I pulled my first Greenscreen in 1996 working on “Star Wars: Episode 1” pre-viz. The footage was rough (pre-vis handycam footage) and the After Effects Color Difference Key was a complete mystery. Now, the Pixel Corps, we shoot an average of two hours of greenscreen footage a day, mostly 4:4:4 uncompressed. While the tools have progressed significantly, the process itself, and the rules, have changed surprisingly little.
In this article, I’ll cover the major issues you need to solve when shooting greenscreen footage. There will be future installments to discuss theory and keying technique. I will argue about 90% of the professionals out there do not use most of the information I will lay out here… and it makes their process much more difficult. I will say, everything we do in the Pixel Corps is designed to lower costs and accelerate delivery speed while constantly improving quality. We’re not particular to be particular. We do everything because it shaves time or improves quality.
While some people still use a green cloth and a few shop lights, they are saving money on the front end only to pay for it later, in time and quality, on the back end. This “future” payment can often dwarf the cost of the original shoot. While you can’t always get a great screen, most bad screens that we see are shot as talking heads…where there is absolutely no excuse to shoot a rough backing. In reality, you really don’t need a super expensive set-up, but a few elements make a difference. The thing to realize is… 80% of the quality of your key happens on-set. If you shoot garbage on set, you will spend countless hours just to survive the composite. If you shoot clean, well lit, plates on set– you will key quickly and easily.
Let’s address Greenbacking first (this is the Japanese Term for “Greenscreen” and in some ways, more accurate…as the Japanese often are). I call it “Green” not “Blue” because, well, you should almost never use a bluescreen. Kermit the frog may thought it wasn’t easy being green…he obviously wasn’t a compositor. Blue is the most disrespected, beaten and shortchanged channel in the digital pipeline. When compression hits…it hits the blue record the hardest. It leaves it blocky, grainy and chattering. You can feel bad for the Blue Record but you shouldn’t use it for your keying operations…even out of sympathy. Green, on the other hand, is usually crisp and clean because it actually gets most of the YUV signal in the conversion. Compression will still affect it but not nearly at the level that it affects the Blue.
So, we’ve decided on green, but what kind of green? You can get Rosco, Composite Components, Wescott, or the green paper at the Pharmacy. At the Pixel Corps, we only use Composite Component’s “Digital Green” (and no, we don’t get paid to say that). On paper, it shouldn’t really work as well as Rosco…but it does by a significant margin. I would not suggest Wescott pop-open screens or anything that feels like corduroy or cloth. Both of these will suffer from reflectance issues and patterns. They are cheaper, and there’s a good reason. If you love Roto and lots of extra hours working on the keys…knock yourself out. But I’m lazy and prefer to make the capture process as clean as possible.
Once you’ve chosen the screen flavor, you need to decide the screen type. The best solution, when you can implement it, is a smooth green painted wall. It’s the easiest to light evenly (which is 60% of your challenge). If you can’t muster a green wall, Lycra is the next best choice. Again Composite Components Green stretched tight is your best, though not cheapest, bet. You can go out and buy a fancy frame for it but we often use 1” PVC pipe that’s 1.5 feet longer and wider than the greenscreen. The nice thing about PVC is that you can get in any city…making it easier to travel. If you are going to go with paint, plan on 4-5 coats. You need it to be really green and really consistent. We also don’t “touch-up screens” we repaint the wall every month or two and we really talk care of it. It’s expensive, but not as expensive as “fixing it in post.”
Alex, you say “If you are on a PC, Adobe’s OnLocation has a good set of scopes to work with.”
OnLocation CS4 runs on Mac OS.
Otherwise, this is a great article. Thanks for the information. I just added a comment to “Keying overview and tips” section of After Effects Help on the Web to point to this article:
http://help.adobe.com/en_US/AfterEffects/9.0/WS3878526689cb91655866c1103906c6dea-7bfda.html
Posted by Todd_Kopriva on 03/27 at 09:44 PM
The nice thing about the super-green Kinos is that they are very, very efficient. I regularly light 12x12 green screens with one bulb on either side… and it’s often too much!
Posted by Art Adams on 03/27 at 10:19 PM
Hi - nice article. Good to see someone trying to get people to shoot better greenscreens.
