19 second mark shows they are using monocular structure from motion.

Some Japanese Uni has a working solution(HDL inside FPGA) doing same thing too http://hackaday.com/2014/01/12/autonomous-quadcopter-fits-in...

you can try it yourself, code is on github https://www.youtube.com/watch?v=E35xbo3r8rA https://github.com/nymanjens/ardrone-exploration