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Getting Started With MultiTouch
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Get the most recent information on -> multitouch getting started

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Multitouch Techniques
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FTIR and DI are the two main technologies used in NUIGroup (Information on alternative techniques will be posted at a later time). Specific Parts (brands, types, etc.) and techniques for completion can be found on the rest of the forum.

<div style="padding:8px; background-color:#BFD6FF; border:1px solid #C3C3C3; padding-top:8px">Frustrated Total Internal Reflection (FTIR):</div>

Infrared light is shined into the side of an acrylic panel (most often by shinning IR LEDs on the sides of the acrylic). The light is trapped inside the acrylic by internal reflection. When a finger touches the acrylic surface this light is “frustrated” causing the light to scatter downwards where it is picked up by an infrared camera.

A silicone rubber layer is often used as a “compliant surface” to help improve dragging and sensitivity of the device. When touching bare acrylic, one must press hard or have oily fingers in order to set off the FTIR effect. With a complaint surface (like silicone rubber) the sensitivity is greatly improved.

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<div style="padding:5px; background-color:#FFF0DF; border:1px solid #C3C3C3; padding-top:5px">FTIR Parts List:

o Acrylic/Polycarbonate/Plexiglass
o Infrared LEDs (or similar infrared light source)
o Compliant Surface (silicone rubber ie. Sorta Clear 40, Elastosil, Lexel, etc. )
o Projection Surface (Rosco Grey, Vellum, Mylar, other, etc.)
o Infrared Camera/Modified Webcam
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Diffused Illumination comes in two main forms. Front Diffused Illumination and Rear Diffused Illumination. Both techniques use the same basic principles.

Rear DI:

Infrared light is shined at the screen from below the touch surface. A diffuser is placed on top or on bottom of the touch surface.  When an object touches the surface it reflects more light than the diffuser or objects in the background; the extra light is sensed by a camera. Depending on the diffuser, this method can also detect hover and objects placed on the surface.

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<div style="padding:5px; background-color:#FFF0DF; border:1px solid #C3C3C3; padding-top:5px">Rear DI Parts List:

o Clear sturdy clear surface (Glass, Acrylic, Polycarbonate, Plexiglass, etc.)
o Infrared Illuminator (Infrared LEDs, Infrared Illuminator, etc.)
o Diffuser/Projection Surface (Vellum, Mylar, Lee Filter, other, etc.) *Note: The Diffuser/Projection Surface must let some light through.
o Infrared Camera/Modified Webcam
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Front DI:

Infrared light (often from the ambient surroundings) is shined at the screen from above the touch surface. A diffuser is placed on top or on bottom of the touch surface.  When an object touches the surface, a shadow is created in the position of the object. The camera senses this shadow.

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<div style="padding:5px; background-color:#FFF0DF; border:1px solid #C3C3C3; padding-top:5px">Front DI Parts List:

o Clear sturdy clear surface (Glass, Acrylic, Polycarbonate, Plexiglas, etc.)
o Diffuser/Projection Surface (Vellum, Mylar, Lee Filter, other, etc.) *Note: The Diffuser/Projection Surface must let some light through.
o Infrared Camera/Modified Webcam

Optional:
o Infrared Illuminator (Infrared LEDs, Infrared Illuminator, etc.)

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<div style="padding:8px; background-color:#BFD6FF; border:1px solid #C3C3C3; padding-top:8px">Laser Light Plane (LLP): </div>

Infrared light from a laser(s) is shined just above the surface. The laser plane of light is about 1mm thick and is positioned right above the surface, when the finger just touches it, it will hit the tip of the finger which will register as a IR blob.

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<div style="padding:5px; background-color:#FFF0DF; border:1px solid #C3C3C3; padding-top:5px">LLP Parts List:

o Clear, flat surface (Glass, Acrylic, Polycarbonate, Plexiglass, etc.)
o Infrared Laser(s)
o Line Generating Lens
o Projection Surface
o Infrared Camera/Modified Webcam
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<div style="padding:8px; background-color:#BFD6FF; border:1px solid #C3C3C3; padding-top:8px">Diffused Surface Illumination (DSI): </div>

DSI uses a special acylic to distribute the IR evenly across the surface. Basically use your standard FTIR setup with an LED Frame (no compliant silicone surface needed), and just switch to a special acrylic. This acrylic uses small particles that are inside the material, acting like thousands of small mirrors. When you shine IR light into the edges of this material, the light gets redirected and spread to the surface of the acrylic. The effect is similar to DI, but with even illumination, no hotspots, and same setup process as FTIR.

