Video with person 'writing on glass' effect I'm trying to find someone who can make videos similar to one shown here:

                
            

At 49 seconds, the actress writes on glass to explain the concept.


                
            

Does someone know what this effect is called so I know what to ask for?

Also, if you know how to do this sort of thing, please reply.

Thanks so much,
There is a much better way to do that. That example video is a pretty poor illusion.


It can be done "for real" with with real light. But it looks
different. It has to be dark. 
You can use motion tracking to create an animation path. 
You can also use manual key frames. There are lots of tutes on the intertubes about that. Here is one: