Multitouch Display - Part Two
Table of Contents
(last update November 13th 2007)
Part One
Part Two
Part Three
Ok, I want to build one! What I need?
If you reached this point, probably you're starting to think that, maybe, you want to build your own multitouch display. Otherwise, you're still thinking "oh my, but it sound to me too strange that I can build one at home, without an ultra-tech lab!". If your on this position, I suggest you to jump to the section where I explain how I built mine, so you can figure how simple it is compared to what you're probably thinking of; otherwise, if you already decided to build one, this is the right page to read on.
Building a multitouch is something very near to bricolage with some electronics and, of course, come computer skills. In order to have something useful, more or less you should be able to build a frame made of alluminium and/or wood and solder some LEDs together, nothing special.
Another thing that probably is useful to point out is what you need, in terms of tools, to build an FTIR multitouch. More or less this tools should be enough:
Choose your destiny
At this point, you probably already understood that the main choice is beetween the FTIR and the DI approach. Of course this is your decision, I can just say that, probably:
Design considerations
The second step is choosing the size of your setup. To keep things simple you will go for a display with a 4:3 aspect ratio, other ratios or even different shapes (circle, triangle) are possible but all the things gets more complicated. Keep in mind that the size of your installation will grow a lot choosing bigger display size because you have to consider also the distance that the projector needs to cover the whole display area. You can use the good Projection Calculator from ProjectionCentral.com to get an idea of the distance you need beetween the projector and the display in order to cover it all.
Alternatively there are some new projectors that are designed for very close projection. They can cover bigger screens from a very short distance. As example, you may take a look at the 3M DMS 700 series or at the Toshiba TDP-EX20U.
Probably you want just to start playing with photos with your multitouch immediately and you are not taking care of this kind of aspects but you should spend five minutes also thinking about how you will use your multitouch display. What I mean is about deciding if you want something like a normal table, a coffee table, a kiosk or a wall. There are some constrains also depending on this decision, for example if you're going to build an hi-tech coffee table you are limited in the display size by the height of the table itself. This is because even using a mirror to double the projector light path, your display size will be limited to the maximum area lighted by the projector at a distance from the display equal to the double of your coffee table height. If you're going for a kiosk you will need to make some test to find the size that will be suitable, in this case you should be able to reach with the same hand the entire area of the display while standing in front of it without moving your feet unless, that's a better case, you're thinking about a multitouch kiosk that may be used by more than one person at time. If you're going for a multitouch wall...you're lucky!
The size of the display is important also regarding the thickness of the plexiglas to use. You need a piece of acrylic glass that is at least the same thickness of the LED's you will use otherwise you'll waste a lot of infrared light and probably you'll get also some counter effects. In addition, the bigger size you want it, the thicker the plexiglas have to be. This is because if you're going for a big display, your plexiglas must be thick enough to not to bend under your finger touches. To determine this you have no other chance than go to the shop where you'll buy your plexiglas and make some test. Just to give you a rough idea, I used a 72x54 cm plate, 10 mm thick and it is pretty solid, I think that I could have been using a 8 mm with no problem at all.
Infrared LED and filter
Once you've decided the size and the kind of your setup, you need to consider how to illuminate it with infrared light. You'll use IR LEDs of course, the great guess, at this point is "how many?". Actually there are no formulas to calculate this. This is a number that depends from many factors like: size of display, power of LEDs, sensibility of the webcam, quality of the IR-pass filter, ambient light, multitouch display case etc. Let's spend some words about those factors.
If your display is big, you'll need more LEDs, this is obvious, especially for the sides where you're going to place your LEDs. But, let's suppose you're going to make a rectangular, 4:3 display and you're thinking about to place LEDs on the top and the bottom sides (like me). The space beetween the LEDs is depending also by the size of the vertical sides! Yes because the plexiglas will attenuate somehow the IR light, so, in order to illuminate also the center of the display, you need to "inject" more light at the top and bottom side and you can do this just by using more LEDs and decreasing the distance beetween each other.
The power of LEDs, of course, is important as well. If you will use powerful LEDs you will need less pieces even if probably this will lead to an uneven infrared light distribution.
The goal is to have visible "blobs" of IR light reflected by the finger touches. The important word here is not "blob" or "IR light", the question that matters is "visible". This means that the blobls need to be lighted and clearly visible inside the surrounding image. In other words if there is no ambient light you have more chances, if your webcam is very sensible you have more chances too, if you're using a great IR-pass filter the chances are raising as well. So, basically, if one of this factor is missing in your setup you have to balance putting more LEDs or using better materials elsewhere.
