From the incredible imaginations of sci-fi movie directors to the impactful stills of photographers like Joel Grimes, composite imaging is used for making the impossible look plausible. In most implementations, it has a look, a captivating, ethereally futuristic, effortlessly cool look. Most of this is down to the use of HDR backgrounds (CGI environments are usually "true" HDR by definition) and atmospheric effects. But what about capturing the image to begin with? For that, we need two (or more) plates.
Plates are the images or footage used to make up the various layers of the composite illusion. In a basic still composite, this consists of a subject plate and a background plate. The background plate is generally an HDR (high dynamic range) image to allow fine tuning of lighting. It might be a beach, abandoned factory, futuristic mall, whatever suits the subject at hand.
The subject plate is simply the subject of the image, dropped out of whatever was really originally behind it to leave transparency everywhere that the subject isn't.
HDR environments are fairly straightforward to capture or generate. What about the subject plate? What techniques are used to create a subject that can be seamlessly integrated into this background? Well, that's the topic of this article.
Green, blue, white, black... composites can be achieved with all of them to a greater or lesser extent. Why am I choosing green? Well, chroma green (more on that in a moment) is a colour rarely found in nature, particularly human skin tones, so people stand out nicely in front of it.
Unlike a white seamless, where the subject may contain a number of specular highlights, pale skin or white clothing, a single hue of green allows for software chroma keying. On seamless, the subject must be cut out manually, whereas keying is a largely automated process.
How so? Keying effectively chooses a single colour in an image and simply deletes it. It was more complex back in the days of film, where there were chemical processes to go through to create a matte. The matte is the area of the image to be cut out, just like a layer mask in Photoshop.
Nowdays, however, it's as simple as clicking the colour you want to remove. Sometimes there's some clean up work to do around finer details like hair, but for the most part, with good lighting and modern algorithms, keying is quite simple.
Choosing and Preparation
For a greenscreen, you need a chroma green sheet, or some chroma green paint. The paint can be applied to walls, props, cars, whatever else you might want. What is chroma green? Basically, it is pure green, equating to 0,255,0 or #00ff00 on your monitor. With inks, paints and dyes the bright pure green we associate with Microsoft Paint isn't always achievable because of lighting and colour gamuts, but generally the idea is to have a single hue as close as possible to pure green.
Green screens are available in a variety of sizes and price points. I bought a screen which hangs on a backdrop rail, around 10x6 feet, for about $20. You can get more expensive ones which theoretically have more consistent hues, larger ones, popup ones for one-person field use which are handy, but a little pricy, or a gallon of chroma green paint seems to average around $60.
What you should get really depends on your personal needs. For the purposes of this exercise, I'm primarily shooting stills of a single person, but wanted extra size for possible future video use. Hence the fairly cheap one, although I checked the reviews first as cheap shouldn't mean useless.
Mine came folded up into its bag, so it needs ironing. Creases cause issues for the key, because they disrupt the lighting. The keying algorithm is looking for a single hue and tone. Creating shadows and bright spots mean that the background is broken up into a variety of tones, which will disrupt the key and you'll have to keep adding more tones to the key.
This will reduce accuracy by increasing the chances of something in the subject being caught in the key. For example, a dark green gemstone or ribbon could potentially be green screened as long as the screen is evenly lit and the keyer isn't looking for a variety of green tones and hues.
The shaded areas of the screen will likely take on a different colour temperature than the rest of the screen as they'll be ambiently lit, stretching not only the tonal rage, but also the hue range. If your ambient or subject lighting is tungsten, you're injecting yellow-orange into the green shadows, pushing it towards skin tones. Obviously, this isn't good. So make sure your screen is as flat as possible for the best chance of evenly lighting it.
The main difference between a good green screen shot and a bad one is the lighting used to shoot them. It's vital to have good, clean lighting in order to make the subject as self-contained as possible.
The first rule of green screen lighting is to light the subject and background separately, for several reasons. Basically, the ideal screen lighting and ideal subject lighting setups for optimal keying are completely different, so separate lighting rigs are used.
Lighting The Screen
As I mentioned earlier with the creasing, the light on the screen needs to be as even as possible over the whole screen. Usually this is done with edge lighting; the screen is lit from the top, sides and sometimes bottom with larger sources like KinoFlos or stripboxes which are often flagged or scrimmed to prevent forward light spill.
As the light fades towards the centre of the screen, it's boosted by the also-fading-in light from the other sources for more consistency. These narrower sources tend to be used because they don't protrude across the image plane when they're turned to face the screen.
