors in Computing Systems, April 2010
]

???
Axis: X is faked error rates, Y is the words per minute.

Shows how WPM did change in the study where error rate is being changed.
If you know you are going to make a lot of errors you slow down your motion
The underlying way we're designed to move is with small motions, to eliminate jerk
We accelerate and decelerate smoothly
Trace of motion for different body parts - after the initial motion, which is a rapid aiming motion,
then corrective motions if you misss the target.
Lots of those corrections, that is where you are goign to see people slow down.
Subtle effects here – like the time to fix errors goes up as the error rate goes up…
Act of correcting an error also takes time.
Not worry about in the assignment, but interesting to know.

---
# Example: Arm based input

Student project using a Kinect from 2012.
Watch the movement of his arm...

![:youtube Bubble keyboard implemented by Chinmay N in 2012. Users swipe with their full arm to select letters in on a keyboard, mQ-pCDr3RCw]

---
# Discussion: Implications of Fitts' law

- Why are some things harder than others to click on?
  - Group discussion - fill in [Ed slide](https://edstem.org/us/courses/38124/lessons/59423/slides/331525)

![:img Picture of mac desktop and windows desktop. Of note is the main menu bar which is at the very top of the mac desktop but at the top of *each* window on the windows desktop, 80%, width](img/pm/macvswindows.png)

???
Constantly relevant

Intuitively, things that are closer and/or bigger are faster and
easier to hit (and vice versa)

What is different about the file location in Mac and Windows?

What is the width at the top of the screen.

- Add your thoughts on the bullet line with your group number in this [Google Doc](https://docs.google.com/document/d/10D3XfkzfU5RM426ueamy7fQA2QL-tWXNw7UxwR5Z3fQ/edit?usp=sharing)

---
# Thinking about Design Tips

Follow along on our [Exit Ticket Ed Lesson](https://edstem.org/us/courses/38124/lessons/59423/slides/331526)
to participate.

Recall Fitts law is:

.jax[$$MT = a + b*log_2({Dist \over Size} + 1)$$]

---
# Design Tip #1: Make small targets larger

Answer on [Ed](https://edstem.org/us/courses/38124/lessons/59423/slides/331526)

What does making targets larger do to Fitts Law Predictions?

]
---
count: false
# Design Tip #1: Make small targets larger

Answer on [Ed](https://edstem.org/us/courses/38124/lessons/59423/slides/331526)

What does making targets larger do to Fitts Law Predictions?

Why should you not maximize all of the targets?
]

--
count: false
.right-column50[
Fitts' law involves a logarithmic calculation -- exponentially bigger gains for small targets, not so
much of a gain for larger targets
]

---
# Design Tip #1: Make small targets larger

.left-column50[
![:img CNN website marketing using a small x to close the pop up marketing that obscures the screen,85%,width](img/pm/cnn-small-x.png)
]

.right-column50[
- How is the CNN marketing group using Fitts' law in this situation?
]
--
.right-column50[
- intentionally making it HARDER to remove this window
]

---
# Design Tip #2: Put commonly used things close together

![:img Screenshot of the powerpoint toolbar with grouped things,100%, width](img/pm/powerpoint-toolbar.png)

Answer on [Ed](https://edstem.org/us/courses/38124/lessons/59423/slides/331526)

Is this minimizing distance, maximizing size, or both?

---

# Design Tip #2: Put commonly used things close together

.left-column50[
![:img Image of the drag'n'pop interface which moves icons to the cursor when they are valid targets,80%, width](img/pm/dragpop.png)
]
.right-column50[
## Can also bring items closer to the cursor

(to also minimize the distance)
]
.footnote[
Baudisch, P., Cutrell, E., Robbins, D., Czerwinski, M., Tandler,
P. Bederson, B., and Zierlinger, A. Drag-and-Pop and Drag-and-Pick:
Techniques for Accessing Remote Screen Content on Touch- and
Pen-operated Systems. In Proceedings of Interact 2003, Zurich
Switzerland, August 2003, pp. 57-64.
]

???
Reduces Distance (D)

Not good design to put everything close together

No space to move all targets (contextual filtering helps)

Could do with a fancy container...