I know that this may be confusing so, let's try to come out of this fog. For my FTIR setup that actually, apart from the unfitting silicone rubber, is working quite fine I used 24 Osram SFH485P IR LEDs, 12 on the top side and 12 on the bottom side. The display is 72x54 cm so think about the fact that 12 LEDs for a 72 cm side are quite good because one Osram SFH485P every 6 cm on each side is quite good to illuminate a plexiglas lenght of 54 cm (shorter sides). To be really honest, I said "quite good" because I actually think that some LEDs more would be better, so think if I used 14 LEDs, one every 5 cm.
But keep in mind that this is true also because of the kind of LEDs I used that, as I said, are Osram SFH485P, they differ from the SFH485 (without the "P") for the beam angle that is 80 degrees in the SFH485P compared to the 40 degrees of the SFH485. I thought that this would help to better illuminate the plexiglas. Anyway, check out their complete features on the datasheet (PDF). In addition, I really suggest you to take a look a the infrared LED section of the NUI Group Forum and to the same section of the NUI Group Wiki as well.
Apart from the number of LEDs, there are other factors that will help you to achieve better results. If you use a IR-pass filter (like the one used often in remote control) you'll let your webcam to see just the light coming out from you LEDs which is, in SFH485P case, in the 880 nanometer range, so the webcam images will be less disturbed by the ambient light. As well, if you use a closed case for holding the whole setup, the multitouch will be less sensible to the surrounding light.
Keep in mind that, anyway, the image displayed will be a rear projected image so, if you're planning to use your multitouch in outdoor you'll probably experience low image visibility problems other than poor blob detection precision. In this case.....good luck and let me know!
Electronics and power supply
Once you defined how many LEDs your going to use you can think about how to power up them. You'll connect the LEDs in series like it is shown in the following picture:
Normally a LED is accepting a voltage of 1,2-1,5 Volts (double check on the LED datasheet) so you have to multiply this to the number of LEDs you're connecting in order to have the minimum voltage that the power supply has to give, so you will choose it keeping in mind that it should provide a DC current with voltage and power enough to power up the LED stripe.
Of course you
cannot connect it directly to the LEDs, unless it is providing the
exact voltage needed by the LEDs themselves, you have to use a resistor connected
to the LED stripe to adjust these values. You can easily calculate the
resistor's value with this great LED resistor calculating tool.If you want to save money, you can use the power supply of your PC where you have +5 Volts (red connector) and +12 Volts (yellow connector) but you should check if this current absorbtion is tollerated by the power supply while still powering up the PC itself.
Rear projection
In order to have clear images on the display, you need to use some material as a projection surface. Actually there are a bunch of materials that may do this job but, of course there are good and worse ones.
The preferred solution is to use a rear projection film that is a sorta kind of plastic sheet that is made to do this job. This material will ensure the best image performances. Anyway there are rear projection films that works with FTIR multitouch and other that don't. As far as I know, also from some NUI Group guys experiences, just few models seems to work out: one is the Rosco Grey and another one is the one that I experimented for my multitouch setup: the Peroni Panorama.
Unfortunately the only way to know if a rear projection film is suitable for a Multitouch Display is to test it!
Compliant surface
I'm hundred percent sure that you'll be so impatient to try your new "creature" that your first test will be using just the plexiglas, the webcam and the LEDs. Most probably you will say "oh my, it works!" immediately.
But things, as usual, are more complicated. If at that time you'll have the rear projection screen already available, you will make a test with it also, just placing it on the bare plexiglas.
At this point, if your rear projection film is ok, you'll notice that the FTIR setup continue to work but that it now requires a very hard pressure in order to have clear and bright "blobs" of infrared light detected by the camera. So you will think that it works better without the rear projection screen but, of course, you need something to project on!
But, why the FTIR effect is working better with fingers on the bare plexiglas? This is because the skin has a better coupling with the plexiglas than the rear projection film. The skin of the fingers is very soft and it adapts on the plexiglas surface very well, with very limited micro air bubbles beetween. This helps a lot in "frustrating" the plexiglas surface with a good consequent FTIR effect. So, micro ari bubbles are our enemy, as a confirm, you can have a try with wet fingers, it will work even better!
