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Blue Screen vs. Green Screen


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#1 Thomas Chaves

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Posted 07 September 2009 - 09:48 AM

Hey there all of you cinemaphiles.

I was wondering if anyone out there can tell me what the differences are between green & blue screen. When would you use one over the other? Is one necessarily better than the other? Is one easier to use than the other?


Also, it seems like a lot of the green screen I see on TV looks cheesy. How do you get the most out of it?

Thanks.

Thomas
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#2 David Mullen ASC

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Posted 07 September 2009 - 11:12 AM

You generally pick one over the other just based on whether the object in front of the screen has any blue or green in it, so you want to use a screen that doesn't have that color. Also, blond hair tends to key better against blue instead of green.

For example, I did a shot of a person standing looking out of a window with a greenscreen outside -- we picked green because the room had blue-ish paint and the lighting was blue moonlight. But then we had a later scene with that person standing outside in daytime and we picked a bluescreen because they were wearing green and there were green trees all around.

The only technical issue is that green tends to be cleaner than blue because blue is a noisier signal / layer (some digital cameras have noisier blue channels than others), and greenscreens tend to be brighter/more reflective than bluescreens so there can be more problem with green spill onto the subject.

But there is no reason why a greenscreen would produce a "cheezier" composite than a bluescreen.
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#3 Perrone Ford

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Posted 10 September 2009 - 11:26 PM

The only technical issue is that green tends to be cleaner than blue because blue is a noisier signal / layer (some digital cameras have noisier blue channels than others), and greenscreens tend to be brighter/more reflective than bluescreens so there can be more problem with green spill onto the subject.


While I am *FAR* from a lighting expert and barely qualified to even speak on this subject, I have been doing a bit of research on this over the past year as I begin to extend my range into chromakeying. David, would it be correct to say that given video camera's penchant for green, that doing greenscreen on video would likely produce cleaner keys in most instances? Of course barring circumstances where the subject needs to be wearinng green, or as you say, blondes keying better against blue.

As I've heard it, blue was popular on film because the film stocks were more blue sensitive. But shooting outdoors on production, using blue proved problematic, especially when having to shoot against the sky.

Again, this is all second hand for me as I've only ever played with keying from other's work. I've never shot it myself.
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#4 Keith Walters

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Posted 11 September 2009 - 08:14 AM

Hey there all of you cinemaphiles.

I was wondering if anyone out there can tell me what the differences are between green & blue screen. When would you use one over the other? Is one necessarily better than the other? Is one easier to use than the other?


Also, it seems like a lot of the green screen I see on TV looks cheesy. How do you get the most out of it?

Thanks.

Thomas

There is also a historical element involved, since this technology was orignally developed for film use.
As David said, the blue layer of a film emulsion is the most sensitive, but also, because the blue layer is on the top, it also tends to have the sharpest focus.

The original chemical "bluescreen" process had to be very rigidly standardized because of the complex processing steps required, and so anybody who wanted to use it had to use a particular shade of blue for best results, and adjust their production to suit it.

When colour TV came along, an entirely electronic matting process (incorrectly) called "Chroma Key" was developed, which for many years could only be done live, using the full-resolution RGB signals from the camera tubes. Most studio vision mixers gave the operator a much greater range of keying options than were possible with film, allowing keying of just about any colour. However the blue-screen system persisted, partly because a blue infrastructure (screens, blue costumes etc) was already in place, and also because with tube-based cameras, the blue tube was the most sensitive, just like with film.

With the development of component video in the 1980s, for the first time it became possible to do chroma keying off recorded footage instead of live. However the best results were then obtained using a green background, because the luminance signal is composed of a greater amount of the green channel than red or blue.

On the other hand, if you are keying from 4:4:4 RGB, just about any colour will work, but again, you're likely to find that in the real world, the industry is already set up for green screen, so you might as well use that.

As for green screen looking "cheesy," that's entirely down to the skill of the operator.

I've just bought the entire series of "Lexx" on DVD (a 20 disc set). A huge amount of that show was done with green screen, but it's only when you look at the "making of" bits at the end of each disc that you realize just how much green screen was actually used, and how well it was done. And that was 10 years ago!
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