1. Infovis using VTK and Qt
DOECGF 2005
Andy Wilson
Sandia National Laboratories
Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

2. Outline Driving Problems Why VTK? Adapting VTK for Infovis
Applications: CallView
Lessons Learned
What’s next?

3. Driving Problems
Customers are asking for tools that are more about infovis than traditional scivis.
Map concepts against one another and display as a graph
Patent database
LDRD calls and ideas
Social and semantic networks
Who knew whom, when?
We also want the ability to annotate data inside Paraview.
Pass along annotations like any other attribute

4. Okay, how do we do that? Possible options: Adopt an existing toolkit.
Write our own.
Adapt something we already have.

5. Option 1: Adopt an existing toolkit
Plenty of candidates
Several toolkits presented at a seminar before Vis last year
Don’t need to reinvent the wheel
Java toolkits make portability easier
Not a panacea…
Licensing issues?
If proprietary, how much does it cost?
If open, which license does it use?
We may still have to re-invent other components
How does the toolkit work with Paraview, VTK, others?

6. Option 2: Write our own Toolkit will do exactly what we want
Play nicely with VTK, Paraview, perhaps even Ensight…
No license problems
Large initial investment
Duplication of effort
Difficult to collaborate with others

7. Option 3: Modify existing code
Some initial investment
Reinvent the wheel but not the whole car
Toolkit will do most of what we want
May not be a perfect fit
Take advantage of existing code
There is already a skilled developer community
Portability may come nearly for free
Contribute back to the community
Potential license problems

8. Adapting VTK for Infovis
Why VTK?
We already know how to use it
Widely deployed within Sandia via Paraview
Good working relationship with Kitware
We already have permission to release code as open source within VTK
Portability
Multiple language bindings
There may be other toolkits with most of these advantages…

9. Infrastructure Problems
String support
Add string as a new data type in VTK arrays
VTK assumes that array elements are numbers
Major problem: Strings are variable length!
I/O
Need a robust way to read and write string data
Filtering
What should happen when a numeric filter encounters string data?
Backward compatibility is critical!

10. Problem 1: String data type
We need to be able to treat a string as a POD type.
std::string is close enough
Bigger problem: vtkDataArray and its subclasses assume that their contents are numeric.
Tried subclassing. It didn’t work.
Solution: Change the class hierarchy.
Factor out the numeric assumption
Backward compatibility is critical!

11. Old VTK data array hierarchy
vtkObject
vtkDataArray
vtkBitArray
vtkIntArray
vtkShortArray
vtkFloatArray
vtkCharArray
vtkLongArray

12. New VTK data array hierarchy
vtkObject
vtkAbstractArray
vtkDataArray
vtkStringArray
Other arrays…
What we’ve done here is to factor out the array-management aspects of vtkDataArray into a new class vtkAbstractArray. The interfaces specific to numeric data types remain in vtkDataArray, leaving us free to define arrays of other data types. We’ve also changed a few of the fundamental containers that hold collections of these arrays to use abstract arrays instead of data arrays.
We did this specifically so that we could handle strings, but it’s already having side effects: a few other classes in VTK like vtkCellArray are going to become much cleaner under this new hierarchy.
The best part about this is that the interface to vtkDataArray doesn’t break at all. It gains one or two new methods that are perfectly safe for users and developers to ignore unless they want to use the new array types.
vtkBitArray
Backward compatibility is critical!
etc…

13. Changes to VTK I/O
The elements of a vtkStringArray do not all have the same size.
The components of each element do, though!
Flatten each string array into two separate arrays
One long block of characters
An array of offsets
Use standard VTK I/O code to read and write those arrays
Since we’re not tying the I/O code to the standard 8-bit character, this leaves the door open for international character sets…

14. Changes to VTK Filtering
Add a new IsNumeric() method to vtkAbstractArray
Filters that only copy and select data remain unchanged
Filters that work with a specific data type must check for that data type
That’s the way it is already!
Since we’re adding a new data type that doesn’t fit some of the old assumptions, we have to expose a way for people to check for it.

15. Okay, now what?
This relatively minor change lets us pass string data through the VTK pipeline just like any other data type.
That was the only real obstacle to using VTK for infovis.
Now, on to what users really care about: the applications.

16. CallView: Who is the customer?
Customer: Sandia LDRD (Lab Directed Research & Development) program office
Proposal submission season every March
About 15 broad research categories
About 100 specific thrusts (topics of interest)
Typical submission season gets ~1200 short proposals

17. CallView: What is the problem?
The people submitting ideas for research projects don’t know what area & thrust to submit to.
The people in the LDRD office want to explore the submissions.
How well do the ideas match the areas & thrusts?
Are ideas being submitted to the appropriate areas?
Infovis to the rescue!

18. CallView: Server function
Store a set of documents in a local database
Initially, thrust text only
Ideas as well (after submission season closes)
Compute similarities between all pairs of documents
Do force-directed graph layout using the results
Also allow clients to compare arbitrary chunks of text against all documents in a set

19. Use case #1: I have an idea!
Researcher has an idea for an LDRD proposal.
Send idea text to CallView as part of submission process.
Results include a list of the top 5 most similar thrusts.
Researcher is not forced to submit to those: they are only a suggestion.

20. Use case #1: Results page

21. Use case #2: Analyzing all idea submissions
Investment area managers want to see how the ideas map to the areas and thrusts.
Construct a document set containing all ideas and thrusts for a given year
Build a semantic graph using the similarities between all pairs within that set
Allow users to explore that graph

22. Use case #2: Front end

23. Use case #3: Analyzing some submissions
LDRD area managers want to see the ideas most relevant to their investment area
Construct a document set with all the ideas
For each thrust within an investment area, find all the ideas similar to that thrust
Sort the results by similarity
Flag those ideas that are very similar to a thrust but were not submitted to it.
How can they improve the process so that this doesn’t happen as frequently?

24. CallView: System overview
Clients
(web pages, interactive viewer)
XML over TCP, HTTP
CallView
CVServer
(VTK + Qt)
VxOrd
(Graph layout)
STANLEY
(Text analysis)
SQLite
(Documents)

25. CallView: Results
CallView integrated into the standard LDRD idea submission process
Server handled about 1000 requests over the 2-week submission period
Positive reactions from LDRD staff, VPs, and end users
Development continues on use cases #2 and #3
Investment area manager tools should be online right now

26. CallView: Lessons learned
Neat graphical interface isn’t always the right thing
Convey the information necessary in the simplest format sufficient to users’ needs
Sometimes a web page is all you need
Developing distributed systems is hard!
Robust network code is hard to write and debug
Following a specification incorrectly can be worse than not following it at all
There is no substitute for testing on real data.

27. What’s next?
We have enough infrastructure to build general information visualization tools
Many, many applications: homeland security, intelligence, business planning, patent & publication analysis…
String support in VTK is general enough to handle data annotation as well as string attributes

28. Where do I get it?
String support is currently in a branch of the VTK CVS repository. With luck, it will be released with VTK Without luck, it will be in VTK 5.1.
If you want to play with it before then…
CVS branch: VTK-Sandia-InfoViz
If it breaks, you get to keep both pieces

29. Thanks to… Will Schroeder, Kitware
Nabeel Rahal, Keith Ortiz, Brian Wylie, Hank Westrich, and Travis Bauer, Sandia
Desert Sky Software