The rear projection plastic is more rough and less "sticky" than the finger's skin and it last still considerable micro air bubbles beetween itself and the plexiglas surface, limiting the surface frustration and the reflection of IR light.
Once you've understood this, you will understand why a compliant surface is definitively a need. A compliant surface is used to improve the coupling beetween the plexiglas and the rear projection screen so the multitouch display will require a very soft touch to trigger and to work.
There were a lot of tests in order to find materials that were suitable for this job. From dancing pads to mylar to the material that actually is the best candidate: the silicone rubber.
The silicone rubber comes as a very dense liquid that, one mixed with the hardner liquid, cures and become like a rubber. Normally it is used in modellism to create moulds starting from objects to duplicate. So it will be more easy to find if you will search in hobby shops. It is also quite expensive: you can have half of a kilo of silicone rubber for about €30 to €70.
With this great material you will prepare a very thin layer (1 mm or less) above the plexiglas.
As usual, not all kind of silicone rubber are suitable for a FTIR multitouch display. You need to use pretty transparent silicone rubber with a "shoreness" around A40. Actually, accordingly also with all the experiments from the NUI Group users, one of the best silicone rubber for FTIR is the Sorta Clear 40. Personally I didn't found any easy way to buy it here in Italy so I decided to try other kind of silicone rubber.
I used the Prochima Crystal Rubber that is transparent as well and has a shoreness very near to the Sorta Clear. The result was ok, probably this kind of silicone rubber lasts a little bit sticky but anyway works.
If you will face my same problem with the silicone rubber stickyness, you'll have to consider an additional layer made of sketch paper or tracing paper. The purpose of this additional layer will be to separate the silicone rubber from the rear projection film in order to prevent the two from sticking together and setting off FTIR on the whole display surface even without any finger press.
Last note about the silcone rubber is regarding the quantity. In my case half of a kilo was more than enough for my display which was of 72x54 cm.
Infrared Camera
Don't be afraid about this paragraph title: you don't need to buy some fancy and expensive camera to build a multitouch display, you may use a normal webcam and some.....patience!.
Theorically all normal camera are able to see much more than the visible spectrum but in normal webcams, in order to avoid, for example, to underline the veins on you face, the IR light is flitered out.
So, in every "normal" camera there's a light hi-pass filter that eliminate or attenuate the IR light and if you need an IR camera, you have just to replace it with a filter that do the opposite, letting the IR light in and blocking the visible light.
On most consumer webcams the IR blocking filter is made of a very thin film on the camera's optics and can't be eliminated without scratching the optics itself. In this case, instead of removing the IR block filter you will buy another optics without the filter and you will simply replace the original one.
The second step will be to get a low pass filter that let the webcam see IR light but not the visible light. To be honest, the best would be a band pass filter that is a filter that allows just a certain light to go though. This kind of filters are normally used with remote controls and have to be choosen accordingly to the frequency of IR emission of the LEDs you've choose. A band pass filter will prevent the webcam to be disturbed also by other sources of IR light other than the visible light.
A not too bad low pass filter may be easily built with 2-3 layers of exposed photo negatives. It is not the best solution but it works pretty well. Band pass filters may be obtained dismounting old remote controls or IR receiving windows of old TVs or oredered directly from a reseller.
Of course you need to think also about the webcam itself other than just about the IR filter that you will place behind it! So, before clicking on the "buy" icon of the more expensive camera you found, let's spend some words about this component.
As you're building a visual interface for, basically, a computer, you really want to have a very responsive interface. So, also to improve the feeling of using your multitouch system, it has to be fast. A powerful PC will help a lot in quickly translating the webcam images to blob events but, of course, you need also a fast camera that will detect quickly the finger movements. So, one of the most important factor when choosing the webcam to use is the framerate.
You will start to have acceptable results with a webcam capable of at least 30 FPS at the resolution you will work.
Higher framerates, also with webcams that just duplicate frames, will enhance the responsiveness of your FTIR setup.
The webcam image resolution will be important just if you're planning to build very large multitouch displays. For example, if you're building a 60x45 cm display, a webcam capable of 640x480 pixels is more than enough. With this resolution you will have 600 mm / 640 pixels = 0,9 mm, it means that you will have a resolution of one millimeter!
