"Glassified is a modified ruler with a transparent display to supplement physical strokes made on paper with virtual graphics. Because the display is transparent, both the physical strokes and the virtual graphics are visible in the same plane. A digitizer captures the pen strokes in order to update the graphical overlay, fusing the traditional function of a ruler with the added advantages of a digital, display-based system. We describe use-cases of Glassified in the areas of math and physics and discuss its advantages over traditional systems. Anirudh Sharma, Lirong Liu, Pattie Maes"
http://fluid.media.mit.edu/node/226