Monday, 30 November 2009

Multi-Touch Screens Research

The technology I chose to explore in the next project is the multi-touch screens, screens that recognize two or more "clicks" at the same time. Weird as it might sound, it is much more cheaper to build your own multi-touch screen rather than to buy one, since that would cost a few thousands. In my research I found that there is a whole community of DIY multi-touch screens' enthusiasts ( who since 2006 have build a great content with tons of information, "How-To"s and tutorials. That made me thinking "So what if I decided to do so?"

The disposable Multi-Touch technologies by now are explained and compared here. I spent some time looking after which table would be the best (robust and easy to build) to try implementing. I ended up with the FTIR (Frustrated Total Internal Reflection) technology which is also one of the most popular in this community.

The multi-touch screen with the FTIR technology works this way (quoting from here):
 "Infrared light is shined into the side of an acrylic panel (most often by shinning IR LEDs on the sides of the acrylic). The light is trapped inside the acrylic by internal reflection. When a finger touches the acrylic surface this light is “frustrated” causing the light to scatter downwards where it is picked up by an infrared camera."
Click to enlarge

So the camera that captures only IR light, when we touch the FTIR screen shows something like that:

Straight-forward and easily understandable.

The Screen

There are 2 ways to place the IR LEDs to light the acrylic. Either with soldering and wiring about 100 LEDs or by using a LED Ribbon. There is a very big difference in the cost between them but the cheaper one takes ages and requires familiarity with electronic stuff. I haven't make up my mind which one is the most efficient yet.

The Surface

Now is a whole lot of story, to create a surface that would push the acrylic only where our finger is (it won't stay pressed, and it would be very sensitive for cases of dragging etc). Thankfully, everything has already been done! So the surface of the table is constructed by the following materials in this order (click to enlarge):

Rear-projecion foil and Infrared blocking foil.. hm I'm not surprised that I don't understand what should I put there. But according to my research the best combination, should be something like:
  1. Polyester drafting film, 
  2. Vellum, 
  3. Silicone & Zylol with this technique, 
  4. 3/8th clear Acrylic

The cabinet
In the FTIR table, the IR light is captured by the IR camera and the projection is done with a front-side mirror, since the short-throw projectors are much more expensive. So the placement diagram is something like that (click to enlarge):

IR Capturing


The List
If we were to make a short list of what is needed we would have:

1. LED Ribbon (or soldered and wired LEDs)
2. Polyester drafting film, Vellum, Silicone & Zylol, clear Acrylic
3. IR Camera (PS3 Eye), floppy disk
4. PC: Core 2 Duo and 2 GB of memory
5. Short-throw projector (or home-theatre projector and a font-side mirror),  heat-absorbing glass
6. (Wooden) Cabinet

Thankfully, there's lots of free and good software for multi-touch tables available in the web.

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