Very clever!! Would love to know how they do that!
We really are a clever lot. Like that a lot.
It's nice to see how we are being more and more creative with the use of lights. LED/projection technology rocks.
I would say that's done with a laptop + a piece of open source software, a projector & a camera connected to the laptop that records the movements of the roller.
A group did something along the same lines but wrote their tags up the sides of buildings using a laser tag. Like this Linky