Green screen – how does it actually work?

The secret to pulling your subjects out of the real world and placing them into a digital domain is chromakey. That means going green with a green screen. Green screen is a useful tool in cinematography. It opens up a world of endless possibilities for what can be put on screen.

Through the magic of video effects and technology, you can superimpose your subjects onto virtual backgrounds. You can place them over animated digital backdrops or transport them to a desert oasis.

You can shrink down a full-grown man so he can stand on a tabletop, use visual effects to make him fly through the sky like a superhero, or simply simulate your own TV weather report.

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But to do it right, you’re going to need a lot of green. No, we’re not talking about money. The green we mean comes in the form of a green screen.

Chroma key allows you to take subjects out of the real world and placing them into a completely new world. In this article, we’ll cover all the essentials you need to know to pull off clean chroma keys and composite digital backdrops and virtual backgrounds into your video editing.

What is chroma-keying?

Chroma keying is the process of isolating a single color or brightness value in an electronic image and using software to make that value transparent, allowing another image to show through the affected areas. Luminance keying, or luma keying, is the process of keying out a brightness value or range, like black or white. Luminance keys are often used for applying mattes. Color keying, or chromakeying, identifies a specific color to remove.

Why green?

Many people use the terms chroma-keying and green-screening interchangeably, but the principle that powers chrominance keying is not limited to the green parts of the spectrum.

In the visual effects world of Hollywood, blue screens are far more common than green. In fact, you can key out any color; red, yellow, purple or pink, blue and yes, green.

So why is that odd and ugly shade of green the hue of choice for television and video? The biggest factor is contrast.

In order to isolate one area from the rest, the background color must be distinctly different. Bright green beats blue partially because it is not a color commonly worn by talent. Any clothing that matches the background color too closely will also key out. It’ll punch a hole in your subject’s body, or make them invisible altogether.

Any clothing that matches the background color too closely will also key out. It’ll punch a hole in your subject’s body, or make them invisible altogether.

We narrowly escaped a chroma key crisis years ago when I was working at a Northern California TV station. One St. Patrick’s Day our wacky weatherman showed up to work dressed head-to-toe in a bright green leprechaun suit. Fortunately, we quickly pointed out the problem and he was able to find a change of clothes before he went live.

Green screen requirements

You can erect a simple chromakey setup almost anywhere with just a few basic tools. In order to shoot footage that will key cleanly, you need a green background, a source of bright, even lighting, and a tripod to lock your camera in place. We’ll cover each of these elements in detail.

Buy or build a background

The most obvious need is for the background itself. Fortunately, there are many options, and many of them are inexpensive. In short, anything that’s bright green will work, and anything that works is valid. I’ve produced professional green screen scenes using giant dedicated sets, large professional fabric chromakey backdrops, sheets of material purchased by the yard from a fabric store, smooth walls or pieces of paneling painted with a gallon of dinosaur-green paint from the kids section of the hardware store, even sheets of green poster board taped together. (NOTE: Be sure to avoid textured walls if you’re painting it green. Texture causes shadows.)

What do you need to look for when buying or making a green screen?

The only requirements are that your background is large enough to fill your screen, smooth enough to take light evenly without showing wrinkles or casting shadows, and bright enough to contrast well with your subject.

If you intend to chromakey a sock puppet your backdrop may be relatively small. If you need to chromakey a full-body shot of an adult actor, you will need a much larger background.

Wall painted green to be a green screen
Anything that’s bright green will work as a green screen, just be aware of textures

Also, if you are experimenting with chroma keying for the first time, you can test the process on a small scale before you build a big set. You can quickly create a functional miniature table-top set with an action figure and a sheet of posterboard. However, it doesn’t take much more effort to stretch a sheet of green fabric between two step ladders or paint a small section of wall.

Light it smoothly

Even a professional-quality cyc wall won’t key well without proper lighting. If there is a secret technique to getting good keys, it is in lighting the wall. The goal is to light the set as evenly as possible using soft light. Any variation in lighting will read as gradient coloring and will complicate your key in post. Achieving even lighting can be more difficult than you might think.

Hard light sources cast narrow and focused beams that create circular hot spots and leave outer portions gradually darker. It helps to move small lights farther away so they cast broader, soft light beams.

