September 30, 2015

5 Secrets to Pulling Off a Hollywood-Level Chroma Key (Besides Good Lighting)

Typically when we talk about how to chroma key, we put a lot of emphasis on how to construct your lighting. But, what are some other tips that will help make yours look clean and professional?

Director/Cinematographer Matthew Rosen shares with us some of his "secret" techniques in the video below. And trust us -- there's no talk about how to light for chroma key. We promise.

Choose the right chroma color

Chroma green is typically the color filmmakers choose when the decide to do some compositing, but not necessarily because it's better or worse than blue (or any other color). Though there are plenty of technical advantages to using green, your choice should be based on the qualities and obstacles of each specific shot you're capturing -- and most of the time blue is found in the shot more often than green. Here are some tips Rosen points out:

  • If there's green in your shot, choose a blue chroma color. If there's blue in your shot, choose a green.
  • Green is twice as reflective as blue, so it tends to contaminate your shot more.
  • If your background is blue or green, use those respective colors for your key color.
  • Most modern cameras have sensors that use the green channel to carry luminance, so shooting on a green screen could result in twice as many pixels.

Don't overexpose the chroma screen

You're gonna need to watch this part of the video to get a good explanation on how and why you need to expose shots differently for chroma keying, because I simply can't give you one. (Lots of percents, RGB parade talk, and zebra stripes.) Simply put, a healthy exposure for a typical image just won't cut it for chroma keying -- it'll be overexposed.

The cleaner the screen, the cleaner the key

Other than green or blue fabric, the next most important tool to have on you when chroma keying is an iron. Seriously -- or whatever it is that will help you flatten those creases and folds in your screen. And I KNOW we said that there will be no talk about lighting, but you can't really talk about having a flat and even screen without mentioning that lighting plays a major role in making it that way.

The less you compress, the better the key

This is simple: using a format that compresses your video less, like RAW or ProRes, will result in a better key, because it retains the minute details of your image. 

One-click keys

Essentially, if you've done everything right -- you've chosen the right chroma color, exposed correctly, shot against a clean screen, and chosen the highest possible compression format you can -- you should be able to isolate your chroma color in one click when you begin the post process. See, every aspect of chroma keying is important, because they all influence the final result. A one-click key means you put in the work to do it right the first time and avoided the "fix it in post" nonsense.

Do you have any "secret" tips on how to pull of a pro chroma key? Let us know in the comments below!      

Your Comment

24 Comments

Camera placement is really important. If the thing you are keying is going to end up being small on the screen, the prospective will match better if you shoot it further away. Matching the angle, hight, placement, f-stop, and focal length of the shot you are compositing onto is important.

September 30, 2015 at 11:01PM

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a lot of really great info here, reinforces that we really need to use our RGB Parade/Waveform monitors while shooting to avoid headaches later.

September 30, 2015 at 11:11PM

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Gvickie Xiong
Editor/Cinematographer/Director
803

That was awesome. So much information without a lot of setup or blather. This is how it's done.

September 30, 2015 at 11:36PM

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>>>and chosen the highest possible compression format you can --

I think you meant to write "and chosen the LOWEST possible compression format", so highest BIT-RATE with the LEAST compressed footage.

September 30, 2015 at 11:44PM

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Guy McLoughlin
Video Producer
32474

..the subject needs to be far enough away from the key screen as to be able to be lit separately... in other words have the screen as a totally separately lit element to the subject.. this also avoids green spill ..

September 30, 2015 at 11:55PM, Edited September 30, 11:56PM

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Rob Englander
business owner/video producer/DOP/editor/miracle worker
193

One more green-screen tip...

Always check for lighting "bleed" from your green-screen background by turning OFF the lighting on your foreground subject, and see if the edges of your foreground subject are being lit by the green-screen background light. Ideally there should be NO green edges on your foreground subject so that when you key out the green color it won't effect the edges of your foreground subject.

Sometimes you need to set up large black barriers on either side of your foreground subject to ensure that they are NOT being lit by the green-screen background light.

September 30, 2015 at 11:57PM

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Guy McLoughlin
Video Producer
32474

Very good info. Cut straight to the meat and bones.

