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Written May 25, 2010


In television and motion picture production, sometimes your “talent” is standing in the studio but you want to make it appear that he’s standing in Tappan Square.

Therefore, in the studio you put him in front of a “green screen.”  Then a function called Chroma Key electronically replaces every green pixel with the correspondingly-located pixel from the outdoor scene.

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 doesn’t turn all the pixels along the line yellow, just those pixels whose original color is grass green.

In the early days of television, the background screen was usually not green but blue.  This color looks much more pleasant in the studio. 

On television, however, a blue screen doesn’t work very well if the talent is wearing blue clothing.

The blue-clad portion of the talent becomes transparent.  A man wearing a blue tie appears to have a hole in his chest.

Other colors are also possible.  To perform her song “I Will Always Love You” on the Porter Wagoner Show in 1974, Dolly Parton wore a dark red dress while the background cyclorama was lit in a complementary color, dark cyan.

That enabled the director to occasionally replace the background (and part of the shiny microphone screen in the foreground) with a different camera angle.

Three recollections:

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 couldn’t resist playfully raising the veil over the stagehand’s 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 (as simulated here).

When they were finished talking, we used our newfangled Quantel digital effects box to split the foreground picture.  The stadium in the background was fully revealed (and we should have slowly zoomed in on it at the same time).  The left announcer slid off to the left, and the right announcer slid off to the right, leaving space for my “Coming Up” 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.

Therefore, we tried a white screen.  We put a translucent rear-projection screen behind the talent and flooded it with enough light to make it brighter than his white shirt or anything else he was wearing.

Then we used an “inverse key” function to replace the brightest pixels with the pixels from another image.

That worked, more or less, though the foreground was rather dim because it couldn’t contain any pure white.



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