Thinking about how inaccurate a finger could be, you easily will realize that a finger detection resolution of 3-4 millimeters is still acceptable, so you would like a webcam resolution higher than 640x480 pixels just if you're thinking about a display of (640 pixels X 3 mm) 2 meters by 1,5 meters or bigger.
List of materials
Ok, at this point you should have a pretty clear idea of what you're going to build so, now that all things are defined, let's try to write down what you need to buy during the week to keep you busy during the week-end:
(last update November 13th 2007)
Part One
Part Two
Part Three
Ok, I want to build one! What I need?
If you reached this point, probably you're starting to think that, maybe, you want to build your own multitouch display. Otherwise, you're still thinking "oh my, but it sound to me too strange that I can build one at home, without an ultra-tech lab!". If your on this position, I suggest you to jump to the section where I explain how I built mine, so you can figure how simple it is compared to what you're probably thinking of; otherwise, if you already decided to build one, this is the right page to read on.
Building a multitouch is something very near to bricolage with some electronics and, of course, come computer skills. In order to have something useful, more or less you should be able to build a frame made of alluminium and/or wood and solder some LEDs together, nothing special.
Another thing that probably is useful to point out is what you need, in terms of tools, to build an FTIR multitouch. More or less this tools should be enough:
- sanding paper
- a saw with fine teeth to cut alluminium
- a sawing guide to make 45° cuts
- a drill tool with some drill bits
- glue for the wood
- normal and angular morsel to glue the frame in the right position
- a sharp cutter
- scissors for wires
- a soldering iron
- insulating tape
- a drawing pen
| Back to top |  |
Choose your destiny
At this point, you probably already understood that the main choice is beetween the FTIR and the DI approach. Of course this is your decision, I can just say that, probably:
- DI is easy and quick to setup especially for "quick and dirty" tests. If you want to refine it the things become a little bit more complicated (but not too much) for finding the right illuminator and its position, the right diffusing material or method. DI is the only way if you want to play also with fiducials
- FTIR is more complicated to setup because of the compliant surface. Without the compliant surface it works anyway with the results you can see in my first multitouch test but, anyway, you need to press really hard on the display to get results.
| Back to top | |
Design considerations
The second step is choosing the size of your setup. To keep things simple you will go for a display with a 4:3 aspect ratio, other ratios or even different shapes (circle, triangle) are possible but all the things gets more complicated. Keep in mind that the size of your installation will grow a lot choosing bigger display size because you have to consider also the distance that the projector needs to cover the whole display area. You can use the good Projection Calculator from ProjectionCentral.com to get an idea of the distance you need beetween the projector and the display in order to cover it all.
Alternatively there are some new projectors that are designed for very close projection. They can cover bigger screens from a very short distance. As example, you may take a look at the 3M DMS 700 series or at the Toshiba TDP-EX20U.
Probably you want just to start playing with photos with your multitouch immediately and you are not taking care of this kind of aspects but you should spend five minutes also thinking about how you will use your multitouch display. What I mean is about deciding if you want something like a normal table, a coffee table, a kiosk or a wall. There are some constrains also depending on this decision, for example if you're going to build an hi-tech coffee table you are limited in the display size by the height of the table itself. This is because even using a mirror to double the projector light path, your display size will be limited to the maximum area lighted by the projector at a distance from the display equal to the double of your coffee table height. If you're going for a kiosk you will need to make some test to find the size that will be suitable, in this case you should be able to reach with the same hand the entire area of the display while standing in front of it without moving your feet unless, that's a better case, you're thinking about a multitouch kiosk that may be used by more than one person at time. If you're going for a multitouch wall...you're lucky!
The size of the display is important also regarding the thickness of the plexiglas to use. You need a piece of acrylic glass that is at least the same thickness of the LED's you will use otherwise you'll waste a lot of infrared light and probably you'll get also some counter effects. In addition, the bigger size you want it, the thicker the plexiglas have to be. This is because if you're going for a big display, your plexiglas must be thick enough to not to bend under your finger touches. To determine this you have no other chance than go to the shop where you'll buy your plexiglas and make some test. Just to give you a rough idea, I used a 72x54 cm plate, 10 mm thick and it is pretty solid, I think that I could have been using a 8 mm with no problem at all.
| Back to top | |
Infrared LED and filter
Once you've decided the size and the kind of your setup, you need to consider how to illuminate it with infrared light. You'll use IR LEDs of course, the great guess, at this point is "how many?". Actually there are no formulas to calculate this. This is a number that depends from many factors like: size of display, power of LEDs, sensibility of the webcam, quality of the IR-pass filter, ambient light, multitouch display case etc. Let's spend some words about those factors.