I’d take issue with you on your disparagement of bluescreen.
You assume that the key mostly depends on the chroma backing channel. And therefore, say that blue is a bad choice, because that colour channel (in film or video or HD) is noisy.
I fact, the way I see it (and from my experience), the quality of the key doesn’t depend on the cleanness of channel recording the backing area, but rather, it depends on the colour difference between the fg and the bg. The key, therefore, depends on all three colour channels - in the case of greenscreen, r and b versus b, in the case of bluescreen, r and g versus b, And, in the majority of cases, blue will have far greater difference from the fg than green will.
A well lit, flat bluescreen will in most cases give a much nicer one click key than greenscreen. For similar reasons to the above (colour difference), bluespill is also easier to remove than green.
IMHO! 
Posted by paddy on 03/31 at 11:45 AM
replace one sentence in the middle with:
The key, therefore, depends on all three colour channels - in the case of greenscreen, r and b versus g, in the case of bluescreen, r and g versus b,
Posted by paddy on 03/31 at 11:46 AM
I haven’t shot a blue screen in years and years. The reasons are several:
Blue is the noisiest channel because silicon is least sensitive to blue. Under tungsten light the blue channel has to have gain added to it in order to match the levels of the others, as there is very little blue in tungsten light compared to red and green so it is always noisier. Hit the blue-only button on a monitor while looking at a picture (instead of bars) and you’ll see it easily.
For corporate work, people tend to wear more blue and little or no green.
The green used for keying is brighter than the blue is, so it requires less light—which saves money.
You’re absolutely right that it’s the difference between chroma channels that results in a good key: at least 40 units of separation between the backing color and the next closest color is how I was taught to do it. But I’m sure there’s a difference in edge quality between the noisy blue channel and the relatively clean green channel.
When I first started shooting green screen on video we aimed for 40-50 units, since we thought that, like on film, you should expose for “middle gray” values for maximum saturation. That came from the blue screen days, where you can get a bright blue or a saturated blue but not both at the same time because a bright rich blue is not in film’s color space. Electronically, though, you can really pump up the screen for maximum color difference, which is why 60-70 units works so well.
Posted by Art Adams on 03/31 at 12:57 PM
All things being equal, I know that a nicely saturated bluescreen will give a better result that a nicely saturated green. That is - in the conditions that I have been working in: 35mm (and more recently HD) features using Keylight in Shake, and before that, Cineon.
As I said above, a good key is only partly to do with the backing colour. If the colour difference off blue is greater than off green (and it will be, for most subjects (blue suits excepted)) then that will mean a LESS noisy key. A key is defined by all 3 channels together, not just one.
Green has its own issues. Because it is closer in hue to skin, hair etc, it can be really hard to get rid of green spill without getting a nasty desaturated look, and grey/white speckles in the blacks.
If lighting with tungsten, then, yes, you will need slightly more light for bluescreen than green. Especially if you are doing what you should be doing, which is to put heavy blue filtration on the screen lights. But if you *aren’t* using tungsten on bluescreen (and, to be honest, why would you?!) then there is no reason why you would need more light for bluescreen than for greenscreen.
Re your notes about bright, saturated blues not being in film colour gamut. Maybe in terms of print, but not neg! I live for bright, saturated blues on film! Doesn’t happen often enough, but it does if I have anything to do with the shoot! 
Posted by paddy on 03/31 at 03:23 PM
Actually, under tungsten conditions you can light blue screen with uncorrected HMIs and end up with a lot of saturation.
When I was talking about a noisy blue channel I was speaking about shooting digitally (something I should have made more clear). I haven’t shot blue screen on film in a LOOOOOONG time. And yes, you’re right, blue is much easier to key flesh tone against, and blonde hair, etc., but I haven’t seen blue in years and years. I’m not sure why, but it just isn’t done anymore.
Posted by Art Adams on 03/31 at 03:51 PM
Something to look for on the waveform monitor on page 3: the fact that the edges drop down a little bit is something you can fight and fight and fight, only to discover it’s not the lighting: it’s the lens. Most video zooms show marked edge drop-off starting at fairly modest focal lengths. Usually this isn’t a big issue. Just know that while it looks like the scope is telling you the left and right edges are falling off in brightness, they probably aren’t.
