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It all started while we were researching an article on future user interfaces. Touch interfaces are hardly futuristic at this point, but multi-touch hardware like the Microsoft Surface or the iPhone is just starting to become a big deal, and we decided to see what big things are going on in that field. What we found that surprised us the most wasn’t anything about the future of multitouch; it was about something that people are doing right now.
There is, it turns out, a whole community of very smart folks out there on the internet perfecting the art of building DIY multi-touch surfaces. The process isn’t exactly simple, but the results we saw were stunning: multitouch surfaces with responsiveness rivaling Microsoft’s $12,000 offering, built in a garage on a shoestring budget. “Future UI article be damned,” we thought, “we’ve gotta build one of these for ourselves.”
And so we did. We documented the whole process, from start to finish, so that you can try building one of your own, if you’re so inspired. We’re not going to claim to have done everything perfectly the first time, so think of this article as more of a build log than a definitive how-to. Still, we’re very pleased with how the table turned out. We’re so pleased, in fact, that we put together a video showing the table in motion. Check it out, and read on to find out how we did it!
First, some acknowledgments are in order. Virtually all the techniques used to create this table were discovered at the Natural User Interface Group website, which serves as a sort of repository for information in the multitouch hobbyist community. If you find the technology shown in this article interesting, you owe it to yourself to check out their forums, where you can participate in the development of multi-touch hardware and software.
Before we can get into the actual, physical construction of the table, it’s important to understand just how it works.
There are several different ways to make a multi-touch surface, but we’ll focus on the one that we employed: the FTIR screen. An FTIR (short for Frustrated Total Internal Reflection) setup involves three vital components: a sheet of transparent acrylic, a chain of infrared LEDs, and a camera with an IR filter. The LEDs are arranged around the outside of the sheet of acrylic so that they shine directly into the thin side surfaces.
Once the IR light is inside the acrylic, it strikes the top and bottom surfaces of the acrylic at a near-parallel angle, and is subject to the effect known as total internal reflection. This causes it to be wholly maintained in the acrylic. This is a little tough to describe in words, so we’ve made a simple diagram:
The net effect of the setup described above is a sheet of acrylic full of internally reflecting infrared light. When a finger is pressed against the acrylic, it causes some of the light to be reflected down, through the acrylic and into the cabinet, where it is detected by the webcam. This effect, called frustrated total internal reflection is a little complicated, and involves something called an evanescent wave, but you don’t really need to understand why it happens, just that it does happen, as illustrated in this diagram:
The webcam, which has been modified to detect only infrared light, sees this reflected light as white spots on the screen; something like this:
From that picture, software running on a nearby computer extracts a map of where fingers have been pressed on the screen, and uses that data to control multi-touch applications.
An FTIR multi-touch table’s screen is comprised of three basic components: The acrylic sheet, the LED lighting, and the projection surface. Each one requires a bit of work, so we’ll discuss them one by one.
The foundation of the screen is the sheet of acrylic which serves as the medium for the infrared light. Why acrylic? Acrylic has several properties that make it a good fit for our project. First, it has the right optical properties, allowing for an excellent FTIR effect. Additionally, it’s lightweight, strong, and very clear (more so than glass).
We constructed our screen from a 24” X 30” X 3/8” acrylic sheet, which we bought at local plastics dealer TAP plastics. Acrylic can also be purchased on the web, although high shipping costs mean that it’s best to try and find a local plastics dealer. For a 24” X 30” sheet, 3/8” is thick enough to prevent any noticeable sagging in the sheet, even when firm pressure is applied to the middle of the screen. A larger screen would, of course, require thicker acrylic for stability.
Most acrylic comes with saw-cut edges, which are rougher and therefore more opaque. Because we wanted to shine infrared light into the acrylic from the sides, we needed to do something to guarantee a clear edge. We had two options:
1. Buy laser-cut acrylic. Most large plastics stores will have the capability to do this, though they generally charge extra. Because we here at Maximum PC always have our mind on our money and our money on our mind, we decided to go with option number 2:
2. Buy saw-cut acrylic, then polish the sides ourselves. To do this, we simply gave the edges a thorough sanding with 200-grit sand paper, then went back over each edge with Dremel rotary tool using the buffer attachment and a polishing agent. This certainly took a little while, and involved inhaling more acrylic dust than is probably healthy, but the result was a nice, clear edge.
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January 18, 2012 at 5:52am
Hi Alex,
Thanks for all the tutorials and explanations provided. We really appreciate. We are working on developing MT with Sony PS3 camera fallowing your instructions. We managed to Install touch Lıb and test PS3 camera with advised software.
However, I can not get a sign whether LED's get 12v and working and test whether they Camera sees IR lights? We will appreciate if you can kindly advise us.
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I'm trying to realize multitouch screen with using Acrylic glass covered with projection material (tracing paper) and IR LED strip that is sticking around the glass. I'm experiencing the problem - IR camera doesn't see position of my fingers placed on the projection material, however it sees my fingers w/o this tracing paper (on bare acrylic glass). My conclusion is that tracing paper blocks IR. For now I don't use any compliant surface. Do you think that adding compliant surface would help ?
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