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Old February 13th, 2006, 06:36 PM   #1
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ChromaKeying - Blue vs. Green?

I'm planning on making a fairly large chroma screen to mess around with and practice on. I went to WalMart and bought a couple sheets of blue and green construction paper and tacked them to the wall. After messing around with it a bit, I discovered a few things. They each seam to have their own strengths and weaknesses, but the green reflected everywhere making it harder to key out objects. Blue is a more common clothing color though. The colors of paper are fairly close to what I've seen used officially, though the green might be a bit brighter and more 'neon' than usual, and the blue a bit darker.

It seems green has been the traditional color of choice, but I know a lot of places also have blue. I only have enough room to make one, so I need to choose carefuly.

What information can you provide about this and what suggestions do you have?

Thanks
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Old February 13th, 2006, 07:23 PM   #2
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Color is secondary to using good lighting and camera configuration, but after that, the best screen color to use really depends mostly on what you intend to put in the foreground.

The general rule for video is to use a green screen, unless the subject is bright. Shades of yellow, white, blond, red hair .... and of course green, will key more easily against a blue screen, but just about everything else will fair better in front of a green backdrop.

If you're in control of selecting foreground AND background, the recipe for quick easy keys is to place dark subjects, clothing, hair, skin, etc. in front of a green screen.
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Old February 13th, 2006, 07:27 PM   #3
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They tout green as having more color information for DV cameras to pick up, but I've done perfect keys with both colors. I'd do green though, personally.
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Old February 13th, 2006, 08:19 PM   #4
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in general without regard for color in sets/actors ..

green works better with video 4:1:1 & 4:2:2 color space ....

camera that shoots 4:4:4 color space can do BOTH eqully as good ...

many film stocks tend to work better with Blue ...

both film and video can key both .
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Old February 13th, 2006, 08:21 PM   #5
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I have heard that green is best because Digital Video get's it's luminance channel from Green. Therefore G100% will always be brighter than B100%

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Old February 13th, 2006, 10:43 PM   #6
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Oh, I intended to also ask about lighting. What would you suggest for lighting? What I had planned to do was get enough pieces of paper to cover the wall and mount them using double sided tape so there's nothing on the surface. I was going to get some track lighting to put along the top of it similar to what I've seen in a studio, but I don't want to have to fix anything afterwards either. I also don't want to pay a lot for lighting either.

What suggestions would you have for lighting a screen?
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Old February 14th, 2006, 12:05 AM   #7
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BTW, you might save yourself some trouble and time by getting some Rosco Chroma Key Green latex paint. B&H has a gallon in a dented can for $30.
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Old February 14th, 2006, 01:30 AM   #8
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Quote:
I have heard that green is best because Digital Video get's it's luminance channel from Green. Therefore G100% will always be brighter than B100%
DV (i.e. miniDV) has a luma component that derives from a combination of red, green, and blue.
Rec. 601 luma co-efficients: Luma (Y) = 0.299 R + 0.587 G + 0.144 B

Green is weighed more than red and blue, so 100% green is brighter than blue. But really that doesn't have anything to do with what color will key better.

The blue channel with miniDV equipment is usually the noisiest.

2- The lighting forum here has discussions on the best way to light a green screen.

*You can also search cinematography.net for fairly extensive (and non-conclusive) discussions on chroma key color.
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Old February 14th, 2006, 04:52 AM   #9
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This might also depend on what you are keying with in post. In the previous version of my editor, the screen had to be well light and flat to key well. Since they added some compositing tools found in another of their products, you can fix the key before you use it. So, it may depend on what you are using too.
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Old February 14th, 2006, 05:08 AM   #10
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Hmmm, Keying comes up a lot - isn't it about time to set up a "Keying and compositing for DV/HDV/DVCproHD forum?"
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Old February 14th, 2006, 07:39 AM   #11
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Quote:
Originally Posted by Don Donatello
in general without regard for color in sets/actors ..
green works better with video 4:1:1 & 4:2:2 color space ....
Your second sentence is precisely why it is so important to consider your foreground.
Green works better as the CHANNEL to base your mask on ...... but don't confuse this as stating that green is always the better background screen color.

Consider blond hair, caucation skin or simply the colors white or yellow.
The green channel value for these pixels is very high.
In fact, under typical lighting and exposure, the green channel values for these colors is as high as the green channel for a green screen!
As a result, with no contrast in the nice clean, hi-res green channel, you are left with calculating a key based on contrast in the more compressed and noisy blue channel. Not fun.

A blue screen, on the other hand, has a very low green channel value, providing EXCELENT contrast in the clean, less compressed green channel for bright foregrounds.
Very handy for clean compositing.

The reason to use a blue screen for bright foreground subjects is NOT to key on blue, but rather to enable you to exploit the video quality in the green channel for your composite !!!
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Last edited by Nick Jushchyshyn; February 14th, 2006 at 09:45 AM.
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