For the last build I did a run-down of the overall parts list (found here) and for the new build I think it’ll be valuable to do the same as there are a few key differences. I think it might be surprising to see just how many off-the-shelf parts can be used in these builds. Future posts will go into greater detail on some of these items – particularly the computer and the projector.
Going from “top to bottom” the parts list for a DSI-based table goes like this:
- Projection layer/Touch surface – sits on top of everything and allows a projected image to be displayed
- DSI material – Some form of “end lighted” acrylic – it goes by various brand names (Endlighten being the most well known) but all exhibit the property that light entering from the edges of the material will be directed/reflected out of the face(s) of the material
- Infrared (IR) light source – In the case of a DSI build this typically means some kind of string of IR LEDs. I’ll be looking at a 5M reel of LEDs – totally beats making it yourself. This reel of LEDs will be wrapped around the edge of the DSI material.
- Mirrors – used to extend the throw length of the projector or to alter the path of it’s projection cone – might or might not need one of these in this build
- Projector – Magic image making machine!! Too much to talk about here, look for a future post.
- Camera(s) – These need to be filtered to pass only IR light, and more specifically only a certain frequency of IR light typically either 780nm or 850nm though sometimes higher frequencies are used.
- Computer – The heart of the system – performance matters.
- Framing and other construction materials – I’m still hoping to use T-slot aluminum extrusions for this but I’ll probably get some 1×1’s to build a mock-up design first
So what matters here? What’s different from the last build? Mostly the first 3 items.
First off, the projection layer – DSI materials are transparent, otherwise it’d be hard to get any of that light sent in from the edges out the faces, so they require a separate projection layer to show an image. This should sit on top of the DSI layer to keep the projected image as close as possible to the users fingers. This avoids the strange sensation of feeling “separated” from the interface and the frustration of having an “offset” between where touches are registered and where they appear to be displayed. This sensation is more exaggerated the greater the viewing angle gets. All this means is that this layer should be as thin as possible. Typical thicknesses are 3mm-5mm.
Next the DSI layer – This is the key to making the whole system work and is the defining trait of this kind of multi-touch build. I’ve covered it’s properties several times already so I’ll skip that for now. What’s important to note here is that this layer is the load-bearing part of the tabletop itself – it’s thick. It needs to be, so that you can get a lot of light into it from the sides which will come from…
Infrared LEDs – The lighting is now LED-based rather than laser-based. This is safer, first off, but it is more costly. This whole setup is more costly though so you’re either bought into that or not. You can certainly save some cash by buying a lot of high-powered IR LEDs and wiring them all up yourself but unless you’re really good at that kind of thing (I’m passably good but I’m also really lazy) it’s probably worth it to just buy something ready made that you know will work. A key difference is that rather than having a light plane above the table surface (where the LLP gets it’s name) the light plane technically IS the table and the IR is directly injected into it. You can make an LED Light Plane table (LED-LP) that works much like the LLP tables but my take on those are that you get the worst of both worlds – the cost of a DSI or FTIR table (lots of LEDs), and no fiducial recognition.
So that’s what I need to buy, well, technically I’ve already bought a good bit of it and that’s what I’ll talk about next time. I’m going to cover the major components and the thought process that went into buying them.
Till next time…
There are several core technologies available to build a multi-touch device – Frustrated Total Internal Reflection (FTIR), Diffused Illumination (DI), Laser Light Plane (LLP), and Diffused Surface Illumination (DSI) are among the most popular.
Previously I had intended to build my table based upon LLP technology as this was the most cost effective of the solutions and despite requiring a little extra TLC and setup provided very strong illumination and finger tracking. However, there are a few downsides to the technology, not the least of which is the chance of blinding yourself (no, really) with a powerful laser that doesn’t emit any visible light. This means you won’t realize you’re in trouble until it’s probably too late. The warning “Do not stare into laser with remaining eye” is ever so relevant here. If you’re careful you have nothing to fear but this is a concern. Another is that there’s no really good way to do fiducial (object) tracking with LLP. Some ideas were put forward but most are experimental and there is little software support for such a setup. Since I wanted to be able to track fiducials this was a negative, but I figured I’d get the table built first and then deal with this shortcoming later on.
Well, as my previous post pointed out, I’m starting a new build and with that new start comes a new technology choice as the basis for the build – DSI.
DSI utilizes a special acrylic with tiny micro-mirrors embedded throughout the entire surface that cause light injected into the edges of the material to be redirected out of the front and rear faces of the material. A simple writeup of the technology can be found on the NUI Group Wiki here. As you would expect a material that exhibits these kinds of properties doesn’t come cheap which is what put it out of reach on my previous build. With a larger budget this time around DSI has become a viable option. Given proper infrared illumination DSI provides solid finger tracking without the occlusion concerns of LLP and, most important of all, DSI provides a good basis for fiducial recognition and tracking.
Once I get a bit more design work out of the way I’ll be ordering the materials for this part (the actual surface of the table) – but for now I still have a PC to finish building and a projector to play with.