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<div style="padding:5px; background-color:#FFF0DF; border:1px solid #C3C3C3; padding-top:5px">DSI Parts List:

o EndLighten Acrylic
o LEDs
o Projection Surface (tracing paper, lee filter, geriets optitrans , etc);
o Infrared Camera/Modified Webcam
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Technique Comparison
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A frequent question is “What is the best technique?” Unfortunately, there is no simple answer. Each technique has it’s pro and cons. The answer is less about what is best and more about what is best for you (which only you can answer).

The General consensus is:  If you need object, fiducial, and hovering recognition than rear DI is most likely for you. If you only need a robust and reliable finger recognition, than FTIR is a great choice. Front DI, although the simplest to build, is rarely used as a permanent setup since it heavily relies on the surrounding environment.

FTIR:

Pros:

o An enclosed box is not required
o Blobs have strong contrast
o Allows for varying blob pressure
o With a compliant surface, it can be used with something as small as a pen tip

Cons:

o Setup calls for some type of LED frame (soldering required)
o Requires a compliant surface (silicone rubber) for proper use
o Cannot recognize objects or fiducial markers
o Cannot use a glass surface

Rear DI:

Pros:

o No need for a compliant surface, just an diffuser/projection surface on top/bottom
o Can use any transparent material like glass (not just acrylic)
o No LED frame required
o No soldering (you can buy the IR-Illuminators ready to go)
o Simple setup
o Can track objects, fingers, fiducials, hovering

Cons:

o Difficult to get even illumination
o Blobs have lower contrast (harder to pick up by software)
o Greater chance of ‘false blobs’
o Enclosed box is required

Front DI:

Pros:

o No need for a compliant surface, just an diffuser/projection surface on top/bottom
o Can use any transparent material like glass (not just acrylic)
o No LED frame required
o No soldering (you can buy the IR-Illuminators ready to go)
o Can track fingers and hovering
o An enclosed box is not required
o Simplest setup

Cons:

o Cannot track objects and fiducials
o Difficult to get even illumination
o Greater chance of ‘false blobs’
o Not as reliable (relies heavily on ambient lighting environment)

LLP:

Pros:

o No compliant surface (silicone)
o Can use any transparent material like glass (not just acrylic)
o No LED frame required
o An enclosed box is not required
o Simplest setup
o Could be slightly cheaper than other techniques

Cons:

o Cannot track traditional objects and fiducials
o Not truly pressure sensitive (since light intensity doesn’t change with pressure).
o Can cause occlusion if only using 1 or 2 lasers where light hitting one finger blocks another finger from receiving light.

DSI:

Pros:

o No compliant surface (silicone)
o Can easily switch back and forth between DI (DSI) and FTIR
o Can detect objects, hovering, and fiducials
o Is pressure sensitive
o No hotspots
o even finger/object illumination throughout the surface

Cons:

o Endlighten Acrylic costs more than regular acrylic (but the some of the cost can be made up since no IR illuminators are needed)
o Blobs have lower contrast (harder to pick up by software) than FTIR and LLP
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Great summery but I think there’s a mistake at
Rear DI
Cons:
Enclosed box is not required.

Shouldn’t it be: Enclosed box is required ?

Very informative, thank you!

Thanks zuppaman. Good eye. If anyone finds any other errors, let me know.

One more T-shirt for Cerupcat ! (with 2 wings on it )

Woho, that’s my hand!

From the difference in contrast on FTIR and DI, I assume that tracking sliding gestures is easier with FTIR. Right?

Thanks for the guide.

I don’t know, my webcam is picking up my fingertip blobs as bright as an IR LED pen. I am using Rear DI at the moment just using 2 desklamps to illuminate the surface evenly. Not good but it works for now!

@ Calvin Muller: If you look at the two images above, you’ll see what I mean by contrast. FTIR will almost have a completely black vs completely white contrast in the raw image. Rear DI on the other hand will never have the black part (although it may in rare occasions have the same white brightness), but therefore less contrast.

i understand what you were saying cerupcat, but with the awesome touchlib library.. the blobs are as bright as anything using DI i was just stating my situation and not criticizing anything you said above.

Add this as a showcase between using silicone and not using it http://nuigroup.com/forums/viewreply/11221/

Excellent post cerupcat. Maybe augment with links to other forum posts on how to build each type of display? This way this thread can become a 1 stop shop.

Add this to the list.

So, this sounds like it could be a very stupid question, but how does the image show up on the surface without it getting in the way of the camera, and is it easier to use projection or lcd?

your camera has to be modified so it only sees IR and not the visible spectrum… once thats done it doesnt matter what you project it will only see the IR reflected from your inputs…

Taha

I’ve updated the main post to add LLP and DSI techniques.

Vlado, i’ll probably add the LCD laser when I have more time. The LCD isn’t really technique in itself. It’s still FTIR.