Why Green Screen? History & Benefits

[sophiecentaur]

In the olden days (1960s 70s and 80s, at least) they used to use a blue background for doing picture inlay of an actor into a remote background scene by switching between scenes as the scan moved over the picture. It was pretty crude at first but they eventually got the soft edges to look reasonable. News readers gradually lost their twittering edges against a background of Westminster.

Now, everyone uses a 'green screen' to achieve the same thing and it all works much better. Why did they change the 'switching' colour? The original argument behind the choice of blue was pretty convincing.

 

[es1]

No idea if it is true or not but seems reasonable:
"In the digital world, however green has become the favored color because digital cameras retain more detail in the green channel and it requires less light than blue. Green not only has a higher luminance value than blue but also in early digital formats the green channel was sampled twice as often as the blue, making it easier to work with. The choice of color is up to the effects artists and the needs of the specific shot. In the past decade, the use of green has become dominant in film special effects. Also, the green background is favored over blue for outdoors filming where the blue sky might appear in the frame and could accidentally be replaced in the process. Although green and blue are the most common, any color can be used."
http://en.wikipedia.org/wiki/Chroma_key

 

[sophiecentaur]

Ah yes - the chrominance bandwidth thing.
R-Y and B-Y are band limited in PAL TV. Switching on the green information will be more precise.
I have just though, also, that with digital processing, it is much easier than with analogue processing to choose a particular area of colour space and to switch on that region only.
Skin tones all have a significant amount of green in which may have thrown the old analogue decision making circuits. Never looked too closely at what happened to Frank Sinatra's eyes on TV.

PS. don't a lot of outdoor scenes have a lot of green grass in? :)

 

[NobodySpecial]

Blue was originally used in film because film was more sensitive to blue.
Green was used in TV because color cameras are more sensitive to green (the imager is covered by a grid of color filters, each 2x2 square of pixels has R,G,G,B - so twice as much green info as Red/Blue) and there isn't a lot of green in people's skin so you don't affect the look of the actor.

Today the image is digitally processed and a lot of the improvement in background separation is done by complex edge detection algorithms. There are even TV cameras that have a laser rangefinder to map the scene depth and separate people based on distance automatically.

 

[turbo]

Check this out. Think all those pricey-looking on-location shots on your favorite TV shows are real? Think again.

http://www.switched.com/2010/02/21/demo-reel-shows-just-how-much-green-screen-is-on-tv/

 

[sophiecentaur]

You have to hand it to them, though. They get the lighting, the perspective, the dynamics and a lot of other subtle things right most of the time. Dead clever. Yet it's still cheaper than location shots or they wouldn't do it in many cases.
That movie shows you only the presence of green bits behind the actors - it doesn't show / just assumes the carefully mapped studio and the accurate placements of camera. That's the really good bit.