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Old May 20th, 2006, 02:01 PM   #1
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Join Date: Apr 2006
Location: Buffalo, NY
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Green Screening insights please?

Greetings all,

I'm posting this because I want to learn more about green screening, and my school's cirriculum briefly touches upon it... next semester... and that's just too long to wait.

so, I'm wondering what nuggets of information you all have regarding this process... and, specifically, if I can make my own setup instead of purchasing one...

- I've been told that equal lighting is required, so as not to cast shadows on the screen itself...

- I've also been told that I should light the subject with amber gels...

Initially I will be using the footage as a mock football show for cable access, but I do want to use the resulting video for pD shows...

Thanks for your time,

I appologize if this is in the wrong section. If it needs to be deleted, so be it.
--------------------my story----------------------
Media Study student at S.U.N.Y. Buffalo
Documentary / Real Time Graphics concentrations
Scott Blajszczak is offline   Reply With Quote
Old May 20th, 2006, 04:03 PM   #2
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Join Date: May 2006
Location: SF, USA
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Hey, I've just started working with green screens too.

Mainly I've been doing things like painting a skateboard blue and trying to make it look like people are floating in the air (idea from Yeah Right!).

I've found that definitly as you said you've heard, lighting and shadows present the most problems.

One thing that I've found helpful in some situations are garbage mattes. You'll have to check those out.

Also, I know this sounds basic, but just making sure you're actors don't have any blue/green on them. just in case!

So that's about all I've got.

I was wondering if anyone has any advice on the invisible skateboard tricks that Spike Jonze used, I'm having a lot of trouble doing it in Avid Liquid 7.1
James OClaire is offline   Reply With Quote
Old May 20th, 2006, 06:38 PM   #3
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Location: Honolulu, HI
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I'm also a novice at green screen but have done a lot of still-image compositing with Photoshop and Primatte.

Shadows: You can actually cast a shadow onto the green screen and have it survive in the matte process. Given the right circumstances, this can actually be useful.

Cameras and formats: The worst problem is the sharpening that all cameras apply to the video. It creates a subtle outline that makes it nearly impossible to get a subtle edge on keyed subjects. Many cameras will allow you to turn this feature down or to even turn it off, and that helps a great deal to solve this problem.

4:1:1 and 4:2:2 color: DV's color space means a very rough green channel, and that means very crude mattes. I tried Graeme Nattress' "GNice" filter which converts 4:1:1 to 4:2:2 and it helps a lot when trying to key DV. But it only works well if you can avoid problem #1 as stated above and turn off the in-camera sharpening circuit.

Lighting: To make it work, the light illuminating the background plate needs to match the subject in terms of direction, quality and color. The human eye is really good at detecting anomolies and, while most might not be able to directly state what's wrong with a shot, they'll have a subconcious feeling that something's not right. An example is our show where the host opens up each segment. We've been told several times that it's a really nice green screen job -- except that it's not. We light the host with a very strong reflector, and that disparity of lighting makes it look like he's in front of a fake background.

Lighting for the screen: This is where even lighting is needed. It needs to be as even as possible and absolutely consistent. So it almost doesn't need to be said that the exposure needs to be set manually and all the automatic features turned off. And, as James mentioned, garbage mattes can help a great deal. Actually the only place you really need to key is right along the edges of the subject. The rest can be garbage matted. And I recall seeing a tutorial showing how to do a very tight, animated garbage matte. Might have been something done by Andrew Kramer?

Andrew has a DVD that tells you how to do this sort of thing. Great price, too. I'm ordering one soon. Check out the online demos he has to get an idea of his instructional style.
Dean Sensui
Exec Producer, Hawaii Goes Fishing
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Old May 26th, 2006, 04:46 PM   #4
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Location: Portland, Oregon
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Regarding amber gels - these are very handy for the backlights, if they are stronger than the screen reflection off the subject they'll really help a clean key.

Most of my green screen work has been putting people in artificial environments made in 3d programs - found that lots of backlight and amber gels on the backlights really helped the sense of depth.
Seth Bloombaum is offline   Reply

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