If your display is big, you'll need more LEDs, this is obvious, especially for the sides where you're going to place your LEDs. But, let's suppose you're going to make a rectangular, 4:3 display and you're thinking about to place LEDs on the top and the bottom sides (like me). The space beetween the LEDs is depending also by the size of the vertical sides! Yes because the plexiglas will attenuate somehow the IR light, so, in order to illuminate also the center of the display, you need to "inject" more light at the top and bottom side and you can do this just by using more LEDs and decreasing the distance beetween each other.
The power of LEDs, of course, is important as well. If you will use powerful LEDs you will need less pieces even if probably this will lead to an uneven infrared light distribution.
The goal is to have visible "blobs" of IR light reflected by the finger touches. The important word here is not "blob" or "IR light", the question that matters is "visible". This means that the blobls need to be lighted and clearly visible inside the surrounding image. In other words if there is no ambient light you have more chances, if your webcam is very sensible you have more chances too, if you're using a great IR-pass filter the chances are raising as well. So, basically, if one of this factor is missing in your setup you have to balance putting more LEDs or using better materials elsewhere.
I know that this may be confusing so, let's try to come out of this fog. For my FTIR setup that actually, apart from the unfitting silicone rubber, is working quite fine I used 24 Osram SFH485P IR LEDs, 12 on the top side and 12 on the bottom side. The display is 72x54 cm so think about the fact that 12 LEDs for a 72 cm side are quite good because one Osram SFH485P every 6 cm on each side is quite good to illuminate a plexiglas lenght of 54 cm (shorter sides). To be really honest, I said "quite good" because I actually think that some LEDs more would be better, so think if I used 14 LEDs, one every 5 cm.
But keep in mind that this is true also because of the kind of LEDs I used that, as I said, are Osram SFH485P, they differ from the SFH485 (without the "P") for the beam angle that is 80 degrees in the SFH485P compared to the 40 degrees of the SFH485. I thought that this would help to better illuminate the plexiglas. Anyway, check out their complete features on the datasheet (PDF). In addition, I really suggest you to take a look a the infrared LED section of the NUI Group Forum and to the same section of the NUI Group Wiki as well.Apart from the number of LEDs, there are other factors that will help you to achieve better results. If you use a IR-pass filter (like the one used often in remote control) you'll let your webcam to see just the light coming out from you LEDs which is, in SFH485P case, in the 880 nanometer range, so the webcam images will be less disturbed by the ambient light. As well, if you use a closed case for holding the whole setup, the multitouch will be less sensible to the surrounding light.
Keep in mind that, anyway, the image displayed will be a rear projected image so, if you're planning to use your multitouch in outdoor you'll probably experience low image visibility problems other than poor blob detection precision. In this case.....good luck and let me know!
| Back to top | |
Electronics and power supply
Once you defined how many LEDs your going to use you can think about how to power up them. You'll connect the LEDs in series like it is shown in the following picture:
Normally a LED is accepting a voltage of 1,2-1,5 Volts (double check on the LED datasheet) so you have to multiply this to the number of LEDs you're connecting in order to have the minimum voltage that the power supply has to give, so you will choose it keeping in mind that it should provide a DC current with voltage and power enough to power up the LED stripe.
Of course you
cannot connect it directly to the LEDs, unless it is providing the
exact voltage needed by the LEDs themselves, you have to use a resistor connected
to the LED stripe to adjust these values. You can easily calculate the
resistor's value with this great LED resistor calculating tool.If you want to save money, you can use the power supply of your PC where you have +5 Volts (red connector) and +12 Volts (yellow connector) but you should check if this current absorbtion is tollerated by the power supply while still powering up the PC itself.| Back to top | |
Rear projection
In order to have clear images on the display, you need to use some material as a projection surface. Actually there are a bunch of materials that may do this job but, of course there are good and worse ones.
The preferred solution is to use a rear projection film that is a sorta kind of plastic sheet that is made to do this job. This material will ensure the best image performances. Anyway there are rear projection films that works with FTIR multitouch and other that don't. As far as I know, also from some NUI Group guys experiences, just few models seems to work out: one is the Rosco Grey and another one is the one that I experimented for my multitouch setup: the Peroni Panorama.
