Another, actually quite nice, approach for multi-touch sensing, here shown on a laptop… Sadly the youtube video has been removed. So I will try to explain the principle instead:
Read the whole story at I STARTED SOMETHING
Last but not least this guy shows how to achieve the same (multi touch sensing) with a bag of dyed water and an isight, hehe : $2 Multi-touch. Yeah, ok, some C-coding involved as well…
To get you started with this topic (maybe for your own experiments), here some rough ideas:
Basicly you will need some acrylic glass, a hand full of white leds, reflective tape, black tape, a cam and some decent software which is not too hard to code:
The effect providing us the ability to actually “see” the points where you are touching the surface is called FTIR or “frustrated total internal refraction”. More information can be found at the wikipedia entry
To sense the coordinates where the object interacts with the surface the process would be to project from the rear onto the surface. The cam will capture the resulting projection on the surface. If you start to subtract the projected and captured images the result will be null.
But if you touch the surface the resulting image will be the areas as shown in the images:
If you normalize the image you will have a rougher image (vary the resolution according to your CPU capacity) which you can easily analyze.
