It's one of the oldest tricks in the video arsenal, yet it's still one of the most useful and effective. We're talking about the selective subtraction of a color or color range from an image, which can then be replaced with a new image. This is the green screen effect.
To be more accurate, it's called a chromakey, though green screen is used so often it's used rather synonymously. Red and blue work just as well under certain circumstances, but are usually avoided for reasons we'll get into in a moment. You can also use luminance to create this key effect, where anything over (or under) a specific brightness will be taken out. Technically this works slightly differently though, so the powers that be have given it a different name, luminance key. Nevertheless the premise is the same, and that's really what we're concerned with.
Green for the Green
Let's give a brief overview of how to use a green screen. A solid color is placed as a background and lit as evenly as possible. Your foreground subject is then placed in front of this backdrop. The rules are simple: the subject must not contain any of the background color (green), must not be hit by any green light, and must never cross the edges of the green screen backdrop. Anything that is to be rendered "invisible" must be colored and lit as close to the background as possible.
Once shooting is done, the image is taken into the edit system or green screen software, where the background color is targeted and removed, leaving a transparent area in its place. This transparency can then be substituted with whatever is desired, real or imaginary. That's the basics. It sounds easy but there's actually a good deal of both art and science behind it. For the rest of our time together we're going to concentrate specifically on green screen materials, but extensive details on all things green screen can be found at www.videomaker.com/learn
Too Green or not too Green?
Only primary colors are used for keying, because they do not contain elements of any other color. If you tried to key on purple for example, anything blue and green would be subject to removal. Green is the color most often used when dealing with people because it is not very prominent in anyone's skin. Blue can be substituted on those occasions when there is green in the subject that cannot be replaced, (like a plant). Likewise red can be used when green and blue cannot. Red is not a very popular key color though, predominantly because it is a prominent skin pigment.
There are actually specific shades of green, blue and red that have been determined to be optimal for the process. Manufacturers of chromakey products have zeroed in on these colors and created a variety of products to help you produce the best effects possible (though again, red is all but nonexistent these days). Green screen materials can consist of paint, cloth, paper or cardboard, and more, each with its own benefits and quirks. It is also common for people on a limited budget to make their own green screen backgrounds out of consumer-ready material. Take a look at what chromakey materials are out there and contrast them with some popular homemade counterparts.
Fabricating your Green Screen
Probably the go-to system when one thinks of a chromakey effect, fabric backdrops are a great balance of durability, quality and versatility. Green screen cloth is easily stretched across an adjustable frame and can be positioned or carried as needed. The right fabric offers the perfect balance of reflection without glare, which will minimize lighting headaches big time. Fabric is more durable than other options, and ages well. It can be sized and shaped simply by folding, and can be draped over objects for concealment. It is, however, prone to wrinkles, whose dark lines can be a big headache. Professional green screen fabric is often comprised of a thick cotton or cotton/muslin blend on a foam backing (think neoprene or felt). It is more resistant to creases than a generic fabric, and great care is taken to dye the fabric evenly. Most fabrics are washable, but permanent staining is always a risk.
In general use, it can be hard to prevent shifting and stretching, so it's not the best choice if you're going to be skateboarding through your key. Large fabric setups can be heavy and difficult to transport. It's also one of the most expensive selections, especially when you factor in the rigging equipment you'll require to hang it.
The obvious improvised solution is to nip out to your local fabric store. You can use anything from a green sheet or blanket to a nice custom cut swath of cotton muslin. The more you pay for your fabric, the happier you'll be with the results, and you can attach some stitched loops or grommets for hanging your green screen fabric. There will come a point however, when your time and effort will equal the cost of the professional system. It will also be harder to create larger home-brewed screens without seams.
Painting the Scene
A long-term solution, for those with a fixed space is to paint the key color onto a background. The cost per foot is comparatively very low, and with the possible exception of a roll of paper you'll probably be able to cover the largest area for the least amount of cost. This savings will only increase with time, as it can be one of the longest-lasting methods. You'll also be able to make any shape a keyable surface in no time. Furniture, boxes, etc. can all be painted and used on set. Like paper, green screen paint tends to get dusty and marked up easy, but the paint is made to withstand limited cleaning. In any case, if a little scrubbing doesn't help, you can simply lay down a new coat over the spotted areas. You'll also never have to worry about wrinkles.
Paint really comes into its element when one is making virtual sets for green screen. It's one of the only practical ways to "apply" the key color to objects. Other methods obscure objects by placing green in front of them. Thus, the painted objects are perfect for interaction. With paint you'll have hard surfaces. If you need to roll or ride over it, or hang something on it, you'll have no problem.
Paint however is permanent, and has arguably the longest setup time. It's also more prone to hot spots than fabric and can be harder to light evenly. Extensive planning is needed if you want to design a system that will satisfy every time.
We know, in this area you do-it-yourselfers are set, right? Match the swatch at the local paint store to the professional color key paint and save some money. After all, paint is paint, right? Be aware though that you may find yourself laying down multiple coats to get the same vibrancy as, specialized green screen paint from a company like Rosco. You also might notice a difference in reflectivity when lighting. That is unless you spring for a more expensive brand of consumer paint. Again though, at what point do you just give up and spring for the real deal?
One of the few systems that can act as both a cover and an application is paper. It can be wrapped around objects (like a boom pole), making them keyable without ruining the actual object. It is easily shaped and piecemealed into any arrangement. You can likewise buy huge 10-foot wide rolls of it and hang it in your studio, to set up at your leisure. Consider it an expendable though, because its lifespan is likely to be the shortest of any method here. Paper tears easily and is extremely vulnerable to dusty feet and dirty hands. It has the dubious distinction of being low maintenance (when you have a dirty sheet, you rip it off and roll out a clean area) and yet hard to keep over time. The edges of the roll can get frayed and bruised if your rolling is not perfect, and it can become dry and brittle as it ages. Paper is also easily discolored. A simple wet spot can cause enough contrast to ruin a clean effect.