Posted by Art Adams on 03/31 at 04:01 PM
Agreed, you tend to see more green than blue nowadays.
I do remember when greenscreens started to be used, thinking… why? It seemed almost like a fashion at the time, and to me, still does.
My point is that blue should be (re)considered, because in many cases, it can give a better result. A certainly, on big film shows with vfx supervisors who are prepared to do their homework, you still quite often get bluescreen. I believe “300” was mostly blue. King Kong also. Narnia, much of the recent Potters, Watchmen etc.
(BTW - film is just like video in terms of the blue channel ( emulsion layer) being the grainiest. Doesn’t matter - the improved colour separation with foreground still out-weighs this in most cases.)
Horses for courses, I guess.
Posted by paddy on 04/01 at 05:26 AM
You don’t think we’re getting enough separation in the images shown?
With Green on Green, you get about as much as you want for most shots. We can usually key glass without much work.
I find blue always affects the edge noise…requiring work that lowers the detail. It is easier to despill but we don’t have much trouble with that using either Steve Wright’s Unspill macro or Conduit’s Unspill node.
For me it used to be a conditional process but after a few days of side-by-side tests, we decided to stop using bluescreen about 4 years ago.
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Posted by Alex Lindsay on 04/01 at 08:27 PM
The cases shown are in highly controlled conditions, where it is possible to get a very flat screen, perfect lighting, green on green etc. In those conditions, you could probably key off any colour at all.
I’m talking about larger sets/locations, with much messier conditions, a variety of fg material (not just heads), unevenly lit screens etc.
All things being equal, IMHO, on film or
HD I’d choose blue first over green. The reasons why it was the first (only) choice back in the optical days still apply, with digital technology.
Posted by paddy on 04/02 at 03:56 AM
There are still folks that I respect a lot that prefer blue. I just have never worked on a green plate that wish was blue (we shoot a fair bit in “uncontrolled” environments too). I have worked with many blue plates that I wish were green.
I think the big difference with Optical and Digital is the unspill process. Unfortunately, most keyers don’t take advantage of this.
I will say the one place I would be tempted to shoot blue would be in direct sunlight…where the lower luma is often advantageous.
Posted by Alex Lindsay on 04/02 at 09:29 AM
True, green material (esp Composite Components, whose material I adore) can get overexposed in direct sunlight.
Re despilling: I’m a Keylight man. I’ve never come across a better keyer.
Posted by paddy on 04/02 at 10:45 AM
yes, the example shown here has been performed in the ideal conditions, had it not been ideal the results i guess would have been much different!
Posted by mikejons on 04/20 at 12:35 PM
There’s a valid reason to shoot green when using digital sources. Even though the key is a result of a three channel difference, the backing color is the most important when defining the edge.
Chroma subsampling causes blockiness on the edges, if the backing color is other than green (green is carried along the full resolution luma signal, blue and red are not).
This blockiness can be seen especially well with 4:2:0 sources like DV, HDV, XDCAM HD/EX and so on, but it’s there with 4:2:2 sources too. the color channels can of course be filtered to minimize the artifacts, but that doesn’t fully make up for the artifacts. Only with 4:4:4 sources the playing ground is level for all colors.
I made an example image using 3D rendered sources a few years back, compressed to HDV and DV, to point out the difference in optimal situation. Some of the text i wrote back then was a bit, well, guesswork, but the images are still valid, and show the results of varying BG colors well.
http://www.kolumbus.fi/erkki.halkka/HDVKeying/Compression_and_keying.html
Posted by Halsu on 07/22 at 03:35 PM
Yes, good point I guess. I have never comped blue or greenscreen with less than film scans, or 4:4:4 HD.
Posted by paddy on 07/22 at 03:44 PM
Alex: Thank you for the in-depth articles. To Alex and anyone else familiar with green kinos: I ordered some green kinos after reading this article and was surprised to find they are four pin lights when I opened the box, the picture on film tools website showed them as two pin. I say all this to ask: the article mentions affordable fixtures for kino bulbs from Walmart, are these affordable fixtures mentioned four pin or two pin?
Posted by ozdogink on 01/17 at 05:56 PM
