Messing with your head

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.

Ok, so here’s the deal…

My previous table build was cut off at the knees when my wife chose to get “separated” (I just love euphemisms!) from her previous employer before likely getting booted out later on anyway. We had to buckle down and deal with being a single-income family which did not leave much room for the indulgences required to build one of these things.

She now has a new employer and I now have a new project with a new budget.

The good news is that I did not buy very many of the parts for the previous build before it got shelved so not much was wasted. It also means (since such a great amount of time has passed) that I can get far more for my money today than I would have previously. Things like i7 did not exist back then! Also, high(er) resolution projectors with a short-throw have come down in cost opening up some new options.

Previously I was aiming for a budget of about $2000-2500, but my current plan is to double that amount. This lets me utilize a different (read “more costly”) touch detection technology and not have to scrimp and fret over every purchase decision as I was last time.

Future posts will detail the technology changes, parts selections, and project assembly, that is, if I can stay diligent about posting the progress on here, but I should have an updated parts list and cost tally in place soon (look in the upper right hand corner for the link).

Until next time…

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

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

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 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:

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

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.

-Rex

Selecting a projector for a project like this isn’t as easy as it might seem.  There’s a huge selection of choices available and price certainly plays a factor when trying to decide between them.  One of the key requirements for a projector going into a multi-touch table is the throw length and in this case length really does matter.

While I am waiting for some other pieces to arrive I decided to try and find a suitable projector for this project.  I really need to come up with a name for this, “project” isn’t very cool sounding.  As previously stated one of the key factors when looking for a projector to go inside a table is the throw length.  Specifically you want a projector with as short a throw length as you can get.  A projector with a short throw length will create a larger image at a given distance than a projector with a longer throw length.  There are “short throw” projectors available but they typically involve complex optics and have correspondingly high prices.  They can also complicate your table design since the steep throw angle often (but not always) requires placing the projector in such a way that it requires significantly more space on one side of your design than the other.  With all of this in mind I set out to find the best projector for my table – that I could reasonably afford.

Requirements

Before you get into this too deeply there’s a few key things you need to know:

• What is your desired screen size – how big do you want your projected image to be?
• What is your desired enclosure size – how big do you want your table to be?
• What resolution and aspect ratio are you planning to run your table at – 800×600?  1024×768? 1920×1080?!
• What environmental conditions will you be using the table in – Dark room?  Full sunlight?  Something in between?
• Your budget – how much can you spend?

Once you have answers to those items you’re ready to go.  In my case, I’d like a roughly 45″ diagonal screen size at either a 16:9 or 16:10 aspect ratio.  This amounts to an image about 25″ high and 38″ wide (slightly different depending on which aspect ratio).  Because of the aspect ratio I want I am looking for a projector with 1280×800 or 1280×768 resolution.  I had initially hoped for a 1080P resolution but then reality hit (see the budget part).  As for the enclosure, I don’t want my table to be more than 30″ high which is typical desk/table height (between 28-30″) and I don’t want it to be more than about 34″ wide – since it won’t fit through the doorways/stairways in my house if it’s wider.  I imagine the table will be roughly 50″ long but that doesn’t really factor in here – the key dimensions here are the height and width.  I also want the table to be usable in fully lit rooms, perhaps not full sunlight (that causes it’s own issues with IR tracking) but certainly partially sunlit rooms are on the list for me.  My budget for the projector is $500 – which is , as I quickly realized, too low.  More on that later.

Projector Resources

Now that I had some parameters to work with I set about finding suitable models.  A couple of great resources for this are:

http://www.projectorcentral.com

and

http://www.aboutprojectors.com

Between them they cover a huge range of manufacturers and models and give pretty detailed specs on all of them.

Reality Strikes

I spent about 2 days combing through looking for projectors matching the specs I wanted.  Once I had the list narrowed down a few things became apparent – my budget was woefully small, and nothing even within the realm of twice my budget was going to get me the screen size I wanted within the size contraints of my enclosure.  The first of these issues was addressed by setting my budgetary expectations to a more realistic level.  While there are several 800×600 and even a few 1024×768 projectors around that can be had for just over or around $500, there are not many 1280×800 projectors that can.  While there are many 1280×800 projectors that list under or near $1000, none of them are true “short throw” projectors – those list for somewhere around $1500-$3500.  Projector brightness or lumens is also a factor here.  Some projectors output 1200-1600 lumens – this is still really bright, but maybe not bright enough for full daylight viewing or when paired with high contrast projection materials.  I wanted something with 2000+ lumens.  This, also, means more money.

The obvious first step after this dose of reality is to think “well, maybe I can find a used projector and save some money – I’ll hit my budgest that way”.  And that seems perfectly reasonable.  The problem with that idea at the level of projector I am buying is that replacement bulbs cost 1/3 – 2/3 the price of the whole projector.  So if I were to find a $1000 projector for $500, but had to buy a replacement bulb (within a few weeks or even months), I might end up paying $800 in the end (bulbs typically range between $200-$300).  At that point I’d be better off just springing for the new model, getting a warranty, and knowing the bulb should last a good, long time.  So I upped the budget to $800-900.  Really only two projectors made this cut (though there are lots of projectors in the range of $800-$1200).  The first is the Dell 1609WX, the second was the Epson PowerLite Home Cinema 700 (hereafter known as “The Epson”).  The list prices for these are $849 and $799 respectively.  Features and specs are very similar with a few differences – here’s a comparison.

Final Evaluation

Here’s how I made my selection-

1. Price – the Dell lists and pretty much sells for $849.  The Epson lists for $799, but sells for as low as $749.  Advantage Epson.
2. Throw distance – The Epson throw ratio is at 1.5, the Dell, 1.55 (smaller is better).  The Epson has a better zoom.  The difference in throw distance for the screen size I want ends up being about 3″.  That seems insignificant but as you’ll see later, every inch counts.  Advantage Epson.
3. Display Technology – The Dell is DLP, the Epson is LCD.  Most would say DLP gives a better picture with greater contrast, and it’s probably true, but the rated contrast gives the edge to the Epson (but never trust those numbers folks!) so how to decide?  In this case, because of the potential for strange mounting orientations LCD is a safer bet.  DLP has moving parts and they are engineered to work properly witht he projector sitting on a table, or hanging from the ceiling – both cases projecting horizontally.  In my case I may need to mount the projector vertically and that means possible issues with the DLP color wheel.  Advantage Epson.
4. Size and weight – the smaller it is, the easier to mount/position.  The Dell wins here but the margin is slim, still, inches count.  Advantage Dell.
5. Brightness – The Dell is rated at 2500 lumens, the Epson, 2000.  Since both meet my requirement of at least 2000 I needed to look a little deeper.  Heat is going to be a concern in this box and honestly 2000 lumens might be overkill anyway.  In most cases the Epson should run cooler since it should have a less powerful lamp = less power draw = less waste heat.  Advantage (conditionally) Epson.
6. Connectivity – This is pretty much a wash as all I cared about was a digital video input but since the Epson supports HDMI it does reduce the total number of cables required if I decide to use the projectors speaker (and I probably will), it’s smaller physically than a DVI connector and easier/cheaper to find right-angle connectors for.  Advantage Epson.

All in all, the Epson solidly wins.  So now I have a projector.  There’s only one problem…  To get the screen size I want requires 56″ inches of throw and I have a box no more than 30″ tall.  Fun.

Tune in next time for the answer of how to fit 56″ of crap in a 30″ box.

Remus out.