Designing Effective Algorithm Visualizations Sami Khuri Department of Mathematics and Computer Science San José State University

Designing Effective Algorithm Visualizations

Sami KhuriDepartment of Mathematics

and Computer Science

San José State University

www.mathcs.sjsu.edu/faculty/khuri

Outline

• Definition

• History

• Design process:– Analysis phase– Design phase

• Evaluation

• Concluding remarks

1. Definition

Algorithm Visualization – is the process of abstracting a program’s data,

operations, and semantics, and creating dynamic views of those abstractions.

– is a depiction of an algorithm using an integrated set of multimedia tools, such as graphics, text, color, sound, code, animation and video.

2. History

1980s:

Visualizations can serve as effective learning aid for students.

– Balsa (Marc Brown, 1988)

– Zeus (Marc Brown, 1991)– Tango (John Stasko, 1990), Xtango, Polka– Later: Leonardo (Crescenzi et al., 1997)

History

1990s:

Empirical studies: Do algorithm animations really help?

– Results are unsatisfactory...• Badre (1991)

• Hundhausen (1997)

• Stasko (1998)

History

Today:

Algorithm visualizations do enhance learning,– if existing visualizations are effectively used

• Naps (2000) • Faltin (1999)

• Stasko and Lawrence (1998)

– if new visualizations are better designed• Hansen, Narayanan, Schrimpsher

3. Design Process

• Good algorithm visualization is not simply a matter of mimicking paper-based tasks or doing what is technically easy.

• Even simple tools can improve algorithm understanding, if they are the right ones.

• A lot of time and effort can be wasted implementing a system that is not needed or is unusable.

Design Process• We advocate:

a user-centered approach

(What do we NEED to show?), instead of

a designer-centered approach

(What CAN we show?)

• Start with an analysis of the users, their needs, tasks, information, and resources.

3.1. Analysis Phase

• Users’ analysis

• Needs analysis

• Task analysis

• Information analysis

• Domain analysis

• Resources analysis

3.1.1. Users’ Analysis

Who are they?

– No algorithm visualization will be universally superior across all kinds of users.

– Understanding who the users are allows us to determine content, organization, depth, breadth, interaction and presentation methods.

Users’ Analysis

Algorithm visualization system might be used by:

– Students – Teaching Staff – Researchers – Visualization developers

Users’ Analysis

Students – Will watch and interact with algorithm

visualizations. – Will need a lot of help information.– Will need a system that is forgiving to their

invalid input.– Will need facilities for setting up desired speed

and detail level, and for reversing actions.

Users’ Analysis

Another possible approach is to divide users into:

– Novice users– Expert users

Users’ Analysis

Novice users– Need a system that is very easy to use.

• For example, if the algorithm visualization system requires the user to annotate his/her program to call animation routines, the novice might not have the skills to do so.

– Need clear and detailed help files.– Do not need a system displaying many

different views of an algorithm.

Users’ Analysis

Expert users– Need a system that complements and

supplements their thinking, rather than mimics internal representations.

– Need a very flexible system allowing to see different levels of details

• if they try to improve performance of an algorithm, for example.

3.1.2. Needs Analysis

Do the users need algorithm visualization?

– An assessment should be made as to whether presenting an algorithm by visualizing it is the most effective way to do so.

• For example, instructors can use a phonebook to illustrate binary search.

• If instructors do not have access to the WWW in class, they cannot use a web-based visualization system.

3.1.3. Task Analysis

What will the users do with the algorithm visualization tool?

– Create new animations.– Interact with existing visualization to understand the

behavior of an algorithm.– Debug their program.– Each task demands a different kind of visualization

tool. It is difficult to build a system that satisfies them all.

Task Analysis

How will the algorithm visualization fit into the existing curricula?

– Classroom demos– Assignments – Hands-on laboratories– Self-directed work outside of class– Office hours– Distance learning

Task AnalysisClassroom demos

– Algorithm visualizations should function well as demonstrations.

– They will not need a lot of written explanations.– They need a facility for adjusting the speed and

reversing previous steps.– They should run smoothly to avoid unpleasant

situations where they “freeze”: • Web-based tools are not always the best for this task.

Task Analysis

Homework assignments– Algorithm visualizations have to run smoothly

on different platforms for everyone.• Better to create Java applications, not applets:

– Web-based Java execution is unpredictable at this point.

