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Advice and Tips for Green Screen Project

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Kris Veenis
Advice and Tips for Green Screen Project
on Jul 17, 2012 at 2:35:56 am

Greetings,

I have been asked to do some greenscreen compositing work on an ongoing video project I have been working on for awhile.

The basic background of the project is this (please bear with me as it is a bit complicated and I want to make sure I explain everything about the project so I can get as much response to my questions as possible): I am being asked to create a marketing template for a internet based cooking program for sponsorship purposes, basically in return for sponsorship money from a particular grocery store, we will create a virtual "shopping tour" with each grocery chain's store brand products. The problem though is that the stores that have signed up to do this do not want to pay our proposed budget to have us travel to them and have us film directly in their 'home' stores. A solution that we found is that we struck a deal with an independent grocery chain that will allow us to shoot the majority of the static plate shots that we'd need for background(s) to make the final video look like it was filmed in a grocery store and then the individual chains that have signed up for sponsorship have agreed to fly in a spokesperson (most likely a nutritionist) from their store to us where we will film them in front of a green screen doing their particular pitch about their store and the nutirional value of certain types of foods which we would highlight.

Having never done extensive green screen work before I have a few questions.

1) the budgets for these shoots are fairly razor thin. Their may be money in the budget each time to rent a camera but I was told not to count on this. The camera(s) that I personally own that I could use in this project are as follows I have 1 panasonic gh2, 1 Sony Z7U, and (2) Sony Z1u's. If anyone can give me a run down on what some of the pro's and con's of using any of the above cameras would be as well as which one they might recommend and any problems that I should expect to encounter using cameras that do not employ the 4:4:4 color space.

2) We will be shooting the greenscreen at a local film school in Pittsburgh PA. The greeen screen takes up the back wall of a large room that measures roughly 30' across and their is greenscreen that extends onto the floor of the room for about 12' forward from the wall. There is also some overhead studio lighting in the room that can be moved and adjusted, although I am not 100% sure of the wattage of these lights. I am also able to bring in additional lighting and/or sign out loose lights from the school. Is there any advice anyone can give me regarding how I can light to get a consisten even look on my subject so that I minimize the problems that I might have when I go to pull the greenscreen out during compositing?

Any answers to the above questions that I posed as well as any additional thoughts that anyone can lend me regarding green screening and compositing would be greatly appreciated.

Best,

Kris Veenis
Filmmaker/Videographer
Co-Producer: The Shot Felt 'Round the World
vimeo.com/krisveenis


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Angelo Lorenzo
Re: Advice and Tips for Green Screen Project
on Jul 17, 2012 at 6:27:04 pm

Well, I would lean towards the GH2. Why?

- Both Sony cameras use HDV which is an MPEG2 based format with a bitrate around 25mbps

- The Panasonic GH2 uses an H.264 codec at about 24mbps. Since the compression is much more efficient I think you'll find you have less compression artifacts. Hacked GH2's also have higher bitrates.

All the cameras, hacks not withstanding, record with 4:2:0 chroma subsampling which is really not ideal for green screen work.

If you have any kind of budget, consider getting an external recorder that can record from HDMI (if the GH2 displays a clean signal with no info overlays), which is capable of 4:2:2 recording. Something like the Atomos Ninja.

In terms of lighting the green screen, it's difficult without knowing what you have. Hopefully you have some spacelights http://blog.adoramarentals.com/tag/continuous-lighting/ otherwise you'll just have to take fresnel lights, put them on full flood, mount some silks/frost if you can (2x2 frames for instance), and space them accordingly.

My advice, on something where you're scraping the bottom of the barrel, I strongly suggest you avoid shooting full body shots on green. These cause all kinds of keying issues even if well lit and if not keyed correctly, they can really lower the apparent quality of a production. To simplify the keying process, any time you aren't shooting full body shots, place black duvetyne to cover the green floor, this will greatly wrangle green spill.

Angelo Lorenzo
Fallen Empire - Digital Production Services
RED transcoding, on-set DIT, and RED Epic rental services.
Fallen Empire - The Blog
A blog dedicated to filmmaking, the RED workflow, and DIT tips and tricks


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Kris Veenis
Re: Advice and Tips for Green Screen Project
on Jul 17, 2012 at 7:06:51 pm

Angelo,

Thanks for your detailed response. A few follow up questions:

1) Thank you for confirming that out of my options my best bet would be the gh2, this is what my research had lead me to believe but I wanted to hear back from someone to help me confirm what I had been guessing. Question about shooting format for the GH2, I am assuming from what I am reading that if I'm shooting with one of the high bit rate hacks, which I plan on doing, should I be shooting 24p rather than 1080/60i, Is this correct?

2) if shooting with an external recorder like the atomos via the hdmi out on the gh2, "you said to be shooting with a clean signal without any display overlays?" By this do you mean that I should have all the indicators on the lcd like the histogram, ect. turned off?

