One important thing required to make a multitouch surface really usable is the surface on which the image from the projector is cast. Without it fingers can be tracked but we just do not get any real feedback underneath them.

Searching the Internet showed that a lot of people decide to use expensive rear projection surfaces. This is indeed a good solution … if you have a lot of money and a local retailer. Which we did not have. So instead we decided to go with the second best solution and used tracing paper.

Besides that just using a projection surface of any kind again complicates things. Because the surface is usually placed between the acrylic and fingers it ruins the FTIR effect needed for blob tracking. This can be solved by coating the projection surface with a thin layer of silicone that produces a similar rubbery surface to that of a finger. When the pressure of a finger is applied on the projection surface the coating on the other side triggers the FTIR effect and everything works fine again. This kind of surface is then called compliant surface.

Of course the silicone has to be fluid enough that we can distribute it over the surface evenly. For that many people have used some factory prepared stuff which we were not able to find in Slovenia. So we decided to use a bit denser silicone in tube (used to seal some cracks and such) and dissolve it. Silicone is dissolved using toluene or xylene. Both solvents are extremely flammable … and hard to get under their original name in Slovenia. Fortunately there exists a much easier to get solvent branded as Nitro that is actually just another name for 100% toluene. Dissolving the silicone was pretty easy after that … just make sure that you are doing it outside. Some images of the process can be seen in my gallery together with images of our first test.