– One has to test all the applications on the actual equipment (under different environments) ahead of time. Impossible to do!

– They should be available for download• running them on the web server can be too slow.

3.1.4. Information Analysis

What are we trying to convey?

– Do we want to emphasize the data structures rather than the algorithm or vice-versa?

– Create a profile of resource utilization.– Search through an execution trace.– Contrast two algorithms.

Information Analysis

Which aspects of an algorithm should we illustrate?

– Source code

– Data

– Data structures

– Execution

– The trend is toward the ability to visualize all aspects of an algorithm.


Source code visualization– View the source code through a flowchart.– View the code by displaying the pseudocode and

highlighting lines currently being executed.– Present currently executing portion of the code

in a window.

Data visualization– Extract and display only scalar variables such as

values held in an array.


How should information be represented?

– Direct representation – Structural representation– Synthesized representation– Analytical representation– Explanatory representation


Direct representation– At any given instant, the data structure could be

constructed from the display, and the display could be constructed from the data structure.


Structural representation

– Only the important aspects are represented.• For example, a computation running on a network of

processes, where each process is either “active” or “inactive”. All other attributes of the processes are concealed since they are not relevant to the viewer.


Do we display current information or illustrate history of what has happened?

– Displaying history:• helps explain the algorithm like a textbook example,

• lets users become familiar with the algorithm’s dynamic behavior at their own speed.


How will we move from one display to the next one?

– Incremental transition– Discrete transition

There is no pedagogical advantage of either

technique.


Incremental transition– Transition occurs smoothly.– It is effective when users (especially novice)

are examining an algorithm running on a small set of data.

– This type of transition is difficult and tedious to implement.

– Developers need to decide how slow or fast the transition should occur.


What is the largest data set the algorithm visualization should be

able to handle?

– Limit the size of input data.– Large input files are allowed.


Limit the size of input data– If visualizations are used to explain steps of

algorithms.• For example, viewing visualizations of large trees

will require scrolling to see the entire tree and might confuse more than educate.

– If visualizations are used by novice users.– If visualizations are used in homework

assignments.


Very large size of input data

– Is useful for researchers or instructors in classroom when comparing running times and behavior of different algorithms.

– Cluttering the screen with information is not a problem.

– Data items should be presented in the simplest form, dots, for example.

3.1.5. Domain Analysis

What is the scope of the algorithm visualization?

– General-purpose algorithm visualization system

– Specialized algorithm visualization system– Single-purpose algorithm visualizations

Domain Analysis

General-purpose algorithm visualization systems

– Systems that can (ideally) animate any algorithm.

– These type of systems should provide a large set of ready-made pieces of visualizations and general tools to create them.

– The greater the number of algorithms that can be animated, the more desirable the result.

Domain Analysis

General-purpose algorithm visualization systems

– Some of the general-purpose systems are BALSA, TANGO and XTango, Zeus.

– Although general-purpose algorithm visualization systems are becoming more powerful, the task of using these tools to create visualizations of algorithms still involves a lot of work.

Domain Analysis

Specialized algorithm visualization systems

– Systems that specialize in visualizing algorithms from a certain field of computer science, such as computational geometry.

– Restricting algorithm visualization system to one field only eases the process of creating visualizations, since graphical representation of data structures will be implemented once only.

Domain Analysis

Single-purpose algorithm visualizations

– Programs that visualize one algorithm or a group of related algorithms in detail.

– They are coded from scratch for each algorithm.– They are easier to create, understand and

maintain.

Domain Analysis

Is there a potential for the domain to enlarge over time?

– Design for growth.– Use object-oriented design.– Provide extensive documentation.

3.1.6. Resources’ Analysis

Given the above information, do we have the necessary resources to create the algorithm visualization?

– Time.– Human resources.– Computer resources.

Resources’ Analysis

Time – Designing and implementing algorithm

visualizations takes longer than expected.

Human resources– Need a team of developers with skills in

context, graphical design, networks, etc.

Resources’ Analysis

Computer resources

– CPU speed, RAM, monitor size and resolution, audio/video input and output, color display panel and projection.

– If the prepared visualization will be available over the internet, it might need large amounts of storage space, and more importantly, high-speed connection to the net.

3.2. Design Phase

• Choosing specification method.

• Choosing techniques for visual representation of information.