---
# Design Tip #2: Put commonly used things close together

.left-column50[
![:img Image of the drag'n'pop interface which moves icons to the cursor when they are valid targets,80%, width](img/pm/dragpop.png)
]

.right-column50[

Why not do this to all targets?
]
--
.right-column50[
- Other usability goals matter too -- e.g. grouping related things
- It is not good design to put everything close together
- There is not enough space to move all targets (contextual filtering helps)

How would you implement this?

]

---
# Minimizing distance and maximizing size!

We can make things *Closer* and *Larger* at the same time

![:img Image of mac with doc icons magnified,100%, width](img/pm/mac-expanded.png)

---
# We're beating Beating Fitts' Law!

How else can we do this?
--
count: false

Just don't use a mouse! Shortcut buttons; scroll wheel

Or CHEAT... manipulate the interface
- Minimize distance
- Increase size

---

# Menu Selection

.left-column50[
Answer on [Ed](https://edstem.org/us/courses/38124/lessons/59423/slides/331526)

Do these menus minimize distance or maximize size?

]
.right-column50[

Select from a menu bar at top of screen

![:img Picture of the apple menu opened on a mac,100%, width](img/pm/mac-noedge.png)
]

???
Problems?

- Can be tiring to reach edges of *very* large screens
- Not all interfaces can do this (e.g. web designers)

---
# Design Tip #3: Make use of Edges: They are Infinite

.right-column[
![:img Picture of the Windows menu in Windows 95 Windows XP and Windows 8 showing progressive improvement in the relationship between look (namely a border around the icon) and feel (can click anywhere making it an infinitely large button which is easier to hit from a Fitts law perspective), 100%, width](img/pm/windows-comparison.png)
]

???
- Windows 95 bug: bottom-left corner is offset by a few pixels, can’t click on it
- Windows XP fixes it by making Start button go all the way to the corner
- Windows 8 uses styling to make start menu circular, but still makes entire corner clickable
---
# Recent Example (Zoom)

Which of these do you think is a better button for maximizing size using the infinite edge?

.left-column60[
![:img Old zoom leave button which looks only like text but really is an infinite edge, 30%, width](img/pm/old-zoom-leave.png)
]
.right-column30[
 
![:img New zoom leave button which looks bigger but loses the infinite edge, 50%, width](img/pm/new-zoom-leave.png)
]

---
# Recent Example (Zoom)

Which of these do you think is a better button for maximizing size using the infinite edge?

.left-column60[
![:img Old zoom leave button which looks only like text but really is an infinite edge, 30%, width](img/pm/old-zoom-leave.png)   
![:img Old zoom leave button which displays its infinite edge/larger button when hovered over, 30%, width](img/pm/old-zoom-leave2.png)

Although it looks like only the text is clickable, it turns out the whole edge was. Now we only have a "smaller" but brighter button to click.
]
.right-column30[
 
![:img New zoom leave button which looks bigger but loses the infinite edge, 50%, width](img/pm/new-zoom-leave.png)
]

---
# Design Tip #3: Make use of Edges: They are Infinite

Problems with infinite edges?

- Can be tiring to reach edges of *very* large screens
- Not all interfaces can do this (e.g. web designers)
- Does this work for mobile devices or other touch screens?

---
#  Menu Selection

.left-column50[
Answer on [Ed](https://edstem.org/us/courses/38124/lessons/59423/slides/331526)

Do these menus minimize distance, maximize size, or both?

]

.right-column50[
![:img Left -- A pie menu consisting of a circle broken into eight
wedges containing menu items like copy and undo; right-- A picture of
a standard linear menu ,100%, width](img/pm/linear-pie.jpg)
]

.footnote[
Left example: OneNote 2013, Right example: Firefox. Taken from [Fitts
law and user experience](https://www.smashingmagazine.com/2012/12/fittss-law-and-user-experience/)
by
[https://www.smashingmagazine.com/author/anastasios-karafillis/](Anastasios Karafillis)
]

---
# Why are marking menus rare & unfamiliar?

Right click in place and then select (starting at :19)

![:youtube Illustration of advantages of marking menus,dtH9GdFSQaw?start=19]

.footnote[
Kittenish, G., & Buxton, W. (1994, April). User learning and performance with marking menus. In CHI (Vol. 94, pp. 258-264).
]
???
Minimizes D
AND
increases W
(only angle matters)

---
## Design Tip #4: Use marking menus instead of context menus for expert tasks

Under some circumstances only...