3) I am not certain I will have budget to rent spacelights, but I will have access to a number of fresnels as well as at least 3 1000watt softboxes and I can obtain the frames and frost diffusion as well. With that in mind in my reading I have found suggested that I as evenly as possible light the green screen and then also try to light the subject carefully to prevent extra spill so that the subject can be as fully defined from the background as possible, is this correct? My main concern since I haven't done extensive greenscreen work is in not creating enough detail seperation between the subject and greenscreen and that causing it be be harder to key out the greenscreen in post, any thoughts on general lighting techniques would be greatly appreciated.

4) To your last point about avoiding shooting full body shots, is this because it just creates a situation where there is physically more subject matter to deal with, why in general does this situation make the subject harder to key is it because of greenscreen color spillage? Related question, would you recommend then to generally stay in close up or medium shot on the subject?

Best,

Kris


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Angelo Lorenzo
Re: Advice and Tips for Green Screen Project
on Jul 17, 2012 at 8:24:52 pm

1) I can't say. I'm not familiar enough with that camera to know the quirks/drawbacks with some of the extended hacks.

2) Yes, exactly. You'll want to make sure it's capable of outputting an image that displays nothing over it like a red recording circle, runtime, histogram, etc.

3) Since you don't have to light theatrically per se (this seems like talking head stuff) then there are some tricks to aid in separation. A hair/rim light is always a nice choice and you can combat some of the spill by putting on a 1/4 or 1/8 magenta (aka minus-green) gel on those rim lights or, if it makes sense and you have a 1/8th, the fill light.

If you have 20x20 solids, you could hang those on the far end of the green screen walls to reduce spill further. Anything that isn't directly behind the announcer will be garbage matted out so always consider that solution.

4) There are a number of technical issues. Number one is probably the skill of the person pulling the green screen key. Full body shots have a lot of spill and lower edge detail - does your production have the expertise and time to make these shots look good or nearly as good as the easier mediums/close ups?

You could shoot with the camera on its side (portrait orientation) if your host isn't a big hands talker. This buys you much more resolution to pull a key with.

Secondly, if this is the gateway to eventually selling this or signing a contract with a customer, try to play it safe. Shooting a medium wide or closeup exposes you less to more difficult green screen situations. Try shooting a full body, but shoot an alternate in the medium in case issues arise in post. Keep your bases covered.

Angelo Lorenzo
Fallen Empire - Digital Production Services
RED transcoding, on-set DIT, and RED Epic rental services.
Fallen Empire - The Blog
A blog dedicated to filmmaking, the RED workflow, and DIT tips and tricks


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Kris Veenis
Re: Advice and Tips for Green Screen Project
on Jul 17, 2012 at 8:41:28 pm

Thanks again for your responses Angelo, yes I think that mostly the subject will be stationary talking head, there may be limited movement towards the camera and the subject will be picking food objects off a pre-dressed table with items for each specific section of the store which will be filmed also in close up, but there won't be any extreeme movements such as running ect. One thing I was reading in a few online tutorials was to try to place the subject(s) at least 5 feet in front of the green screen to help combat spillage and create more separation. The room I'll be doing the green screen in is fairly large so I believe I should be able to do this, any other, any other thoughts that you might have in relationship to subject distance from the green screen?

Lastly, I was thking of using pre-visualization software during the shoot but upon further investigation this won't be possible if i tie up the hdmi out with an external recorder, which also apparently disables the other outputs on the camera as well. In this case since I read that it was suggested that I expose the subject during the greenscreen session for near the same exposure as the background plates I will shoot. Any advice on techniques on how to do this effectively?

Best,

kris


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Angelo Lorenzo
Re: Advice and Tips for Green Screen Project
on Jul 18, 2012 at 10:10:25 pm

I don't think there are any hard and fast rules about placement. I'd say the further, the better. This allows more room for placement of rim lighting and depending on how much light you're pumping in, you may still get a lot of spill at 3-4-5-6 feet. You'll have to play that by ear.

In terms of exposure, whatever your subject is reading on the key side, that is what your green screen should also be reading. Unless you're lighting for something really high key or low key, your screen should be +/- 1 stop from where normal scene exposure is. A good spike in the middle of this histogram if you don't have a light meter is a safe bet.

Angelo Lorenzo
Fallen Empire - Digital Production Services
RED transcoding, on-set DIT, and RED Epic rental services.
Fallen Empire - The Blog
A blog dedicated to filmmaking, the RED workflow, and DIT tips and tricks


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Kris Veenis
Re: Advice and Tips for Green Screen Project
on Jul 19, 2012 at 3:10:35 pm

Many thanks Angelo, the advice you have given me is very helpful and insightful

Best,


Kris Veenis


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