The technique of inlaying on a green background: explanations and tips
What are the elements to take into account to achieve perfect keying ?
Those of you who know or follow SmartCuts know how common the use of the green screen is in our video productions, especially since the creation of our studio equipped with a 360 ° green screen.
A little magic trick carried out in a few clicks, the inlay on a green background, also called Chroma key, is a technique invented by American artist Lauwrence W. Butler in 1940 and very often used in cinema since. Some current films with a lot of special effects are almost entirely shot in a studio with a green background. The way it works is to remove one color from the shot, leaving all others intact, so that where the removed color used to be, you are left with what is now called an alpha channel, or transparency.
This technique is widely used by professional and amateur videographers. With a green cloth neatly placed in the background of your subject for the shoot, you can then project your subject into the setting of your choice, either live or during post-production.
You might be wondering what makes this possible and what you should watch out for to ensure the smooth running of this easy but demanding technique. This is what I will outline for you below in a few lines.
Green background or blue background?
You may have seen this keying technique done with a blue background. This is because blue is a color that works well for keying, as it is a color that approximates the complementary color of flesh. The blue background will be useful if the protagonists or the objects filmed in front of the background will have green colors on them, in order to prevent certain details from disappearing at the same time as the background.
However, the green background is used more in digital video while the blue background is more used in film. In digital, green is the brightest color, and it will contrast better with the foreground colors.
How to choose the right green?
Of course there are several kinds of green. And you won't get the same result with different greens. For best results, it is important to use a green that is as neutral as possible, that is to say that contains as little red and blue as possible (depending on the RGB color code).
It is also important that the green be as vivid as possible, as bright as possible, so that it contrasts the best with your subject. Thus, with a neutral and bright green, the keying will be easier as the background you remove will stand out well from the rest of the image. What’s recommended is what’s called Disney Green.
Back drop materials
There are many different options of what to use as the background material. Each has its advantages and disadvantages. It's up to you to choose what will best suit the use you want to make of it, depending on whether you need a large area, transporting it, washing it, ironing it, etc.
The green painted wall is of course best suited, as it has the advantage of not wrinkling and allows for a larger surface. Fabric is frequently used because it allows a large surface, but you must stretch it as much as possible to avoid creases, so iron it before use.
Other options include linoleum, strung up as a curtain and painted green. This gets rid of the wrinkles problem, but linoleum is heavy, bulky and fairly delicate, so not ideal transporting a green screen like this.
Nowadays, pop up green screens, which use wrinkly free, very light textile stretched within a foldable frame are very popular. These are essentially like flat pop-up tents that you can pack up into a small case and then pull out quickly when needed. They don’t wrinkle, they’re light to carry, but they are limited in size. These work well for single-person interviews or presenter-driven video performances, especially if you’re framing waste-up, rather than full body.
Enhancement of the subject
In order to facilitate the embedding of another background instead of the green background, it is essential to detach the subject as much as possible from the green screen.
The first thing, as I said before, is to make sure that your subject is wearing zero green. For example, if you interview someone who is wearing a green t-shirt, the t-shirt will disappear along with the background in postproduction. This can be a desired effect and the technique is often used. You can make your character's head float for example. But if that isn’t the desired effect, you may get charged for murder. 😉
Avoid materials that reflect too much light ; for example chrome. The legs of a chrome or steel chair will reflect the green of the background and this will be problematic when keying.
Then, position your subject as far away from the background as possible so that s/he stands out more. This way, you can more easily illuminate your subject separately from the background. Also, you will get less bleeding. We’re not talking blood here, but rather the green from the green screen reflecting onto the subject, and therefore causing trouble in post-production when keying out the green.
Proper lighting is essential for a good key. Properly illuminate the green (or blue) background so that it is sufficiently bright and even, without shadows. Then adjust your lights on your subject as you would usually do in the studio, taking special care that the shadows do not carry over to the background. If the scenery you are going to embed is not a simple wall but a deeper setting, it will be a problem for it to be "contaminated" by your protagonist's shadow.
Chroma keying - the secrets of the magic
Keying is the term used to describe the process of background removal in post-production or live. In short, you will order your editing software to replace the green (or blue) color with transparency. Thus, the background that you will have below will be visible in the background of your subject. It sounds simple, but it can also be complicated if the lighting is not properly controlled, especially for complex areas like the hair. The edges of the subject should be sharp and well demarcated from the background.
At SmartCuts we have over 10 years of experience shooting on green screens. We have custom built our studio specially with this type of technique in mind, so we can achieve near perfect results every time. In post-production, it makes our editors’ lives so much easier. When it comes to inserting a virtual set in real-time for live streaming, we use the software vMix, which I will tell you more about in another article coming up soon.