3.2.1. Specification Methods

Choices available to the developers:

– Predefinition (hand-coded visualization)– Annotation– Declaration (animation by demonstration)

3.2.1. Specification Methods

Predefinition (hand-coded visualization)– Mainly used in application-specific visualization.

– Fixed or highly constrained mapping of the steps of the algorithm to the graphical views.

• The information and its representation cannot be changed after the implementation.

– The main advantage is the execution speed.

– Can be easily distributed.

– Can be very interactive and visually attractive.

Specification Methods

Annotation– Important steps of an algorithm are annotated

with interesting events.– These events call graphical operations from an

existing library.• For example, move rectangles, change colors, etc.

– The main advantage of this approach is the animators ability to define events at any suitable level.


Annotation

– The most significant shortcoming of this method, is the need to access and modify the code of the program.

– The first major system with interesting events was BALSA by Brown.

– Other examples: Zeus and Tango.


Declaration– The developer defines mappings from program

states to graphical objects.– During program execution, changes in the

program state trigger updates to the graphical view of the program.

– This paradigm is instantiated in the Pavane animation system by Roman, Cox, Wilcox and Plun.

3.2.2. Techniques

Some techniques for visual representation of information about algorithms.

– Selection of default visualizations.– Screen design and multiple views.– Color and Sound.– 3-Dimension.– Interaction .

Selection of Default Visualization

Design appropriate default visualizations

– How much complexity does the user of algorithm visualization need?

• The complexity of visual presentations is generally proportional to the amount of information being conveyed.

• Start with relatively small problem instances and gradually introduce larger ones for the users who begin to understand the meaning of the visual patterns.

Selection of Default Visualization

Design appropriate default visualizations

– For pedagogical purposes, the following input data should be provided as default visualizations:

• Pathological input data

• Cooked input data

Screen Design

• The screen on which the visualization occurs can easily get cluttered with the many visual representations of control constructs or data items.

• The user may get lost in the details and not see the overall picture.

Screen Design

The solution to reducing the graphical overload is abstraction:

– Condense complicated parts of the scene into simpler items.

– Omit several phases of an algorithm and present only the final result of several program steps.

– Use semantic zooming.– Use step-wise refinement.

Screen Design

Stepwise Refinement– Give the basic idea first and then progressively

add more detail.– How many details do we want to display?

• Too many levels can have a counterproductive effect and difficulty to the learning task.

• A rule of thumb: a given line of pseudocode should be expandable to a maximum of three levels.

• The fully expanded pseudocode should fit on one screen without scrolling.

Screen Design

Some recommendations – Divide the screen into functional areas, each

containing a different type of information.– Consistently display each type of information in

its assigned area.– Place important information near the top and to

the left. • Eye-motion studies show that our gaze goes to the

upper left of a rectangular display first and then moves clockwise.

Screen Design

Recommendations for designing user interface

– Don’t make the user guess where to click to perform an action.

• For example, unlabeled graphics as controls (buttons, links, etc.) are confusing.

– Try not to have two or more buttons that perform the same action.

– Keep the interface uniform. Have the same controls perform the same action everywhere.

Multiple Views

Multiple views– Each view displays only a few aspects of the

algorithm.– Each view must be easy to comprehend in

isolation.– They are conceptually simpler and easier to

implement than monolithic views.– They eliminate the possibility of forcing the

viewer to remember algorithm states no longer on display.

Multiple Views

Multiple Views

Multiple views

– The important design question is how to synchronize the views. Some possibilities are:

• Display related objects in different views in the same color.

• Display related objects in different views using the same shape (circle, triangle, etc.).

• Display related objects in different views in the same size.

Multiple Views

Segmentation is a useful extension of

the multiple views idea

– If the user selects a physical region in one of the views, the display would immediately highlight the corresponding values in other views.

Multiple Views

Multiple Views

Some recommendations

– If multiple views are needed to visualize a concept, allow the users to page them so that only one or the other can be seen at a time.

Color

Color offers the following advantages– It calls attention to specific data or information.– It identifies elements of structures and processes.– It can portray time.– It increases appeal, believability, memorability,

and comprehensibility.– It reduces errors of legibility and interpretation.

Color

Advantages of using color– It allows denser presentation of information:

• fewer pixels are needed to make a color change visible than to make a change in the shape of an object visible.