October 1, 2015 at 12:50AM

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Alex DePew
Cinematographer
98

Secrets to a Hollywood level chroma key:

1) It's not a 1-click process. You will inevitably need to use garbage masks, key mixes and a variety of techniques for each section of the image to get an optimal matte. That includes simply rotoscoping some parts.
2) You have to learn to treat your edges since getting an alpha channel is half the battle. Learning to pull color out of your core matte and working in unpremultiplied color is essential.
3) When all else fails, roto and paint.

The statement about Green being brighter than blue isn't true. I'm not sure why he's saying that, you can light blue green or red to the same exposure point.

Also one of the most important things to do when getting a good chrome key has nothing to do with the key itself it's ensuring that you're lighting your subject as if they were against the background you intend to put them into. If you light them (including your chroma background) as if they were standing in a bright white room, and then put them into a contrasty dungeon, they'll look terrible no matter what you do. Also please for the love of all that is good and holy, stop putting a red backlight on every green screen actor. It does nothing except give your character a red backlight when you're done keying to remove.

October 1, 2015 at 1:05AM, Edited October 1, 1:09AM

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You're completely right, you can light all three primary colors to the same exposure level. But I believe he is simply saying that green is inherently more reflective than blue, so if you were to light both a blue and green screen with the same intensity of light, the blue screen would have less spill.

This may be less applicable in a studio setting, but is worth noting when you have less control over the intensity of your chroma key light e.g., outside, or using natural light.

October 1, 2015 at 3:24AM

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I didn't see him draw this parallel, but I always thought green screens were brighter on camera because most cameras have twice as many green pixels as the other colors, and use the green channel for luminance. Another reason to use scopes (not eyeballs) when it come stop measuring luminance for a key.

October 1, 2015 at 11:01AM

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Joel Farabee
Shooter
99

Having good distance separation from your green screen and subject is one of my secrets.

October 1, 2015 at 1:11AM

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Bert Beltran
Filmmaker | Editor | Motion Graphics | Drone Op
124

No such thing as a one click key, any 'Hollywood' quality key will be multiple keys on multiple areas. The thing about even green screens is good in practice, but if the area of screen is way off the subject, it doesn't matter as it will be dealt with in a garbage matte, especially if they are sitting still. Only the edges around the character matter.

Shoot a clean plate if possible, they are very helpful to retain natural shadow detail, especially on the floor where actors cast shadows. If lighting with tungsten lamps, spike them with green to help remove the red cast that will be created. Daylight balanced sensors are notorious for noise with tungsten, so use a 80a filter if using only tungsten lamps. Knowing your BG that will be inserted is very important. If it is a dark BG, underexpose the screen slightly or you'll be treating unnaturally bright edges. Blonde hair will key better on blue screens. If using tracking markers, use a slightly darker green or blue tape, it will key out fine and the tracking programs will have no trouble locking onto them. Use a light meter for exposure and try get it within a stop across the areas you will be keying. Write down all your camera info & lens info, so that the compositor can match the Depth of field correctly for the given shot. Know the film back size - its critical to tracking.

Reality is, price up the time it takes to fix and balance that with quality. A crew of 20 is more expensive to keep standing around for 20 min to fix a shadow in the corner which 1 man can matte out in post in the same amount of time. A crew of 20 is worth keeping waiting if the problem is larger and will effect the end result, especially if the results are under expectation and take 20 post hrs to fix.

October 1, 2015 at 1:16AM

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thx. ovaltine power!
fugk i just noticed im subscribed to him
senpai notice me!!!!!!!!!!!

October 1, 2015 at 1:24AM

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False Colour is a great way to judge the evenness of the background too.

October 1, 2015 at 5:21AM

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Not sure there's ever a 'one-click key' that looks good - normally you have one for hair, one for body, etc. etc. - and they all need tweaking in combination with other masks.

And again - basic stuff - but get your subject as far away from the background as possible to minimise spill.

...And as important as anything else is trying to give your subject similar lighting to the scene they're going into. You might have an amazing key, but it will never look natural if (for example) they're lit with a head-on soft light and the scene is high-contrast side-lighting.

October 1, 2015 at 5:24AM

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Alex Richardson
Director
3573

Brilliant tutorial... ProRes has been the most important improvement to my keying.

October 1, 2015 at 9:52AM, Edited October 1, 9:52AM

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Kayode
934

For us with compressed output (7D, 5D without raw) :
Make sure your WHITE BALANCE is pulling the greens out as much as possible. It's easy to correct the subjects white balance after keying. If you get some or too much green in the white, you will have less dynamic range to work with in separating the green from the rest.