Paper excels as a keying element for unusual setups. Back in my college days, we made a video short for animation class where a 4-inch high robot was animated through stop motion and made to interact with a bunch of "thugs". At one point the robot shoots a laser at one of them. As the thug falls, the others scatter and you can see their feet and legs through a huge hole in his chest. This was accomplished by taking a roughly round piece of green paper, attaching it to his chest and making the shirt around it look burned. We then shot him in front of a matte, lying on a floor of green. This was later combined with the footage of the other actors on location scattering in all directions. To enhance the effect we had someone run in front of him on the green screen as well, adding a third layer of depth to the final composite.
Paper can also be very helpful in places where you need to make corrections to your background (like when you need to cover those wires your audio guy just ran across the set). Most people however will find that paper is best used as a portable background for small frame sizes. It's easy to tack a piece of cardboard behind a talking head for an interview. Needless to say, no matter which solution you regularly use, with this much versatility you might want to consider keeping some keyable paper around.
Miniatures are another great place to use an ad-hoc paper solution. Any crafts store will have multiple options for creating a miniature keyable light box. The problem with DIY paper actually lies in finding larger sizes with the exact properties you desire. In the end, don't be surprised if you end up piecing a bunch of poster board-sized paper together to fill larger areas.
Green on the Go
Perhaps one of the best choices for those on the go is a Flexfill key solution. These are basically wire loops with key color fabric stretched over them. They pop open in an instant, can stretch tight over the wire loop, and easily twist up to travel size when done. The frame means they are easily supported with just a couple of clamps, and their light weight means they can be placed in locations where being delicate is a factor. As an added benefit, some have green on one side and blue (or a reflector) on the other. The trade-off for all this however is that they are limited in scope. They cannot be made larger, and are not easily integrated into other systems. For quick use, portability and convenience though, these are a great solution.
A great deal of accessories exist to help enhance your virtual sets and aid in keying. Green gaffer's tape (or its hardware store equivalent, e.g. FrogTape) is an obvious one, as is the above-mentioned paper. You can even find full body suits if you need an invisible performer or stage hand. Green screen lighting kits are available as well. These usually consist of everything you need for a chromakey effect along with two or more soft boxes that can evenly distribute light onto your actors and backdrop.
Your own Key
So far we've only discussed cheap solutions that can replace their professional counterparts. We know there are some of you out there that are rather resourceful though, and could take things a step further. Let's theorize how to make a green screen that could rival a professional system.
Your first job is to find a fabric with the properties you desire, something rather light, slightly stretchy and more matte than reflective. You'll also want something that can resist folds and creases. If you can't find one in the exact shade of green that you want, get white instead and color it with dye or diluted paint. Testing will allow you to find a good mixture that will both coat evenly and let the fabric remain malleable. From there you can again attach grommets or sew in loops on the edges to feed poles and ropes through. An expanding painter's pole on the top makes a great crossbar that can be hung from the ceiling, or clamped to two light stands (remember those sandbags).
Professional chromakey tools are made to be efficient, easy to transport and use, and optimized for quality. These features will come at a premium though. Alternatively, you can certainly produce your own backgrounds and systems with materials found in the consumer market. The areas where you'll notice the most difference are longevity and practicality, and there quickly comes a point where it's just worth making the leap to the pro kits.
If you're going to key often, consider using a professional system. These effects have been around since the dawn of color video, and the tools have been refined to the point that features will be included that you never knew you wanted, but will find most helpful. Fabrics, for example, will be seamless, washable and have mounting points or Velcro. Paints will be washable, and durable. Rigging systems will have access points for clamps. Portable screens will have green on one side, and blue or reflecting material on the other. Everything will be quality controlled so the color is even and will stand the test of time in a production environment. You'll more than make up for the cost by reducing your production setup, and post tweaking times. If on the other hand you only want to use the effect once or twice, and budget doesn't allow for such an allocation of resources, then know that you can go for the cheap green screen effect and get away with it.
There are some middle grounds of course. Some of the professional systems are cheaper than others. Likewise if you really work at it you'll be able to come up with a homemade system that will last for quite a while. Clearly there's a place for both. In the end don't be afraid to establish your green screen production methods using home-brewed systems. When you're ready to get serious though, a quality rig will also serve to re-enforce to your clients that you mean business, and have the right tools for the right job.
Sidebar: What's With Those Spots?
If you've ever watched a making of video involving big budget green screen effects, you've probably noticed the green screen has a pattern of marks on it, or the subject has a suit on with little white spheres attached. These are for advanced forms of keying that involve motion tracking and digital replacement. Simply put, the marks are used to line up the movement of the green screen camera with the movement of whatever will be replacing the green.
If the camera moves in to the subject, the marks on the background will grow larger. If the camera turns, the marks will distort respectively. A computer will be able to read the movement the marks go through and extrapolate how the camera was moving at any given moment. This can then be used to control another camera that will later shoot the background, or feed into a 3D program to control the virtual camera there.
The spheres are used in a similar manner, only in this instance they are reference points for tracking the motion of the subject. A sphere on the end of the nose for example will show the computer which way an actor is looking at any given moment. This data can again be used in a 3D program to make a creature look the same way. Later the actor can be removed, and replaced by the creature, which will now move the same way the actor did. Combine these two together and you have limitless movement options for flawlessly combining what really is and isn't there.
Peter Zunitch is a post-production manager and editor working on every system from 16mm film to Avid Symphony, utilizing many of today's advanced manipulation and compositing tools.