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Old May 21st, 2006, 04:21 PM   #1
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greenscreening ground?

I am thinking about creating a greenscreen set with some ground(dirt) and have the edge recede off where it could be merged with a digital background. I realize the light issues-it would have to be a pretty big room with a giant green screen--but could it work? I am thinking in terms of forced perspective.

This would be for night scenes--shooting as much indoors would be great for scheduling--and since i have to doctor the backgrounds in the location shooting anyway.

I am hesitant about greenscreening the floor--how on earth(no pun intended) do you prevent the the green from splashing back onto the performer?
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Old May 21st, 2006, 06:50 PM   #2
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I would think that it would be difficult to evenly light the green screen without also casting unwanted light onto the subjects.

It might be possible to send the light from behind the subjects so that the green screen is evenly illuminated, yet the subjects are essentially back-lighted so that you can still pull off the day-for-night look. You might want to experiement on a smaller scale before committing to a large-scale attempt at this.

Another option: The ground they're standing on in the set is real, and have the green screen start off somewhere in the back. You'll have to design the set so that the green screen blends in with the foreground set. Perhaps a false edge like a cliff, rocks or foliage. You might also need to motion track the green screen itself if you have any camera movement, and avoid making the background plate appear to be pasted to the middle-ground.

I know this can be done as it's been done in Star Wars and other major productions. The big question is what was done to make it work at that level?
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Old May 22nd, 2006, 12:19 AM   #3
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Its the other option I was thinking about. Have real ground--but have the background of it blend into the greenscreen edge.

Another thing I am considering is partial sets. If the camera is locked down(and even if its not) it should be possible to add anything to the background--digital painting. I would much rather use a real partial set than a greenscreen.

If I used a partial set with ground--and some black tarp to suggest a night sky--and lit the thing--why not add the rest of the set/sky in post?

I dont see how blending edges would be such a problem in a locked down camera set up with photoshop layers etc. But I have yet to try it.
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Old May 22nd, 2006, 12:52 PM   #4
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If the studio were large enough, and you had enough paint or green material, you could definitely have the real set provide foreground and just have the greenscreen deal with the sky and distant background.

In a night scene you would just have to worry about vague shapes in the distance at best, and perhaps stars if needed. The keying would only deal with any objects and characters that moved against the distant background and sky.

If there's no overlap of characters against the distant background, then you could just blend a matte painting into the foreground as you suggested, and that would save the effort of green screen work. I hadn't had the chance to try that, although it's one of the effects I would like to try, even for a daytime shot.
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Old May 22nd, 2006, 01:09 PM   #5
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The best solution is to use the despill operations in your keying tool.
Most of the better tools have a despill operation.

Here's an example where we had the performer LAYING on the greenscreen floor with shortsleave shirts .... lots of green spill on their skin.
The key is just a quick-n-dirty comp using the Keylight filter in AE Pro 6.

The entire environment (including the floor) in the finished shot is CG, with matchmoved camera to match the on-set dolly/crane move.

http://www.techvantics.com/demo/nej_demo_aoau_v02.htm

If the "ground" is to be natural rather than flat, you can typically get away with adding a little breakup to the bottom of the key. In most situations, you only need to do this for a couple establishing shots, then cut to medium or CU shots for the duration of the scene.

Hope this helps.
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Old May 22nd, 2006, 01:40 PM   #6
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Wow
fantastic job.
And what a fine camera you used there too.

I was hoping if I could minimize the shooting schedule/location shooting I could look into renting a higher end cam.

The partial set thing really intrigues me though--the more natural I can make the setting--as in without a matte line around the performers, the better I feel about it...but with greenscreening examples like that it seems quaint and a waste of effort. The only concern is blurring problems I have heard can come with swift movement and greenscreen.

I have very few full view shots which is why I was considering the interior option for the exterior night stuff. But I wouldnt want to try doing an indoor for outdoor day shoot myself with either greenscreen or partial sets.
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Old May 22nd, 2006, 03:42 PM   #7
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A possible alternative is to do a difference key.

