So, I am putting together a DI multitouch coffee table and thought I would post some shots of what I have so far. The box is an old plywood shipping crate that I converted. I need a bigger mirror so I can adjust my projector and camera properly to see the whole screen, but I set it up temporarily to see what I could get. The original IR LED arrays I made weren’t bright enough to see through my diffuser (which is vellum I believe) so I am using a desk lamp aimed at the ceiling for front illumination at the moment until my new LEDs arrive.

I opted to mount the projector outside the table in order to maximize screen space and minimize the height of the table surface. I am going to put some sort of enclosure around the projector eventually so it’s not quite so ugly.

Since the mirror is too small I can only see part of the screen with the camera. Also, the tape that was holding up the camera was slipping, hence the misalignment.

Woo, look at the smoke go!

Alternate desk lamp doing the illumination. Seems to work well with either one or both turned on.

Hey gazpachoking, Nice table! Love the shipping crate idea! I’d leave the rope handles on it
Keep up the good work, can’t wait to see more pics/vids.

I’m not sure I fully understand what’s going on here (sorry guys - new at this!) when using front illumination, the software detects hit points where the shadows from your fingers are, right? But why is there any shadow at all if you are projecting onto the rear of the surface? Doesn’t that prevent any shadows by overpowering the ambient light? Or maybe I’ve misunderstood the process here. Any info would be greatly appreciated as our existing DI set up isn’t working and our deadline looms ever closer so looking at possible alternatives/back up plans!

technodai - 13 May 2008 04:33 PM

I’m not sure I fully understand what’s going on here (sorry guys - new at this!) when using front illumination, the software detects hit points where the shadows from your fingers are, right? But why is there any shadow at all if you are projecting onto the rear of the surface? Doesn’t that prevent any shadows by overpowering the ambient light? Or maybe I’ve misunderstood the process here. Any info would be greatly appreciated as our existing DI set up isn’t working and our deadline looms ever closer so looking at possible alternatives/back up plans!

The projector mostly projects in the visible spectrum, I have 2 layers of exposed negative in my camera which filters out visible light, and only let’s IR light through. My desklamps and the sunlight that comes in the windows contain quite a bit of IR light and as a result create IR shadows on the screen which the camera can see.

Ah I see now. I didn’t realise you were using infra red from the lamps (thought it was picking up the shadows from visible light). Thats cool, we’re having difficulty with our rear DI setup at the moment because the illuminator is pathetically weak. I wonder what would happen if you put a high power torch inside the box but put a visible light filter in front of the bulb? So that the torch/lamp only gave off IR. Of course the IR would be filtered twice, once by the web cam filter and once by the filter as it leaves the torch, but maybe could be used to fix a dead spot? Has anybody tried this? I know you can get filters that clip onto maglite torches so that they only emit IR - they are used by the military for covert lighting. Cheap IR source anyone? Hmmm....

technodai - 14 May 2008 06:43 AM

Thats cool, we’re having difficulty with our rear DI setup at the moment because the illuminator is pathetically weak. I wonder what would happen if you put a high power torch inside the box but put a visible light filter in front of the bulb? So that the torch/lamp only gave off IR. Of course the IR would be filtered twice, once by the web cam filter and once by the filter as it leaves the torch, but maybe could be used to fix a dead spot?

Yeah, my illuminators were also too weak, which is why I am using the front illumination right now. In fact I am a little worried that it will not be bright enough even with my new LEDs which haven’t arrived yet. I tried sticking those desk lamps inside the box (while I wasn’t using the projector,) and my finger blobs were still very weak. I’m considering getting a new diffuser if I still have that problem with the final illuminators installed (maybe frosted glass/acrylic.)

I’m not sure if it will help your set up, but we had to replace our projection material as it was blocking too much IR. We already had the acrylic and as it would have cost too much to get it sand blasted, so we looked around for other ways to achieve the same goal. We tried some frosted film from a local DIY shop but it looked horrible and was not frosted enough to support the projection. Then we found this http://www.purlfrost.com/purlfrost.php

‘White purlfrost’ seems to be just right, it diffuses the IR nicely and serves as a good projection surface. Not too expensive either we bought 2 metres for £22 (plus postage)

Obviously not ideal (if we had known we would have bought frosted acrylic in the first place), but if you are in a bind and need to find a suitable surface for low cost, it’s a good solution. It’s definitely cheaper than buying a whole new piece of acrylic if you already have one!