Unfortunately the only way to know if a rear projection film is suitable for a Multitouch Display is to test it!
| Back to top | |
Compliant surface
I'm hundred percent sure that you'll be so impatient to try your new "creature" that your first test will be using just the plexiglas, the webcam and the LEDs. Most probably you will say "oh my, it works!" immediately.
But things, as usual, are more complicated. If at that time you'll have the rear projection screen already available, you will make a test with it also, just placing it on the bare plexiglas.
At this point, if your rear projection film is ok, you'll notice that the FTIR setup continue to work but that it now requires a very hard pressure in order to have clear and bright "blobs" of infrared light detected by the camera. So you will think that it works better without the rear projection screen but, of course, you need something to project on!
But, why the FTIR effect is working better with fingers on the bare plexiglas? This is because the skin has a better coupling with the plexiglas than the rear projection film. The skin of the fingers is very soft and it adapts on the plexiglas surface very well, with very limited micro air bubbles beetween. This helps a lot in "frustrating" the plexiglas surface with a good consequent FTIR effect. So, micro ari bubbles are our enemy, as a confirm, you can have a try with wet fingers, it will work even better!
The rear projection plastic is more rough and less "sticky" than the finger's skin and it last still considerable micro air bubbles beetween itself and the plexiglas surface, limiting the surface frustration and the reflection of IR light.
Once you've understood this, you will understand why a compliant surface is definitively a need. A compliant surface is used to improve the coupling beetween the plexiglas and the rear projection screen so the multitouch display will require a very soft touch to trigger and to work.There were a lot of tests in order to find materials that were suitable for this job. From dancing pads to mylar to the material that actually is the best candidate: the silicone rubber.
The silicone rubber comes as a very dense liquid that, one mixed with the hardner liquid, cures and become like a rubber. Normally it is used in modellism to create moulds starting from objects to duplicate. So it will be more easy to find if you will search in hobby shops. It is also quite expensive: you can have half of a kilo of silicone rubber for about €30 to €70.
With this great material you will prepare a very thin layer (1 mm or less) above the plexiglas.
As usual, not all kind of silicone rubber are suitable for a FTIR multitouch display. You need to use pretty transparent silicone rubber with a "shoreness" around A40. Actually, accordingly also with all the experiments from the NUI Group users, one of the best silicone rubber for FTIR is the Sorta Clear 40. Personally I didn't found any easy way to buy it here in Italy so I decided to try other kind of silicone rubber.
I used the Prochima Crystal Rubber that is transparent as well and has a shoreness very near to the Sorta Clear. The result was ok, probably this kind of silicone rubber lasts a little bit sticky but anyway works.
If you will face my same problem with the silicone rubber stickyness, you'll have to consider an additional layer made of sketch paper or tracing paper. The purpose of this additional layer will be to separate the silicone rubber from the rear projection film in order to prevent the two from sticking together and setting off FTIR on the whole display surface even without any finger press.
Last note about the silcone rubber is regarding the quantity. In my case half of a kilo was more than enough for my display which was of 72x54 cm.
| Back to top | |
Infrared Camera
Don't be afraid about this paragraph title: you don't need to buy some fancy and expensive camera to build a multitouch display, you may use a normal webcam and some.....patience!.
Theorically all normal camera are able to see much more than the visible spectrum but in normal webcams, in order to avoid, for example, to underline the veins on you face, the IR light is flitered out.
So, in every "normal" camera there's a light hi-pass filter that eliminate or attenuate the IR light and if you need an IR camera, you have just to replace it with a filter that do the opposite, letting the IR light in and blocking the visible light.
On most consumer webcams the IR blocking filter is made of a very thin film on the camera's optics and can't be eliminated without scratching the optics itself. In this case, instead of removing the IR block filter you will buy another optics without the filter and you will simply replace the original one.
The second step will be to get a low pass filter that let the webcam see IR light but not the visible light. To be honest, the best would be a band pass filter that is a filter that allows just a certain light to go though. This kind of filters are normally used with remote controls and have to be choosen accordingly to the frequency of IR emission of the LEDs you've choose. A band pass filter will prevent the webcam to be disturbed also by other sources of IR light other than the visible light.
