Getting Started With MultiTouch
Multitouch Techniques
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.
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.
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
Diffused Illumination (DI):
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.
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
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.
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.)
Laser Light Plane (LLP):
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.
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
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.
DSI Parts List:
o EndLighten Acrylic
o LEDs
o Projection Surface (tracing paper, lee filter, geriets optitrans , etc);
o Infrared Camera/Modified Webcam
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
