Thursday, March 20, 2008

Planning My Multitouch Setup

As it turns out, not all multi-touch surfaces are created equally. There were many decisions that had to be made before getting started. Here is what I decided on.

Touch Sensing
For homebrew multi-touch systems, there are basically two different ways to detect your touches. I will briefly go over each. The first is direct illumination, or DI. Basically, DI uses an infrared source to illuminate the touch surface and an infrared camera to see what is on the touch surface. This approach allows you to see anything on the touch surface, whether it be fingers, a paintbrush, or fiducials.

The other method uses Frustrated Total Internal Reflection, or FTIR. The theory is that when infrared light is shined inside the edge of a material such as glass or acrylic, it will be reflect internally to the other end of the material, never escaping. However, when a finger touches this material, it "frustrates" the internal reflection, and causes the infrared light to escape through the surface of the material. An infrared camera then tracks this on the opposite side of the material. Unfortunately, this method does not allow much else than fingers from being tracked.
The method I chose was FTIR. To me, it looked more stable than DI, not needing as much calibration and not being as susceptible to outside interference.

One of the coolest parts of multi-touch is the illusion it creates of manipulating virtual objects as if they were on the table in front of you. This requires the touch surface, and the display to be on top of each other. Much easier said than done. One method is to use a projector to shine the image on your touch surface. Projectors aren't cheap though, and I didn't have any spares lying around.

I decided to go with a more affordable method, using an LCD monitor. I know what you are thinking. "But you have to track the infrared blobs through the display, and LCD monitors are very much opaque." That is exactly what I thought. As it turns out, the actual LCD matrix in an LCD monitor is clear. It is just a piece of glass(?) that can color itself. Then, behind this is a backlight which allows you to see the image. By ripping apart an LCD monitor (gulp) you can take out the LCD matrix and mount it on your multi-touch table. You just have to figure out your own way to backlight it.

Multi-touch Surface
The final major design consideration is how you are going to display your multi-touch surface. Because I will only be using a 19" display, a wall display wouldn't be very impressive. Also, this being a science fair project it can't be overly large or I won't be able to fit it. My goal is to enclose it all in a box, about 2" on each side.

So that is my planned multi-touch table. I will post more progress, and some photos soon.

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