Physics in touch
Designing interfaces for multi-touch tabletop computers;
manipulating object with and without pysics
The whole point of interface design is to create a natural/intuitive interface. However, since computers are not natural by definition it is hard to decide what is natural. In the classic WIMP interface the desktop has been used as a metaphor to design the interaction with the computer. Commonly a traditional desktop is horizontal and a computer screen is vertical. This and many other shortcomings of the metaphor creates a mental gap which has to be bridged by training. This training has altered our perception of ‘natural’ or intuitive interfaces for computers. For the trained user it makes sense that pressing the right mouse button pops up a context menu. Also it is only possible to move a window by dragging its handle bar and it is not possible to rotate a window. Obviously the desktop-metaphor can only go so far. There is no such thing as a menu on a classic desktop, not can one resize a piece of paper easily. It is not hard to point out the short comings of the WIMP interface, nor is it very difficult to create a ‘better’ interface. Unfortunately the average user is used to the Windows interface and invested a serious amount of time to be able to navigate it. So any new interface that works with similar in-/output devices as the standard PC has to unlearn the Windows way and learn the new way. Obviously this is quite a hurdle for people and as such new interfaces seldom survive let alone go main stream.
Enter tabletop computing…
Since tabletop computing hasn’t developed a standard interface, yet, we still have a chance to create new metaphors which will stick. Apple has managed to get a serious grip on multi-touch interface design. Many people believe that the multi-touch interface has empowered the designer to create such a novel interface. However it is my believe that only a few actions really need multi-touch, zooming being one. I don’t think people really want to use two hands to control a computer when they can do it with one hand. Since tabletop computing is much more akin to the classic desktop we can have another look at this metaphor. To rotate a piece of paper you don’t put your two index fingers on the sheet of paper and start rotating. If you want to rotate it 180degrees you probably don’t even try to turn it out of your wrist. Instead you grab it at a corner and give it a twist. – To be honest, since you can manipulate the piece of paper in 3D it is most likely that you grab the piece of paper from the table and turn it in the air 
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However if we stick to the concept of 2D manipulation we can use it to enhance the current metaphor. Even if you don’t know Newton’s laws of motion, you still expect them to work. Turning a piece of paper by putting a finger in the corner and moving it, whilst counting on the inertia and drag of the paper to rotate instead of translate the paper doesn’t require any degree in physics. Few people would know which forces are at work on the paper, even fewer do care, only the result counts. Why does one not use a multi-touch gesture as used in the iPhone? Basically because it is more complicated. Look at the very simple animation below. The upper user is using physics to turn its object, while the lower user is using multi-touch. As you can see the upper user can keep on spinning if he likes, while the lower user runs into a wrist issue.
manipulating object with and without pysics
While in real life we don’t need to understand the Newtonian laws to be able to use them, in the software world it is not that simple. Since there is no natural gravity within the computer, the Newtonian laws don’t apply naturally either. We need to introduce physics into the GUI. Fortunately physics is very important in current games and a lot of effort has been put in creating physics engines which recreate the Newtonian world of physics in the virtual world of the computer. We are planning to use Farseer, since it is both open source and runs in C#. Having physics at our finger tips we can create much cooler interfaces. Instead of having two seperate actions to translate and rotate an object, you can do it in one fluid motion (upper user). Moving an object is also much easier, just give it a push in the right direction (lower user).
Using physics to manipulate objects – please note that the third hand of the lower user doesn’t touch the table anymore -