- Less than 8 options (Small target areas when too many menu entries are added)
- Expert user (willing to memorize and gain advantage of marking
menus)
- Grouping not important (hard to group radially)

Pie menus are often not implemented in production code
- Needs to be seen as a high priority feature
- Managers need to be able to assign staff to the project
  - Sadly, project constraints define an interface. (**Shouldn't be a thing! Whorfian effects**)

---
layout: false
# Example in Maya

Example of how Marking Menus are used in practice (Maya)
![:youtube Example of marking menu in Maya, r8PQy8cX9dc]

---
# Menus Assignment

Goal: to compare pie menus, linear menus, and a custom menu you make!

We provide support for running the experiment in `AbstractMainActivity` (and a testing harness in `ExperimentActivity`)

You implement a variety of menus

---

# Snapping

.left-column50[
Answer on [Ed](https://edstem.org/us/courses/38124/lessons/59423/slides/331526)

Does snapping to a target minimize distance, maximize size, or both?
]
.right-column50[

![:img video of someone moving emails into folders with snapping,100%, width](img/pm/gmail-snapping.gif)
]

---

# Design Tip #5: Use snapping to minimize distance when likely targets are known

![:img Picture showing drag snapping in gmail trello and chrome,40%, width](img/pm/common-snapping-cases.png)

.font-small[

Gmail | Trello | Chrome
----- | ------ | -----
![:img check, 10%, width](img/pm/check.png)Drag Handle | ![:img x, 10%,width](img/pm/x.png)No Handle | ![:img x, 10%, width](img/pm/x.png) No Handle
![:img cursor,10%, width](img/pm/grab-cursor.png)Grab cursor | ![:img cursor ,10%, width](img/pm/pointer-cursor.png)Pointer cursor | ![:img ,10%, width](img/pm/cursor.png)Default cursor
![:img check, 10%,width](img/pm/check.png)Drop Shadow | ![:img check, 10%, width](img/pm/check.png)Drop Shadow | ![:img x, 10%,width](img/pm/x.png) No Drop Shadow
![:img check, 10%,width](img/pm/check.png)Drop Target | ![:img check, 10%,width](img/pm/check.png)Drop Target | ![:img x, 10%,width](img/pm/x.png) No Drop Target
![:img x, 10%,width](img/pm/x.png) No Natural Movement | ![:img check, 10%,width](img/pm/check.png)Natural Movement | ![:img check, 10%,width](img/pm/check.png) Natural Movement
]

.footnote[Examples courtesy of [Drag and Drop for Design Systems](https://uxdesign.cc/drag-and-drop-for-design-systems-8d40502eb26d)]

???
Also common in drawing programs

Most sophisticated approach: dynamic semantics: Check legality and
consequences of each result at every move don’t catch errors, prevent them
---
# Other Examples

Fan Cursor, Area Cursor and Bubble Cursor
![:youtube Video showing a variety of schemes for making the cursor
bigger including fan cursor; area cursor; and bubble
cursor,bq1x5cRqgUc]

.footnote[Su, X., Au, O. K. C., & Lau, R. W. (2014, April). The
implicit fan cursor: a velocity dependent area cursor. In Proceedings
of the SIGCHI Conference on Human Factors in Computing Systems
(pp. 753-762). ACM.]

---
## Design Tip #6: Separate Motor Size from Visible Size

.left-column[
![:img Picture of two versions of a dialog box; scrollbar; and
menu, 100%, width](img/pm/hiddensize.png)
]
.right-column[
- (a) Visual space appearance of buttons in a dialogue
box.
- (b) Motor space version of button design in (a) with much larger
targets for certain buttons.
- (c) Standard scroll-bar design.
- (d) Visual space appearance of scroll-bar redesigned to occupy smaller
screen space.
- (e) Motor space version of scroll-bar design in (d) with larger targets for active areas.
- (f) Visual space appearance of menu.
- (g) Motor space version of the menu design in (f) with the
distance to more important items reduced by compressing the size of less important items)
]
.footnote[Semantic pointing widget designs from Blanch et al. 2004]
???
How would you implement this?
- Manipulate picking
- Whorfian effects
---
# Small on purpose?

