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: Setup the picture frame
Remove the back from the picture frame and remove the glass plate. Optionally, at this point you can sand one side of the glass with special glass/diamond sandpaper (important note: if you sand glass, use water to reduce glass dust since it can be hazardous to breathe) to diffuse the lighting and make the glass look frosted. Standard sandpaper should work too, but it will take a lot more work.
Use a hot glue gun to securely affix the glass to the frame. If your glass is frosted, make sure the frosted side is facing out (i.e., the glass feels rough when touching it from the pretty side of the frame).