October 1, 2015 at 11:38AM, Edited October 1, 11:39AM

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Joakim
296

Does anyone know if it matters if you use ProRes 422 instead of ProRes HQ for keying?
My capture device seems to have problems with ProRes HQ capturing and I've been using ProRes 422 instead. I can't see a difference myself but maybe there is one. (The camera I feed into it is 4:2:0.)

October 2, 2015 at 12:56PM

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No difference other than a bigger file, given the source. Go with 422.

October 2, 2015 at 5:59PM, Edited October 2, 5:59PM

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David S.
3197

Thank you. This helps me a lot.

October 3, 2015 at 12:30PM, Edited October 3, 12:30PM

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It's "raw," not "RAW." It's not an acronym.

October 3, 2015 at 1:41AM, Edited October 3, 1:41AM

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David Gurney
DP
1514

.

October 3, 2015 at 9:04AM, Edited October 3, 9:35AM

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Indy Cine
Producer-Director-Writer-Cinematographer-Editor
8

A common error nowadays is referring to "Greenscreen Compositing" as "Chroma Key," or "Chromakey."

Common or not, it is incorrect.

"Chroma Key" (correct) or "Chromakey" (incorrect) is not a Film or Filmmaking term.

It is a Video term, and an OLD Video term. It refers to using Set and Background
color values to ELECTRONICALLY SWITCH video areas to be "transparent,"
in a Video Switcher.

Today we do not use Video Switchers or Chroma Key to combine images.
In Filmmaking we shoot Film or Digital against a Greenscreen, and we Composite images together in computers, with software "Layers" emulating "Core" and "Hold-out" matte images, as done for many years on Film.

This particular "History of Compositing" reference to "Chroma Key"
is the only known incorrect information, in it.

A correct History of Compositing can be found in the many papers of the SMPTE.
(Society of Motion Picture and Television Engineers).

For technical processes to work, it is necessary to have correct definitions.
We do not make up our own definitions, of technical devices or processes.
We find nearly a century of already-defined terms, at SMPTE.

A simple search shows if "Chroma Key" is used in the movies.
No, only in old articles about Television.
https://www.smpte.org/s?as_q=chroma+key

This search shows the preferred term for movies is
"Greenscreen Compositing."
https://www.smpte.org/s?as_q=greenscreen+compositing

Indy Cine
http://indycine.com

October 3, 2015 at 9:26AM, Edited October 3, 9:37AM

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Indy Cine
Producer-Director-Writer-Cinematographer-Editor
8

The most valuable point I have learned from Mr. Rosen is to use your bars generator and waveform monitor to analyze the green.

I turn on the bars on my GH4, then turn on the waveform monitor on our Panasonic professional monitor. (This can be a problem if you don't have a monitor that can display a waveform monitor, but it's really a key part.)

I look at the green bar and put a piece of masking tape physically onto the monitor where the green bar falls on the waveform monitor. This tells me what the exposure for the green screen should be. I turn off the bars and light the green screen. All I need is two 500-watt Lowel Totas about ten feet from the green and a little higher than the camera lens, off to the sides of the green, and behind where the actor/subject will sit. I look at the waveform and I should see a straight line across the waveform. If it isn't, I adjust the Totas until it's flat. I adjust exposure with the lens iris/shutter angle (I usually use 180 degrees, although Rosen suggest a slightly faster shutter speed/lower angle to reduce blur if the actor is moving about a lot) until the flat line reaches the piece of masking tap where the green bar was before I turned bars off. Now I know where the camera iris has to be for a good green.

Then I put the subject into the shot and adjust the key light until the exposure is right. I don't touch the iris. Move the light in and out instead to adjust the exposure on the face. This maintains the proper green exposure.

Keep the green way behind the actor -- ten feet at least -- so the green spill onto the actor is minimal. Yes, it requires a good-sized studio to do this, but otherwise you will get green highlights on your actor that will cause problems. Shoot in 4k -- easy to do with the GH4. This gives a lot of pixel density to pull a good key.

Two open face 500-watt incandescents for the green and one big 36x48" soft light for the key and maybe a reflector for some fill and I'm good to go. Editors love it.

October 7, 2015 at 10:41PM

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