Bringing Physics to the Surface

Wilson, et al.

Summary:

This papers looks into how to effectively add real physics to tabletop touchscreen surfaces. The idea is to use physics that have already been developed for the game industry, e.g. Nvidia's PhsyX. Surfaces already mimic physics when users rotate or stretch out pictures with their hands. This is not real physics computation though, they are merely scripts that replicate real physics.

There are three different forces that must be considered when using the physics computations: static friction, kinetic friction, and collisions. There are already methods for computing these forces, but none that compute all three well. Direct force is one method that only detects collisions based on one contact point. Virtual joints and springs connect a contact point to a virtual object via a spring or a joint. This results in a drag-and-drop functionality and does not work well with collisions. Another method is to use proxy objects. This creates an object, such as a sphere or square, underneath the contact point that can interact with the virtual objects by means of friction and collisions. Although this uses all three forces, the results can be unexpected because the proxy object does not match the contacts points.

To compensate for the weaknesses of the previous methods, a new method is introduced: proxy particles. This creates a stream of particles underneath the contact point that accurately represent all contact points. These particles act in the same way that a proxy object works; if the contact point starts on an object, friction takes over. Otherwise, if the contact points starts off of an object and runs into an object, collisional force takes over.

User studies indicate that proxy particles may be the best way of applying physics to touch surfaces. Six participants were given three tests each. The joint method and the proxy particle method had the fastest completion times. The joint method was easy for the users to pick up because it keeps in line with the drag-and-drop mentality that mouse users are accustomed to. However, the participants commented that the joint method was "limiting" and "less satisfying." The joint method also poses problems when creating two separate contact points on an object and pulling in separate directions. When using the proxy particles, users interacted with objects with comparable times to the joint method, but enjoyed it more.

Discussion:

These surface computers are getting popular and the idea of introducing real physics into the technology looks very promising. There are still some kinks to be worked out though and I think the initial surfaces will only contain pseudo-physics. Real physics will have to be introduced later. While the proxy particles worked well for the tests given, it does not look like it is ready to be commercially introduced. Problems will occur once the surface becomes filled with 3D virtual objects and cluttered. There will have to be a logical way to apply 3D physics and move 3D objects around a 2D screen.