– It increases the number of dimensions for coding data:

• one can encode information in both the shape and the color of objects.

Color

Drawbacks or disadvantages– It requires more expensive and complicated

design equipment.– It may not account for color deficiency among

some users:• about eight percent of Caucasian males.

– It can cause visual fatigue and afterimages induced by strong colors.

Color

Drawbacks or disadvantages – It can invoke negative cultural associations to

some or all colors.– Establishing general rules or specifications for

color use is very difficult:• We still lack some important understanding of color

vision.

– Color is subjective:• Different users have different preferences.

Color can be used– To draw attention,

• through highlighting, for example.

– To communicate organization.– To indicate status.– To establish relationships,

• in multiple views, for example.

– To increase user satisfaction.

Color

Color

To draw attention

– temporarily paint a small region of interest with a transparent, contrasting color:

• The highlight color should be transparent.

• It should not interact visually with the data elements on the screen, but simply draw the eye to them.

• Highlight must be temporarily and should be removed quickly when the scene changes. Think about double-buffering in Java to avoid flickering.

Color

Color

Some recommendations for using color

– It is better to be conservative.• Use a maximum of five, plus or minus two colors.

– This allows extra room for short-term memory (about 20 seconds) which can store five words or shapes, six letters, seven colors and eight digits.

• For novice viewers, four distinct colors are appropriate.

Color– Use fovea (center) and peripheral colors

appropriately. • Use blue for large areas, such as background, not for text,

thin lines or small shapes. – Blue-sensitive cones are the least numerous of the color

receptors in the retina, and the eye’s central focusing area, the fovea, contains relatively few of these blue-sensitive cones.

• Use red and green in the center of the visual field, not in the periphery.

– The edges of retina are not particularly sensitive to these colors.

• If red or green are used at the periphery, some signal to the viewer must be given to capture attention (size change, blinking, etc.)

Color


– Do not use high-chroma, spectrally extreme colors simultaneously.

• Strong contrasts of red/green, blue/yellow, green/blue, and red/blue create vibrations, illusions of shadows and afterimages.

Color


– Use familiar, consistent color coding with appropriate references. Some common Western denotations are the following:

• Red refers to stop, danger, hot, fire.

• Yellow refers to caution, slow, test.

• Green refers to O.K., clear.

• Blue refers to cold, water.

• Gray and white refer to neutrality.

Color


– Carefully select colors that will be displayed across media with different techniques for generating color.

• Note, that CRT display screens use additive color mixtures which combine into white, while most hard copy devices use subtractive color mixtures which combine into black.

ColorSome recommendations for using color

– Use color to enhance black and white information. – With respect to learning and comprehension, color is

superior to black and white in terms of viewers emotional reactions.

• There is no difference in the viewer’s ability to interpret information. People do not learn more from a color display, although they may say they do.

• The crucial factor is that color is more enjoyable and color information is easier to remember.

Color


– Keep in mind that many people are color blind.• Any time color is used to convey information, the

secondary cues should also be used.

• Secondary cues can consist of anything from the subtlety of gray scale differentiation to having a different graphic or different text label associated with each color presented.

Sound

Why use sound in algorithm visualizations?

– Sound has the potential to convey information that is difficult or awkward to display graphically.

– Sound can help reduce the visual clutter of current graphic interfaces.

– Sound can be redundant with visual information to reinforce the concepts.

Sound

Some problems when using sound– Sound is more difficult to use than color.

• Most people do not have the same level of sophistication and training aurally as they do visually.

• What happens when more than one computer is using audio in the same room?

• What happens when more than one “view” uses audio?

• What if there are no audio output?

Sound

Some problems when using sound– Sound is difficult to master:

• frequency

• volume

• attack and decay rates

• duration

• timbre

– Poor use of sound is less forgiving than poor use of color or 3-D.

Sound

Some problems when using sound

– Many aspects of sound perception are not yet well understood. More research is needed to determine the best sonic representation of data.

– There is no standard sound hardware and software platform.

Sound

Some recommendations for using sound in algorithm visualization

– Carefully consider the number of sounds.• Use only a few sounds for the most important or

difficult events/commands.

• The threshold for learning and remembering sounds is between 7 and 9 different sounds.

Sound

Some recommendations for using sound in algorithm visualization

– Test your algorithm visualization with a range of users to ensure that the selected sounds are effective.

