1. ElastoLab A Physics Playground for Kids David Buck Simberon Inc. www.simberon.com

2. History Started as a Master’s thesis – 1990 Smalltalk Re-wrote in C++ (1993) Close to release (1996) Rewrote in Smalltalk (1998) Released for sale (2001) Physics re-written in Smalltalk (2002)

3. Demo

4. Physics How do you simulate physics? Easy: –Calculate the forces on each particle. –a = F/m –Add a bit of the acceleration to the velocity –Add a bit of the velocity to the position –Repeat –Euler’s method

5. Problem Falling particles work great Springs explode when the strength gets too high

6. Solution: Runge Kutta Calculate forces 4 times, combine results.

7. Better But really strong springs still explode. Solution: Adaptive Runge Kutta Adapt step size according to error between single and double steps.

8. Barriers Easy – you just reflect the particle’s position and velocity by the barrier

9. Umm… Problem Springs explode and rigid bodies don’t spin. Solution: Step forward to the time of collision then apply an impulse to the particle

10. What time is the collision? How do we know what time the particle hit? What if many particles hit around the same time – which hit first?

11. Solution Use Bisection Method to narrow down time of first collision.

12. Resting Contact? What happens if the particle is sitting on the barrier? Collision time is almost zero.

13. Dynamic Constraints Detect resting contacts and add a dynamic constraint force. The constraint force is a force just strong enough to prevent the particle from penetrating the barrier. Net Force Constraint Force

14. Multiple resting contacts What happens if a particle is in resting contact with two or more barriers?

15. Solution Use Quadratic Programming or Baraff [94] Ok, just stop the particle and hope nobody notices.

16. User Interface Every physics object has a corresponding “Display” object. A “Camera” defines the transformation from world to display and back. Clipping implemented for lines and polygons.

17. Custom Widgets Custom widgets enhance the appearance of ElastoLab.

18. Interfacing to Sound Smalltalk calls a C DLL I wrote The C DLL calls the COM interface to access the DirectX 8 sound facilities.

19. Help Facilities Three kinds of help –Fly by help –Help mode – Windows help files –Guided tour

20. Guided Tour

21. Source Code Management For C++, I just made copies of the source files. Smalltalk: StORE using a Postgres database. ElastoLab consists of 2 bundles, 15 packages, 300 classes.

22. Making it Fast Displaying bitmaps works much faster if you use convertForGraphicsDevice: and cache the result. High level optimizations better than low level optimizations

23. Packaging Runtime Packager used for stripping image. ResHacker used to create one executable. InstallShield Express used to install.

24. Message Board Online message board runs on Web Toolkit. SSL pages link to Smalltalk servlets Small app (4 classes)

25. Wrap Up Smalltalk is quite fast enough to handle ElastoLab. Flexibility of Smalltalk is a big advantage. Packaging can create a single executable that fits into the Windows environment. SSP pages and Smalltalk servlets can be used to build community with a message board.