Effect seen on American Idol

[DVX100Shooter]

I saw an effect on American Idol last year that I would like to try. They had some of the singers superimposed on the side of sky scraper building. Do you think this is just a composite edit and they lowered the opacity of one clip and sorta blended the two together or do they shoot them against green screen and then place them on the building that way?

 

[Epeck]

Alink to the video perhaps? my guess is they overlayed the said clip (greenscreen). Also that effect would probably be better acheived using after effects.