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June 18, 2009


Multi-Touch Table – Mirror, Mirror on the Wall

In the previous installment I discussed selecting a projector.  This time I explain what to do when you finally have selected a projector and realize that there’s no way to get the image size you want within the confines of the enclosure you want to build.  The answer is mirrors.

As I had previously stated I wanted an enclosure size of no more than 30″ high and 34″ wide and a screen size of 45″ diagonal, but the throw length of the projector I choose is 56″ to get that screen size.  Here’s a step by step walkthrough of how to make 56″ of crap fit in a 30″ box.

First off how did I calculate a 56″ throw distance?  I used the projection calculator found here.  The results of which look like this:

Throw calculation for Epson 700

Throw calculation for Epson 700

It’s also a good idea to download the user manual for your projector as this will contain valuable information regarding the throw and placement guidelines. The relevant information for my projector looks like this:

Throw and projection cone from user manual

Throw and projection cone from user manual

The important things to note is that the suggested throw matches well with the previous calculation and that there is no “offset” in the image, the bottom of the image is projected straight out from the center of the lens and makes a nice right-triangle (the +/- (B) data is used for 16:9 mode, not 16:10 and I plan to use 16:10).  Armed with this data – 56″ of throw for a 24″ high screen I can now start “folding” the projection – and I mean literally folding it.  Using a method suggested frequently on the NUI Group forums I translated the projection triangle of the projector onto a sheet of paper.  First I used TurboCAD to make a precise scaled triangle with one leg of 56″ and one of 24″:

An utter waste of TurboCADs capabilities

An utter waste of TurboCAD's capabilities

I then printed this triangle out at 1/5th scale and cut it out.  It’s also advisable to make a scale cutout of your projector itself and attach it to the end of your triangle but I found it much easier to play with different folds without it.  Still it’s good to have.

After a while of attempting various single bounces I realized I’d need to do two bounces (much harder to get right) to get everything inside the enclosure dimensions I wanted.  I spent a good bit longer playing with options here until I came up with the following:

Its origami with light!

It's origami with light!

I was able to measure (roughly) the dimensions and scale them up to get a feel for if this would fit inside my enclosure.  It looked good (it is kinda hard to accurately measure something shaped like that) at first glance but I wanted to use another tool to validate.  There just so happens to be a tool that can do this located here (beware this thing does some awful things to my CPU but it DOES work – eventually).  After working with the tool to mimic my folding attempts I got the following results:

Projector settings

Projector settings

Mirror settings

Mirror settings

If you look in the lower left you’ll see the projected screen size – all I cared about here was 24″ in height, this calculator only does 4:3 aspect ratio calculations but if I hit 24″ high I’ll get a 38″ wide image.  Looking at these results I can indeed use two mirrors to make a 56″ throw fit inside my 30″ high box.  The projector is sized per the user manual and I tried to leave a little room to clear the cables off the back.  The projecter is off to one side a bit, sticking out beyond the projected image but I plan to have a lip around the entire table so I’d just match that offset on the other side so that everything looks symmetrical – I need a place to put the computer and PSU anyway.  Since I’d likely add a lip of 4-5″, I still end up with a total width of 34″ (or slightly less) which is right where I want it.

Hopefully this information helps others go from projector selection, to throw calculation, to paper triangles, to mirrors, and finally to construction.  There has been a good bit of confusion on the NUI Group forums regarding this process so I wanted to detail it out a bit and show how each step relates to the next.

This is just a minor step along the way, I still need to figure out proper mirror sizes, and it’s unlikely that these calculations will all work out quite so neat and tidy when I actually drop it all into a box.  This just gets me to a point where I know I should be able to make it work, and for now that’s good enough.

Next time I’m hoping to have some lasers.  Cover your eyes.

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2 Comments Post a comment
  1. Chandra
    Feb 11 2010

    That’s really great work Rex….It is so useful.

    Thanks for help.


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