In some cases, a number of spotlights are used and pointed all over the screen. While backlighting the screen would save space and could achieve better consistency, it throws too much green light forward, causing green highlights in hair and other glossy surfaces.
Speaking of hair, it's very important that the screen behind the subject's hair is evenly lit, with no gradients or shifts. While the rest of the body is fairly easy to manually mask if that becomes necessary, hair really has to be keyed and it's important to have as perfect a background to key from as possible.
Fluorescent lighting is popular for lighting green screens because the light tends to be heavily tinted green, or in full spectrum tubes, contain some lesser degree of green spike, which helps make the screen that little bit extra green.
Lighting the screen with tungsten or another non-white, non-green source will usually result in a strange muddy colour cast to the screen, resulting in a less clean key. If you have no flo lights, you could gel a white source with mild green to achieve a similar effect. Just ensure the gel is as close to pure green as possible, whatever its strength may be, to cut out the most red and blue.
Lighting The Subject
Tungsten is often used for subject lighting in video work, making the background more green and the subject more orange creates a stronger contrast for the keyer to latch onto. This orange cast is then balanced for in-camera to make the subject appear normal. However, check your camera because some video cameras (and likely stills cameras too) key badly when there's tungsten-induced noise in the blue channel.
Another advantage of the studio distance required to contain this multiple-lighting-setup is that it allows you to use a shallow depth of field to even out any visible texture or mild imperfections in the screen.
That said, do not open your lens up all the way because it's important to keep the entire depth of the subject sharp in order to reduce the deletion of things like stray hairs or textured clothing. Any blurring of the subject into the background will result in a spill of the keyed matte into the subject and subsequent strange artifacts.
Matching the Lighting of the Scene
The subject lighting itself must generally match as closely as possible the ambient light of the environment they're going to be composited into, with some margin for appearing "lit." Usually this consists of a large source in front of them at some height to match local lighting and any sky, and then edge lights creating a rim light allowing them to blend into bright backgrounds or local backlighting.
For example, if the subject is to be put into a direct sunlight scene, the main light would be quite hard and bright compared to the rim lights which would be simulating the large sky source behind them, and coming from the same angle as the sun in the final scene.
Other scenes may require softer lighting, or more complex lighting setups depending on what the subject is going to be composited into. Usually a three-point setup like this is the best way to start off, and then tweaking it to match your final scene more closely.
Sometimes the rim light is taken a step further, and a kicker light gelled with magenta (for green screen) or yellow (for blue screen) is used to negate the green/blue fringing often created in subjects' hair, particularly for very fine, frizzy or styled hair. Any remaining kicker colour can be removed with hue/saturation controls.
Briefly, the subject itself. Naturally, hair, clothing, and makeup must contain as little green as possible so as not to be removed by the keyer. While you can usually set parameters to be more strict, the mask roll-off tends to become less natural and may require more manual masking. So try to stay away from the green end of the spectrum, including greenish yellow and cyan/green-blue. If the subject must contain green, this is where a blue screen comes in instead, and then the same rules apply, but for blue instead of green.
Finally, let's talk about the shooting itself. Avoid lens and motion blurs wherever possible. They can usually be added in later in post if necessary. Much like HDR shooting, it's critical that you capture every detail of your subject to be manipulated later. This usually means closing down the aperture and cranking the shutter speed up, which of course require either a higher ISO or brighter lighting. A smaller sensor size will help with the depth of field issue without having to cut a lot of light out.
If you're shooting video, try to shoot in significantly higher resolution than you're going to master at, for exaple, 1080p or 2.7K for 720p, or 4K for 1080p. The extra detail captured will help the key, and the later resizing will improve sharpness and reduce the appearance of artifacts. This is less vital to bear in mind in stills work, since we're almost always shooting several times larger than the image will finally display anyway. However, shooting a super high resolution medium format camera will make your composites look better!
Generally, you want to use the same focal length to shoot the subject plate as you did to capture the background plate, so there aren't any strange variances in distortion, which our brains pick up on just like "unnatural" lighting. That said, this isn't necessarily set in stone and some photographers use different focal lengths for effect. It depends on your intent, really.
In this introduction to compositing, we've covered the practical shooting aspects of using a green screen to create a good quality matte without having to do lots of cutting out. We know the general range of options for buying or creating a screen and how to prepare it for shooting. We know the general setup for lighting, that the more lights you have, the better, and how to improve the quality of the keyed matte with various lighting tricks. Finally, we covered the obvious issues with the subject's appearance, and how to ensure you capture them as crisply as possible.
So now we're all set to shoot on green screen. In the next installment, I'll be looking at how to process the results and composite them into various backgrounds!
Questions? Comments? Hit up the comments below!
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