For football telecasts, a similar technique displays virtual graphics on the field during a football telecast.
A computer determines where the first-down line should be. However, we only want to paint that virtual yellow line on the turf, not on the players. Therefore, the computer doesnt turn all the pixels along the line yellow, just those pixels whose original color is grass green.
I remember watching a Tennessee Ernie Ford show in 1960 or thereabouts. A comedy sketch was performed in front of a blue screen so that a prop could magically levitate into the scene. It was carried by a stagehand who was invisible on TV because he was covered from head to toe in blue. Ernie couldnt resist playfully raising the veil over the stagehands face, revealing a disembodied pair of eyes hovering above the prop.
I remember televising a football game at the beginning of the 1983 season, Pitt at Tennessee. At the beginning of the show, both announcers were standing in the booth in front of a blue screen, which in this case was a blanket-sized cloth that we had unfolded and tacked up on the wall. Their image was Chroma Keyed over a wide shot of Neyland Stadium. When they were finished talking, we used our newfangled Quantel digital effects box to split the foreground picture and fully reveal the stadium in the background. The left announcer slid off to the left, and the right announcer slid off to the right, leaving space for my Coming Up: The Kickoff graphic. Viewers may have wondered what kind of motorized platforms the announcers were standing on.
But before that, I remember learning television at Syracuse University in 1970. We didn't have color cameras, so neither blue screens nor green screens were possible.