Hardware Platform

This is a tabletop platform that can respond both touch input and physical objects. Some related research uses a Diffused Illumination (DI) approach to accommodate both design goals. In this scheme, the table must be enclosed so that the IR lights fixed on the bottom plate can illuminate the inside of the table. When objects or fingers are put on the top of the acrylic panel, the IR light will be reflected and captured by the camera. However, since a diffuser should be applied on the top surface for rear projection, only the objects very close to the top surface of the table can be tracked well. Besides, fingertips are sometimes lost tracking with this scheme because the blobs of fingers will be connected to the palm’s image due to their similar brightness.

As an alternative solution for multi-touch input, Frustrated Totally Internal Reflection (FTIR) scheme can acquire even clearer and more stable fingertips. It makes the acrylic panel fulfilled with IR light so that the finger put on the top surface will scatter light and creates a bright blob. However, FTIR scheme cannot track any pattern on objects so this method alone is not suitable for our purpose.

Due to the complementary advantage of each scheme, we experimented with the integration of the two techniques. The main approach of this setup is to utilize a compliant surface under the diffuser to improve the finger tracking of DI scheme. However, since DI scheme needs a very sharp image to precisely read the details of the marker, and FTIR requires having a very bright image to recognize even very light touches, we need to finely adjust the iris and other parameters of the camera to get a balance of these two conflicting requirements from different schemes.

The whole setup of our system is shown in the figure. A short throw projector (Benq MP522ST) with an IR-block filter is used so that a direct projection can be achieved without any mirror. In our case, the table with a height of about 1m creates a projection area of 870mm x 652mm. At the center of the base plate, a Firefly MV B/W camera with a piece of IR band pass filter is installed. A Tamron 3.0mm~8.0mm lens (13VM308ASIR) is mounted on the camera which provides manual focus, zoom and iris. The camera gives a resolution of 640x480 at 60fps. Thus we can get an equivalent resolution of the tabletop surface area as 1.5mm/pixel. Surrounding the camera there are four IR lights with caps which are used to avoid hot spots. We also put a series of IR LEDs at the side of the acrylic panel as most FTIR setups do.