The 2012 Venus Transit

It only happens twice a century and, if you missed it, then regale yourself on this video as the next one is 105 years away, in the year 2117.

This video from NASA/SDO is beautiful but what I don’t understand is how Venus can look transparent as it transits across the sun. Optical illusion, perfectly legitimate scientific explanation or just a little bit of digital manipulation as the transit is superimposed on different pre-existing videos of the sun?