A not too bad low pass filter may be easily built with 2-3 layers of exposed photo negatives. It is not the best solution but it works pretty well. Band pass filters may be obtained dismounting old remote controls or IR receiving windows of old TVs or oredered directly from a reseller.
Of course you need to think also about the webcam itself other than just about the IR filter that you will place behind it! So, before clicking on the "buy" icon of the more expensive camera you found, let's spend some words about this component.
As you're building a visual interface for, basically, a computer, you really want to have a very responsive interface. So, also to improve the feeling of using your multitouch system, it has to be fast. A powerful PC will help a lot in quickly translating the webcam images to blob events but, of course, you need also a fast camera that will detect quickly the finger movements. So, one of the most important factor when choosing the webcam to use is the framerate.
You will start to have acceptable results with a webcam capable of at least 30 FPS at the resolution you will work.
Higher framerates, also with webcams that just duplicate frames, will enhance the responsiveness of your FTIR setup.
The webcam image resolution will be important just if you're planning to build very large multitouch displays. For example, if you're building a 60x45 cm display, a webcam capable of 640x480 pixels is more than enough. With this resolution you will have 600 mm / 640 pixels = 0,9 mm, it means that you will have a resolution of one millimeter!
Thinking about how inaccurate a finger could be, you easily will realize that a finger detection resolution of 3-4 millimeters is still acceptable, so you would like a webcam resolution higher than 640x480 pixels just if you're thinking about a display of (640 pixels X 3 mm) 2 meters by 1,5 meters or bigger.
| Back to top | |
List of materials
Ok, at this point you should have a pretty clear idea of what you're going to build so, now that all things are defined, let's try to write down what you need to buy during the week to keep you busy during the week-end:
- a piece of plexiglas, you'll need to have it already cut by the shop because cutting the acryl could be a real pain (you must use special blades otherwise it will tend to melt)
- the number of infrared LEDs you decided (see previous paragraph)
- the resistors you calculated above
- alluminium profiles for the display frame
- the power supply (in case you don't have it already)
- a good webcam (if you don't have it already)
- a non IR coated lens for the webcam (see previous paragraph)
- an IR filter (if you don't want to use photo films, see previous paragraph)
- a rear projection tissue (you can use mylar or tracing paper for the first time)
- transparent silicone rubber to prepare the compliant layer
- wood or other material to prepare a case or a structure to hold your display in place
- a video projector (hopefully you already have one)
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If I understud , you put the compliant surface over de plexiglas , and then over this the rear projection film ... but how do you fixed this rear over de other?.
thanks ,
Hi Nicolas, I answered the same question in the comment of Apollo 4 days ago, look in "Recent Comments" in the homepage :o)
Hi BlaXwan. Nice site!
I have a question about the power supply. I get the voltage/resistance side, but can you just plug in a pc power supply to the mains and tap off 12v from each of the connectors without any control from a pc?
Thanks
@nussbox
Thank you. Surely you can use a PC power supply but remember that it won't start by itself, to start an ATX power supply you have to short pins 14 and 15 together (see http://freespace.virgin.net/matt.waite/resource/psu/atxpsu.htm )
Hi! Great site! I was just wondering if plastic cling wrap fixed on the projection screen would work? Its cheap has all the properties of silicone rubber..
Hi, I'm also trying to build my own multi-touch table, I ran into some problems and was hoping if you could help me out or give some pointers. See http://nuigroup.com/forums/viewthread/2923/, the last two post by the user "edwardaa" is from me.
Hi BlaXwan,
I really like the site, and your great tutorial.
Is the rear projection screen really necessary or can the sketch paper be used as the screen. Also, are there any cheap ways of getting a projector, or making one possibly.
Thanks, Andy
Great Tutorial, loads of detail and media. I live in the UK and finding someone selling (A40)silicon rubber took a while, might I suggest making a list of suppliers from feedback?
UK: http://www.tiranti.co.uk/
Hey BlaXwan,
I'm making a multitouch display for a fourth year project and your site is really great for advice.
My projector must be at least 5 feet away from the screen to cover the display completely. Since I am making a table, this distance is too high. I was wondering if you know any tricks that might help me to shorten this distance (mirrors, lens, etc.) Thanks alot!
Fantastico .. ho trovato le tue istruzioni semplicemente favolose !!! Se ho bisogno ti rompo le scatole,per ora tutto chiaro,avro' un lungo week end !! :-) Di dove sei ?
Ciao,Matteo.