The Portable Plexi Cliffhanger is one of the most simple, cheap, and portable Trackmate systems. It can be setup in under 5 minutes, costs less than $40 (plus a webcam), and fits easily in an over-the-shoulder bag.

Trackmate is an open source initiative to create an inexpensive, do-it-yourself tangible tracking system. Using the project's Tracker software, any computer can recognize tagged objects and their corresponding position, rotation, and color information when placed on the imaging surface.

Using a system like the one shown here, you can track physical objects on a surface and use them to control and manipulate spatial applications on your computer. Since objects are each uniquely tagged, they can easily be mapped to particular actions, information, or relationships. See the LusidOSC project for more details about spatial applications.

Step 1: Gather All the Parts

You will need:
- a 12 inch x 12 inch x 1/4 inch clear acrylic or plexi sheet (often sold as a table protector)
- two 2 inch C clamps
- two thin adhesive-backed rubber feet (in a pinch, just use a few pieces of tape)
- a clamp light (found in most hardware stores)
- a 13W (60W equivalent) compact fluorescent bulb
- a mini camera tripod (found in camera stores, or online)

And finally, you need a webcam. I recommend the PS3 Eye since it is a very nice camera for the price (~$33) and there are now drivers for Windows and Mac. But any decent quality webcam that allows manual functionality (i.e., exposure control and focus) should work with this setup.

You can find most of the parts at your local hardware store (and save the cost/time of ordering online). The parts are also available via Amazon.com; here is an amazon.com list to help you find everything you need.

Step 2: Modify Some Things

Add two rubber feet to both C clamps. This keeps the clamp from making marks on the table and helps it to grip the plexiglass sheet better.

Optionally, you can make the top side of the acrylic sheet frosted by sanding is lightly with fine sandpaper. This helps to diffuse light and makes the surface feel a little less-grippy (this can be nicer for sliding objects on top).

Step 3: Set It Up

Everything is packed up in an over-the-shoulder bag. The first couple images show all the contents together (including a laptop).

Pull out the acrylic sheet (if frosted, make sure the frosted side is facing up) and the C clamps.

Clamp the sheet to the table with about 2-3 inches of overlap to ensure a solid mount.

Place the clamp-light to the bottom of one of the C clamps under the table. Always be careful with where the light is mounted as a hot bulb next to paper, fabric, the table, etc. can start a fire. Using a low-wattage fluorescent bulb makes things a lot safer, but still be careful.

Setup the tripod under the table. Place the webcam on top of the tripod pointed up towards the overhanging plexiglass (the camera should be as close as possible while still being able view all of the illuminated area). Try to align the camera square with the table.

Mounting the camera to the tripod can be a bit tricky. Experiment with different brackets and clamps. I found that a simple twist-tie holding the camera's cord tightly worked as a strong-enough mount for the PS3 Eye camera (although a custom bracket would be even better).

Step 4: Start Tracking!

Once you have all of the hardware setup, run the Trackmate Tracker and place one of the Trackmate layout guides on the surface. The layout guides make it easy to quickly configure the Tracker and make sure the entire surface is being read correctly.

For a complete walkthrough showing how to setup the Tracker software, see the Cliffhanger Walkthrough on the Trackmate wiki.

Your Portable Plexi Cliffhanger is now ready to use. Plug in the camera and get started with Trackmate!