Front Diffused Illumination (Front DI)

HOW IT WORKS:

Light is shone from above the touch surface (the front). When an object stops the infrared light it creates shadows on the touch surface. The camera then converts the black blobs into touches. This setup works in inverse of the other setups, where all non-bright spots are considered blobs. This setup does not work as well as any of the other setups but is a great place to learn multitouch by building a simple MTmini

How do I make an Front DI FTIR setup?

The layers from top down are:

Touch Surface Acrylic or Glass

All that is needed is a strong, durable surface that can take the pressure of user interaction.

Diffuser/Projection Layer

The diffuser stops the image produced by the projector. The diffuser is also needed so that the shadows have a plane to lie on and thus represent the objects placed near the touch surface.

The source of light for a Front DI setup comes from the ambient room lighting, or an overhead light which covers the touch surface.

The webcam is placed on the opposite site of the touch surface so that is can see the blobs.

(The following comparison is from the nuigroup.com/forum thread)

Advantages

No need for a compliant surface, just a diffuser/projection surface on top/bottom

Can use any transparent material like glass (not just acrylic)

No LED frame required

No soldering (you can buy the IR-Illuminators ready to go) or just use ambient light

Can track fingers and hovering

An enclosed box is not required

Simple setup

Disadvantages

Cannot track objects and fiducials

Difficult to get even illumination

False blobs from hovering objects can be a problem if unintentional

Not as reliable (relies heavily on ambient lighting environment)