When you’re lighting a green set the color temperature of the instrument is not as important as lighting your background evenly.

If you have access to softboxes, they are excellent options for casting evenly spread light. I’ve had success lighting green screens with long tubular fluorescent fixtures along the top and sides of the backdrop. You can get a six-foot garage-style fixture from your local home hardware store for less than lunch. These fixtures cast a soft light that is quite appealing for your green screen. When you’re lighting a green set the color temperature of the instrument is not as important as lighting your background evenly.

Light it separately

Another important, but often overlooked, essential is lighting your subject independently of your set. This is important for two “key” reasons: shadows and reflections. Part of keeping your wall evenly lit is keeping your subject’s shadow from falling across it. Position the talent at a distance of at least a few feet from the screen, and light them separately using three-point lighting.

Proper positioning of lights to minimize shadows
Proper positioning of lights to minimize shadows

If you do not have a lot of distance to work with, position your key & fill lights slightly to the sides, not straight on, so any resulting shadows will fall outside the visible frame. Another advantage of moving your subject away from the wall is the reduction of reflected green spill light on your talent. Reflected spill light can rim your subject in a tinted halo that can be difficult to discern with the naked eye, but if your actor is too close to your wall, it will be there, and any green bouncing off your actor will mess up the cleanliness of your key. You can wash away a fair amount of reflected green using a bright backlight, but you will find that distance is your best friend.

Lock your tripod down

Unless you are planning to use motion tracking markers and a complicated visual effects compositing software to motion track your subject into a virtual 3D environment so that the movement of the camera perfectly matches that of the digital background shot, even the smallest camera motion is unacceptable. This is a case where it’s got to be all or nothing. Either commit to motion tracking or use a good tripod, and take your hand off when you roll to prevent any vibration.

The trick is to shoot your subject in a way that blends well with the background. In most green screen scenes the digital backdrops will be a still, a stable computer generated (CG) image or animation, or a clip of video shot from a locked down camera. If the camera moves or shakes, even a little, your subject will appear to bounce around in front of your keyed background scene, and that’s a chromakey faux pas to avoid at all costs.

Frame and focus

For the same reason, do not zoom in or out as you record. This will make your subject appear to shrink or grow in size in relation to the background. Use the zoom to frame the subject where you need them to be. Once you’ve set up the shot, step away from the controls.

I typically recommend setting your focus on the sparkle, or specular highlight, in the eyes, but in a chroma key setup, I make an exception.

Focus is another important consideration. The only way to know that you have the sharpest focus possible is to zoom all the way into your lens’ maximum telephoto setting when you adjust your focus. I typically recommend setting your focus on the sparkle, or specular highlight, in the eyes, but in a chroma key setup, I make an exception. The best keys come from clean, sharp edges, so it’s important that the edges of your talent be in crisp focus when you shoot on the green.

Pay attention to your camera’s compression

The camera you use for shooting green screen footage for visual effects and virtual backgrounds does make a difference. Because chroma keying is a digital process, the way your camera creates the image directly affects your ability to pull a clean key.

Footage with a 4:2:2 ratio is much easier to key than 4:2:0. 4:2:0 typically leaves a ring of pixels around the edge of the subject. With some effort, the halo can be removed, but footage with a ratio of 4:1:1 or below is not an optimal option for chromakey. Check your camera’s specs to find out what type of color subsampling your model uses.

Pull the key during editing

If you have successfully shot your subject in front of a smooth, bright, evenly-lit wall, lit your subject independent of the background to avoid shadows and spill, and shot with a crisp, clear focus, then pulling the key to reveal your digital backdrops or virtual backgrounds should be a relatively easy process.

Setting up the timeline and keying software
Setting up the timeline and keying software

First, import the clip to your timeline and place the footage you would like to show through on a lower track. Then apply a chromakey filter from your effects menu to the green screen shot.

The key filter will offer you adjustment options in the effects control panel. With a little tweaking, you can use these controls to dial in the key, eliminating grainy areas or green edges. Poorly lit green screen footage may still be keyable, but might require multiple key passes. Taking the time to light and shoot the scene well makes things as easy as possible in the edit.

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