– Graphical display results should be synchronized with the audio.

3-D

• Do not use 3D for enhancing the beauty of a picture that is easily shown in 2D.

• Use 3D displays to:– represent the 3-dimensional data some

algorithms manipulate– provide an extra dimension to represent more

information about two-dimensional data.

3-D

3-D can give us one more dimension

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2-D Display 3-D Display: direct visual comparisonof the elements

3-D

Some techniques that may be used in crafting 3-dimensional visualizations

– Rotation– Transparency– Navigation– Exploration of objects

3-D

Attention should be paid to choosing:

– the axis of rotation,– the speed of rotation.

The orientation and rotation speed should be

saved each time there is a pause in animation.

3-D

Transparency

– Very complicated 3D objects may be best understood if the user can interact with them and inspect their contents.

– Make the object transparent.– Use transparency with care, otherwise, there

may be too much detail.

3-DTransparency

– To avoid overload, make only selected portions of an object transparent. We can draw the viewer’s attention only to the important parts of an object.

– An ideal interface will allow the user to look inside an object by:

• cutting it open• slicing it along a selected plane• breaking it apart at natural boundaries and separating

pieces for further examination.

Interaction

• Visualizations are more effective when the user can steer them in appropriate directions.

• Algorithm visualizations should allow for interactive controls.

• Users should be forced to interact with the system at least every 45 seconds.

InteractionSome of the varieties of interaction an algorithm visualization system can

utilize:– Allowing the user to step through the execution or

set its speed,– Allowing the user to design input data for the

algorithm• tools for inputting data should be simple so as not to

overwhelm the user,• input tools should do error checking and report any invalid

input to the user.

InteractionOther forms of interaction

– Allowing the user to control the presentation and amount of information:

• Facility to hide one or more of the multiple views.

• Facilities to zoom in and out.

• Means for changing the data while the algorithm runs, e.g., by dragging a data point to a different location and watching the effect of this change,

– not suitable for some algorithms, e.g. sort algorithms

InteractionOther forms of interaction

– Allowing the user to simultaneously run several algorithms to compare and contrast them.

• This capability is useful for comparing the execution times of two algorithms by running a race.

• Note, that modern windowing environments and operating systems will allow almost any window-based visualization system to be run in parallel.

– Running two versions of algorithm visualization in different windows would not qualify under this category because there is no centralized control or synchronization between both running visualizations.

Interaction

Other forms of interaction

– Allowing the user to step back in the algorithm visualization.

– Allowing the user to design his/her own visualizations.

Interaction

Other forms of interaction

– Allowing the user to check his/her understanding by answering stop-and-think questions:

• if the feedback from the interaction is sluggish such that users’ actions and their feedback become unrelated, then this type of interaction fails.

4. Evaluation

Over one hundred different algorithm

visualization tools have been built in the last

twenty years, yet very few of these were

systematically evaluated.

EvaluationThere are some difficulties which arise when performing a formal evaluation study:

– Obviously, evaluation takes time.– Which visualization should we use for testing?– How do we measure program understanding?– Which criteria should we use?– How do we properly and fairly conduct the study?

Conclusion

• So far, we have only informal evidence that applications of algorithm visualizations are useful - we need controlled studies:– need to conduct formal studies of applications

in classroom settings and use the result to influence the next generation of tools.

Conclusion

• Creating good educational algorithm visualizations is time consuming and generally requires a team of developers with different skills.

• Think about distribution and maintenance of your new algorithm visualization system.

Conclusion

Distribution issues– Make it possible for other educators to try your

tool.• Put the tool on the web.

– Educational algorithm visualizations are shifting toward web-based design:

• It simplifies installation and platform compatibility.

• It provides immediacy of access.

• It allows them to be incorporated into electronic textbooks.

Conclusion

Maintenance issues – There is increasing demand for more

functionality in algorithm visualizations which results in larger programs.

– Turnover in the development and support community is great; development tools become obsolete; source code is even lost.

– All this makes maintenance very difficult and important.

Conclusion

• Color, sound, and 3-D graphics do not merely enhance the beauty of presentation; they can be used to give fundamental information.

• Designing effective dynamic algorithm visualizations is a craft, not a science.

Kiitos!

Documents

Designing Effective Algorithm Visualizations Sami Khuri Department of Mathematics and Computer Science San José State University