What might you make too small and why?

--
count: false

How about a cancel button?

---
# How long will this take?
.right-column-half[
Select 'Open New Window in Process'
]
.left-column[
![:img Cascading menus. User wants to use "Open New Window" then "Open
new window in process" -- second option-- but the most direct path takes them over "Open command prompt" which changes the displayed sub-menu causing problems, 200%, width](img/pm/size.png)
]
--
count: false

.right-column-half[
Steering law ... Fitts law integrated over the path you have to follow
]
--
count: false

.right-column-half[
But it's not predictive at all. Why?
]
--
count: false
.right-column-half[
It doesn't account for actual user behavior
]
---
# Actual User Behavior

.left-column50[
![:img Cascading menus. User wants to use "Open New Window" then "Open
new window in process" -- second option -- but the most direct path takes them over "Open command prompt" which changes the displayed sub-menu causing problems, 80%, width](img/pm/size1.png)
]
--
.right-column50[
![:img Cascading menus. User wants to use "Open New Window" then "Open
new window in process" -- second option -- but the most direct path takes them over "Open command prompt" which changes the displayed sub-menu causing problems, 80%, width](img/pm/size2.png)
]

???
Overly eager cascading menus, wrong submenu
- Common problem w/ web menus (but not Mac or Win)
- could brainstorm solutions (e.g. snapping? set focus somehow?)
- whorfian effects: this is hard to implement

---
# Fitts law has a huge limitation

It is only predictive of **ERROR-FREE, EXPERT** behavior

Related to our discussion of [Interaction Design](people-motor.html#33)
---
# What else might we want to measure?

--
- Time on Task -- How long does it take people to
complete basic tasks? (For example, find something to
buy, create a new account, and order the item.)

--
- Accuracy -- How many mistakes did people make? (And
were they fatal or recoverable with the right information?)

--
- How strenuous (e.g. gaze has lower throughput but is less strenuous
than head pointing Mackenzie 2018)

--
- Recall -- How much does the person remember
afterwards or after periods of non-use?

--
- Emotional Response -- How does the person feel about
the tasks completed? (Confident? Stressed? Would the
user recommend this system to a friend?)

???
Build up a list of good UI design Principles from these basics
- Undo
- Predictability
- ...
What is missing? (e.g. fun)

---
# What else might we want to measure?

.left-column50[
- Time on Task
- Accuracy
- How strenuous
- Recall
- Emotional Response
]
.right-column50[
What is missing?
]
???
(e.g. fun)
- Undo
- Predictability
- ...

--
.right-column50[
- Fun
- Predictability
- Error Likelihood & Recovery
- ...
]

---
# Summary of Design Tips
- Design Tip #1: Make small targets larger
- Design Tip #2: Put commonly used things close together
- Design Tip #3: Use edges. They are infinite in size (W)
- Design Tip #4: Use pie menus for expert tasks (better yet, marking menus)
- Design Tip #5: Use snapping to reduce distance (D) when targets are known
- Design Tip #6: Separate motor and visible size

---
# End of Lecture

Extra information is on the slides that follow

---
# Two Handed Interaction

![:img Picture by Escher of two hands each drawing the other, 40%, width](img/twohand/escher.png)

???
Used to be a norm

Seems we forgotten to include it in our interfaces
---
# Guiard's model of bimanual control

Hand | Role and Action
---- | ----
Non-preferred | Leads the preferred hand (in time)
| Sets the spatial frame of reference for the preferred hand
| Performs coarse movements
Preferred | Follows the non-preferred hand
| Works within established frame of reference set by the non-preferred hand
| Performs fine movement

Aside: Guiard is assuming right-handed. Tendencies somewhat less clear for left-handers,but we typically assume this still applies
]
.footnote[
Seminal work
Yves Guiard, “Asymmetric Division of Labor in Human Skilled Bimanual Action: The Kinematic Chain as a Model”, Journal of Motor Behavior, Vol. 19, No. 4, 1987, pp. 486-517.http://cogprints.org/625/
]
???
non-preferred hand sets context for preferred
coarser, slower movement
tends to move first
???
Applications?

