DIY 30" Multitouch Display: Introduction

First of all, let me say that all credit for this project goes to turkey tek and his instructable, avaliavble here: http://www.instructables.com/id/Interactive-Multitouch-Display/

Having read turkey tek's instructions through a few times now, I have decided to embark on a very similar project.

Goals:

• Diagonal display width: 30 inches (24" width x 18" height)
• Use a single 1024 x 768 resolution LCD projector for the display portion.
  • Note that both the display size and the projector resolution are both in a  4 x 3 ratio so they should scale nicely together.
• Use a single Nintendo WiiMote (tm) along with the FTIR method described in the instructable to track at least 4 "touch points" at once. 
  • If the wiimote proves too troublesome or insensitive I intend to try the modified webcam method described in the instructable.
• After developing a proof of concept, model create a cabinet or stand to hold the display, wiimote and projector in alignment.
• Connect the peripheral to a computer and explore uses of the interface.

Possible problems and concerns:

• How well will the wiimote's (tm) visible light filter perform given the brightness of the projector?
• Will the projector's throw distance to be short enough to incorporate it into a cabinet or will a relay mirror be necessary?
• How well will the wiimote (tm) be able to "see" the reflected IR from the Leds, what wavelength will work best?
• How many leds will be necessary to provide a sufficient amount of IR light? How much power will these LEDs require and how best to break them into groups?
• Will the wiimote (tm) be able to see the IR light through the diffuser well enough to get an acurate position fix?

Parts List

Screen Components:

• 84" of extruded aluminum U channel: Border and LED holder and light baffle.
• 1- 24" x 18" x .220" Optix for the FTIR touch surface.
• 1- 24" x 18" x .080" Optix for the bottom layer of the diffuser sandwhich.
• 1- 24" x 18" Piece of vellum drafting paper for the light diffuser.
• 1- Wiimote(tm)
• 1- Bluetooth dongle and interface software (I have this already from previous wiimote(tm) projects)
• Clear Silicone Sealer and hot-melt glue for screen edge
• About 54 IR Leds, give or take a few on the corners.
  • I currently plan to place the LEDs about 1.5" apart on all sides, although this may change.
• Wood and hardware for frame / cabinet.
  • More detail on this when I get that far.

First surface trial and test

As I mentioned in the last post I decided to try simple spray paint before looking at a more complicated (expensive) projection surface. The goal is to get a surface that maintains the polarity of the light as it is reflected back to the viewer. See the last post for links to a few commercial products. 

For my test surface I am using a 24" x 24" piece of gypsum board, the surface of which is probably too rough for real use but hopefully will make a reasonable test surface.

Surface 1: Cleaned surface followed by two coats of white spray paint to insure that the gray gypsum board color does not interfere. The final later is two coats of Silver Rust-oleum (tm) Brilliant Metal Finish spray paint.

It's worth noting that the metal flakes in this paint are very prominent, much more so then I thought when I bought it. this makes for a fairly rough finish that I do not think will work very well.

Test 1: Despite my misgivings about the surface I decided to go ahead and test the surface anyway. My test consisted of a single old 3-LCD projector, projecting through the circular polarization filter onto the surface. I simply put on a pair of RealD (tm) 3d glasses left over from a recent Avatar showing and subjectively compared the image brightness reflected to each eye.

Results: There was an apparent difference in brightness, indicating that the surface is not entirely eliminating the polarization. However, the difference was only about 10% overall brightness.

Conclusion: I am confident that the polarization filter is working properly but the surface is changing the polarity too much. I think this is due in part to the roughness of the surface but also due to the large metallic flecks in the paint. I will try a similar paint with a smoother finish and then look into a smoother base surface.