5 Elements of a Great Chroma Key

In the greenscreen history lesson I talked about the processes filmmakers have used to make traveling mattes – in this lesson I’ll dive into the necessary elements you need to pull off a great chromakey.

Space

The first element in pulling off a key is the type of space you have to work with. Your space will determine what kind of chromakey shot is possible. When shooting against a greenscreen or a blue screen, you’ll want to pull you subject away from the background far enough so that shadows don’t fall on the screen and you’ll want to minimize the reflection of the screen on your subject.

Get subject away from screen to minimize shadows and screen spill on subject.

Get subject away from screen to minimize shadows and screen spill on subject.

With smaller spaces you should be able to pull off a reasonable talking head shot – that’s where the shot is just the head and shoulders of the subject speaking. For a full body shot, you’re not only going to need a larger space for a screen, but some distance to place the camera so that your subject looks natural and isn’t distorted by a wide angle lens.

GS-Ed-Cosico

Blackmagic Cinema Camera shooting 24mm Lens about 10 feet from actor Ed Cosico

Shooting outdoors is also a possibility especially for certain shots, just keep in mind you’ll have to deal with all the issues that come shooting outdoors including wind, noise, and shadows.

Outdoor Greenscreen shot

Outdoor Greenscreen shot

The Screen

The first question you’re going to ask is green or blue? Blue screen was a traditional color in the film days and is still used today for many productions, but green is the preferred color for digital keying. Why? Because many digital cameras use a Bayer pattern of Red Green Blue photosites where there are twice as many green photosites as there are red and blue. This makes digital cameras much more sensitive to green coloring. Green screen also requires a lot less light than blue screen and is less likely to match the clothing of your actors.

Still you may want to use blue screen in certain cases – say you’re shooting a green monster. In fact on the Sam Raimi’s Spiderman, The Green Goblin had to be photographed on a bluescreen because the suit would have been lost on a greenscreen. Spiderman on the other hand had to be shot on a Greenscreen, because his suit was red and blue..

When it comes to the material of your screen you have several options. The first is to paint your background using a chroma key paint. This is the most permanent although labor intensive way to create a chromakey background though certainly necessary if you are planning on installing a cyclorama.

Cyclorama

Sample shot of aGreen Cyclorma also called an “infinity cyc”

The other option is to hang your screen You’ll need background stands and clamps to hang your screen on. The screen itself can be made out of paper, or muslin background clothe, but I prefer foam backed cloth because it scatters the light more evenly so you can avoid hotspots and the foam keeps the screen from wrinkling when not in use.

Foldable Chromakey

For small setups, foldable chorma key screens are available. These kits are really handy for quick portable setups or outdoor use and built in frame keeps the screen from getting too wrinkled.

Lighting

When lighting for chroma key you have to think of lighting first for the screen itself and then for the subject. In tight situations you can combine the lighting but you could end up with shadows on your screen and that will make for a much more difficult task of pulling a good key.

Start by focusing on getting a clean and even light on your screen without your subject. Here I’m using 2 ikan IDMX1500 dual color fixtures – part of the ikan 5 piece dual color chroma key kit. These are hung in front of the screen using a truss system. These are awesome LED lights that put out a lot of soft even light which is an absolute must properly lighting a greenscreen.


IDMX1500B

Now if you’re an avid DIYer you may try to use long fluorescent tubes to light the screen. Another DIY option, is the Hollywood strip lighting fixture – this one I found on a street curb being thrown out – and it made a nice shadowless screen light.

Hollywood-Lights

To check to see the quality of our lighting on the greenscreen, I’m using the ikan MD7’s built-in waveform monitor.

Waveform Monitor

Waveform shows a relatively tight band – an overall even lighting on the screen.

Notice the tight band.. This means that are screen is evenly lit and there’s not much variation on the screen itself. You want to get this band as tight as possible, not slanted or have spikes which are hotspots..

If you can’t get your hands on a waveform monitor, you can use your camera’s zebras settings. Zebras tell you what part of your image is overexposed, if you lit your screen properly you should see the entire screen turning into zebra stripes at once as you open your iris to over expose the screen.

Once we have a well lit screen it’s time to put our subject in place and work on the subject’s lighting. Keep in mind the final composition and try to light your subject that will match the scene you’re attempting to composite.

IB100

Here I’m using the ikan IB1000 LED lights – part of their chroma key kit. These lights can be set to 3200k which will match great with the tungsten lights I already own. Just take care when lighting your subject to avoid casting shadows on your screen.

The Camera

When I first started experimenting with greenscreen, I was working with DV in standard definition. You had to be exact in order to get an acceptable key, and even then there were some funky artifacts. But I’m happy to say that with HD, getting a reasonably acceptable key is much easier because the pixels are so much smaller. But the type of compression can be a detriment to quality greenscreen.

Most consumer cameras use a 4:2:0 compression for handling color – I talk more what color compression is in the lesson on Non-Linear editing if you want a refresher. 4:2:0 throws away a lot of color data – data that can be very useful for achieving a quality key.

BMCC1

BMCC2

In this demonstration, I’m using the Blackmagic Cinema Camera decked out with ikan’s Tilta Rig. The Blackmagic Cinema Camera is capable of recording in 4:2:2 compression using ProRes or DNxHD formats or even recording 4:4:4 uncompressed 12 bit raw.

My tests have shown that the compressed 4:2:2 is pretty good for pulling a good chroma key but the RAW is simply unbeatable in terms of ease and quality. If you want the best chroma key, by all means shoot RAW – just know that it is expensive in terms of the amount of memory it eats up. I’m not saying you can’t pull a decent key using anything less than 4:2:2 color compression or RAW as in the Blackmagic Cinema Camera, but a camera like this with it’s compression or lack thereof sure makes a the key a heck of a lot easier.

Post Production

Most of the basic keyers you find inside NLEs behave like hardware chroma keyers in that you’ll most likely be let down by the results. But there are a lot of professional keying software solutions available out there that can help you pull pretty good keys even out of so-so footage. My favorite. happens to be one that comes bundled inside of Adobe After Effects called “Keylight” made by The Foundry. This plugin is incredibly robust with all kinds of features like matte choking, spill removal, de-spotting- and even masks that you can apply to fix little chroma key mistakes here and there. Plus After Effects is just a good platform to be working doing your compositing in the first place. Keylight is also available for purchase for other editing platforms.

garbage-matte

Now if you can’t pull a perfect key off the entire frame – don’t worry as there is a technique called garbage matting. Using After Effects, you can create a rough mask around your subject and throw away all the other junk in your frame thats unimportant. This is a great way to work in smaller spaces – just make sure you subject does cross into the garbage matte

Once you have a good clean key, it’s a matter of compositing the subject into your background plate adjusting the color to match, and compositing foreground elements to really sell the effect.

By considering your working space, using a quality screen, lighting it evenly, using a camera with as little compression as you can get, and finally using a good software keyer, you should be able to pull off a great key. Good chroma keying is a skill, and it will take a little practice, but the reward for your patience and experimentation can be quite liberating. – Another tool for you use in your quest to make something great.

 


Quizzes

5 Elements of a Great Chromakey Quiz