- Mouse+keyboard
- Lenses

---
# What does theory say about keyboard layout?

![:img Picture of right-handed user typing on keyboard with leftside
and rightside power keys and mouse on right side (top). Picture of
user  mousing with same set ups (bottom). Of note is the bad angle for
the mouse in the keyboard with the right side power keys,
30%](img/twohand/keyboard-mouse.jpg)

.footnote[From [Evoluent keyboard advertisement](https://turningpointtechnology.com/KB/EV/KB1.asp)]
---
# Analysis of alternative keyboard layout

Task | Leading Movement | Trailing/Overlapping Movement
----|----|----
Delete | Right hand — manipulate pointer with mouse; select by double clicking/dragging | Left hand — press DELETE (probably with little finger)
Select an option in a window (see Figure 15) | Right hand — manipulate pointer with mouse; click option | Left hand — press ENTER (Assumes OK button is default)
Click on a link in a browser | Left hand — navigate to link via PAGE UP and/or PAGE DOWN keys | Right hand — manipulate pointer with mouse; select link by clicking
Open file; open folder; launch program | Right  hand — manipulate pointer with mouse; single click on icon | Right hand — press ENTER (avoids error prone double-click operation)

---
# Other Examples: Tool Stone for Magic Lenses

![:youtube By manipulating a mouse and a cube (one in each hand) the user controls a lens and uses it on screen, V32SnUnAe2E]

Inspired by [1994 CHI paper](https://dl.acm.org/citation.cfm?id=166126)
Published in [UIST 2000](https://lab.rekimoto.org/projects/toolstone/)

---
# Magnification

![:img Picture of a mandelbrot fractal with a lens over part of it magnifying that part, 50%, width](img/twohand/magnification.png)

---
# Font Selection

![:img Picture of a paragraph of text with a lens containing a menu of options for bold italic regular and bold italic, 40%, width](img/twohand/font-selection.png)

???
Fitts' law implications?
---
# Debugging Lenses

![:img Picture of a GUI application with a lens showing information about two lines including their coordinates and the option to turn on and off additional features, 40%, width](img/twohand/debugging.png)

.footnote[
Hudson, S. E., Rodenstein, R., and Smith, I. 1997. Debugging lenses: a new class of transparent tools for user interface debugging. In Proceedings UIST '97, 179-187. http://doi.acm.org/10.1145/263407.263542
]
---
# Advantages of Lenses

In context interaction
- Little or no shift in focus of attention and/or movement
- Alternate views in context and on demand

???
- tool is at/near action point
- can compare in context
- useful for “detail + context” visualization techniques
--

Structured well for 2 handed input
- non-dominant hand does course positioning (of the lens)
- dominant hand does fine work

"Spatial modes"
- Use “where you click through” to establish meaning
- Typically has a clear affordance for the meaning

---
# Implementing Lenses

.small[
<div class="mermaid">
 graph TD
 Root(Root) --> LP(Lens Parent)
 LP --> L[Lens]
 LP --> I(Interactor 1)
 LP --> II(Interactor 2)
 LP --> III(Interactor 3)

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.footnote[Edwards, Hudson, et al., “Systematic output modification in a 2D user interface toolkit”, UIST ’97. http://doi.acm.org/10.1145/263407.263537 ]
???
Can be implemented with special "lens parent" container & lens interactors
- Lens may need to change results of picking (only positional is affected) in collusion with lens parent
- Lens parent forwards all damage to all lenses
- Lenses typically change any damage that overlaps them into damage of whole lens area
- Can pass a subclass of Drawable to "ambush and modify" drawing for output effects
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# Discussion: Should scrolling be dominant or non-dominant?

What tasks is scrolling used for?

Which precedes/follows?

Design tips for motor

- Design Tip #1: Make small targets larger
- Design Tip #2: Put commonly used things close together
- Design Tip #3: Use edges. They are infinite in size (W)
- Design Tip #4: Use pie menus for expert tasks (better yet, marking menus)
- Design Tip #5: Use snapping to reduce distance (D) when targets are known
- Design Tip #6: Separate motor and visible size

Two-Handed input principles
- Non preferred leads
- Sets frame of reference
- Preferred does fine movement