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Green Screen Noise


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#1 Phil Beastall

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Posted 18 February 2008 - 05:50 AM

Why is that when you key out a green screen there is often noise around the person being keyed, or the objects in the foreground? Is there a way to remove it? There's a fuzzy pixelated edge to it
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#2 Walter Graff

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Posted 18 February 2008 - 09:33 AM

Why is that when you key out a green screen there is often noise around the person being keyed, or the objects in the foreground? Is there a way to remove it? There's a fuzzy pixelated edge to it


It has to do with the format you are using, and the software you are using to do the key (and the knowledge you have of that keyer). Yes there are plug ins that offer the ability to choke green screen keys and smooth out the edges and 'fuzz' you see. What format are you using, and what software to do the key?
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#3 Chris Keth

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Posted 18 February 2008 - 06:59 PM

That's just a poor key. A well-lit screen and subject should nearly key itself. With a good compositor, it gets totally invisible.

It sounds like there is green spill on the subject from being too close to the screen.
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#4 Matthew Parnell

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Posted 18 February 2008 - 09:34 PM

Could also be the recording format. The colour space of dv is not good for keying.
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#5 Chris Keth

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Posted 19 February 2008 - 01:28 AM

Could also be the recording format. The colour space of dv is not good for keying.


It's not ideal but I've done it with good results. You just have to be careful with the separation between the subject and screen, mostly. Spill is harder to fix.
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#6 Walter Graff

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Posted 19 February 2008 - 07:11 AM

It's not ideal but I've done it with good results. You just have to be careful with the separation between the subject and screen, mostly. Spill is harder to fix.


It's easy enough to use a 4:1:1 color smoother filter to smooth out any jags as does a matte choker filter. And a simple green spill filter solves most all problems of reflections. If you folks are in NY you ought to attend my green screen seminar in March. I put to rest most all myths (and ther are far too many of them) about green screen shooting. If not, I am finishing up a new article on the myths of green screen that should rock most of the threads on green screen and what causes poor keys, etc. I'll notify the group when the article is up on the website. It has some 40 illistrations so far coving everything from spill, to screen intensity and how you can cut a key with 7 stop differnces each way and how even green shirts can be keyed on green backgrounds. Like the myth that we only use ten percent of our brain, there are so many myths about green screen that we simply accept as truth but no one has shown anyone that they are simply not true. I do this in my DVD on green screen, in my seminars on the subject and soon in this extensive article.
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#7 Benjamin_Lussier

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Posted 23 March 2008 - 11:03 PM

If you're shooting on HD, DVC-PRO 50 or any other 4:2:2 format,

Overexpose the green screen by one stop. It will reduice the noise in the compressed color channels. Also, noise or grain on a green screen shot is very common. A decent compositor should be able to remove it fairly easily. There're many techniques.
Matte contrast, Despotting filters, frame blend passes ect.

Pinnacle systems' composite wizard is great for removing spill, noise ect. in After effects.

I personally find matte Choking... barbaric... hehe. It ruins the matte, removes the motion blur ect... It's the last resort I guess.. Ive never had to use it.

If ur shooting on DV... well.. I just wouldnt recommand that.


Ben
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#8 tylerhawes

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Posted 24 March 2008 - 07:35 PM

A well-lit screen and subject should nearly key itself. With a good compositor, it gets totally invisible.

Really? We've done thousands of greenscreen shots, and I've yet to see one that I would describe that way. On big movies, too. To be fair, I note you said "nearly", but the tone of it makes greenscreen sound easy and simple, and it is actually almost always more work than anyone expected it to be (except for the jaded veterans). I remember on Superman Returns there were many, many rounds of revisions to get the keys right, and that was under the sage Mark Stetson. God help indies! :lol:
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#9 Hugh Macdonald

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Posted 26 March 2008 - 07:17 AM

I would never say exactly "key itself", but I've worked on a number of shots where, if the greenscreen is nice and solid, I can pretty much chuck in the same bunch of nodes that I know I usually use and it'll have got me pretty much all of the way there.

When I started out in this industry, I found keying to be a complete nightmare. Now that I've been doing it for a while, I've done it in so many different ways that there's usually one way that works well. Different compers will have different solutions, but any good comper will be able to come up with a solution (or be able to say pretty quickly if it's going to need rotoing).

Edited by Hugh Macdonald, 26 March 2008 - 07:18 AM.

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#10 tylerhawes

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Posted 26 March 2008 - 11:21 AM

We did a film last year that was shot on Genesis and was 100% green screen, typically with 2-3 layers and sometimes 6-7 layers. So you have about 1200 green screen comps. I know that we had the same Shake node tree on 90% of the shots. It took some time to get it tweaked to the specific lighting setup that the DP used, which was pretty consistant. Once we did that, about 30% of the time a copy-and-paste of nodes would get the shot there. Another 50% of the time I'd say the same nodes worked, but needed some tweaking for the shot. The last 20% of the time, it was almost back to scratch or else significant roto.

The only real problem was that the client had limited budget for post, didn't consult us until 2 days before the shoot when it was too late to implement most of our advice (where we basically told them that they were doomed to a big budget overrun if they didn't change their approach), and had this expectation that you just hit a magic button and the green screen would be perfect. It was a very experienced director and DP, just not a lot of VFX experience. Fortunately they quickly realized the reality and became pragmatic, not expecting perfect keys but basically something that would convince you on a viewing or two, but it was OK if you saw some issues under close examination. IOW, don't sit there looping the shot to find every forensic problem.
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#11 Markus Manninen

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Posted 03 April 2008 - 10:54 PM

Keying is a bit of an art in my book. I've always relied on a great roto department for excellent extraction. The one's I've worked with are great at using the keyers and roto only what they have to. When you find a guy or gal like that, keep them, and don't tell anyone. It's a dirty little secret that we depend on them heavily in teh visual effects industry.

You can't always have the green screen at a "comfortable" distance from your object, and at that point ti really becomes about good compositing. You will have so much spill in your subject that you need to color correct out after you have the mattes, that the two things need to work as one. That's truely an art.

On that subject, I am going in to a post house soon to do the latter for a commercial I shot the live action elements for in December last year. Sigh. Wish me luck. :unsure:

Markus
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