Here's the results I'm getting:
http://www.glennchan.info/Proofs/rta/pprod/

A difference key might be a bad idea. However, as I see it:

PRO:
The spill is perfect as long as your studio floor matches the surface you want. If you want the lighting to match between your environment + your subjects, this would be the best option (although you may be able to get just as good results via a green screen).

Cost-wise, you don't have to pay for green screen material or set material. It can however, easily be much more expensive than the other options if the key doesn't turn out well and you have to roto everything.

CON:
It doesn't work perfectly.
If you have lots of talent colors that match the background plate, that might give you trouble. You'd still be able to rescue yourself by painfully rotoscoping where the difference key messed up.

Notes:
A- A big reason for me doing a difference key was because it would cost me nothing, and I was lazy and didn't want to paint a bunch of flats green.
B- I had access to an Avid Media Composer, which records 4:2:2 HD SDI (1920x1080i) with low compression (6:1?). I'm not sure how a difference key plays out with more noise (i.e. DVCPRO HD, HDV, HDCAM, etc.). The compression noise might make the difference key not very clean.
The camera was a Panasonic HDC-900 or something like that.

--Do make sure that you shoot the background without anyone in it. Even if doing a green screen, this can help. Some keyers are able to combine both green screen + difference key techniques.
*If the camera is moving, then that isn't so easy.
--Do shoot some sort of reference to get the perspective right (i.e. how lines recede into the horizon/infinity).

2- On the Chronicles of Narnia, they did a partial set + green screen for some of it I believe.
http://www.hatchfx.com/matte-paintin...arnia_girl.jpg

3- But anyways, don't listen to me. The only reason I'm throwing the idea of a difference key is more intellectual curiousity than practicality.

I don't know how to do greenscreen work very well. By well, I mean the edges are clean and the various elements match each other. When trying to do a naturalistic environment, that's really hard because you have to match the lighting. You do need to figure out your environments beforehand.

In a practical sense, I'd try to find someone who knows what they're doing and *shoot a test* with the most difficult elements. So test with the cheapest camera you'd shoot with (i.e. some HD prosumer camera), test shoot the hardest scenes (i.e. night, natural environments with complex lighting), use subjects with blonde hair and translucent props+clothing, etc.
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Old May 22nd, 2006, 05:36 PM   #8
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Take a look at the video you can see from this page on the "digital backlot". Some pretty amazing stuff.

You can see how they handle the ground issue.

http://www.studiodaily.com/main/tech...ects/6503.html
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Old May 22nd, 2006, 08:28 PM   #9
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Hi Glenn,
You've noted a couple of the issues with difference keying in your page there Glenn. Another concern is that it can be a bit noisy. Color difference keying and 3D color vector keying appear the to be the most methods these days.

If you're really interested out of intellectual curiousity, I'd highly recommend Steve Wright's book "Digital Compositing for Film & Video" (now in it's second edition) where he discusses compositing down the the root mathmatics.... but with a friendly, approachable delivery style.
http://www.amazon.com/gp/product/024080760X
Steve is a great guy and has tremendous depth of experience in this field. This book is a must read for anyone that is serious about digital compositing.

There's also a great article about compositing at FXGuide.com:
http://www.fxguide.com/article314.html

BTW: Kelly ...
The easiest way to deal with spill is to plan ahead have your performers wear black shoes and pants. This makes keying against green much easier, and it's very easy to tune out any green spill.

Have fun.
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Old May 22nd, 2006, 10:50 PM   #10
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Well.
That digital backlot sample is totally mind blowing.
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Old May 23rd, 2006, 05:43 PM   #11
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The easiest way to deal with spill is to shoot for black and white. ;)

Thanks for the links Nick. I've seen the fxguide articles, there's lots of great stuff on that site.
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