Is Chroma Key just for the pro's ?
Chroma keying is a process used to separate out foreground action from background action in order to create a final shot. It is one of the many different methods used and probably the most common.
Actors are filmed in front of a screen, usually blue or green, instead of the normal background. For the purposes of this article, we will refer to the screen as a green screen. In post production, software is used to make all green areas in the shot, transparent and thus, a new background can be inserted in its place. This technique can also be done live as on the weather reports and the news.
This enables you to put your actors in many different locations or compose shots that would otherwise be expensive, impractical, or even dangerous. Superman was one of the first films to use chroma keying. Christopher Reeves was filmed lying down on a green box with a green background. These shots were then composited with real footage of the sky to create the illusion that superman flies! Many more films have since used the technology; Star Wars, Lord of the Rings, and Harry Potter films (to name but a few) put their actors into make-believe landscapes and situations.
There are many films that use chroma keying to get special effect shots, but equally there are films that use the technique, but subtlety, in order to cut down on location costs. This is one of the main benefits of using a green screen. Many scenes can be filmed in a day, even though in the final edit, they would be in different locations or situations. It saves time and cost during filming. Actors and crew can all assemble in one place to shoot many scenes, thus cutting down on time and logistics. This does leave more work for post production as the scenes all have to be composited with their respective backgrounds. The good thing here is that if there is something wrong with a background shot, then you only need to send out a small crew to get the shot (not the entire crew & cast!!).
Why blue or green ?
As we all know, computers are fairly stupid. We can look with our eyes at something and easily say “I want that removed from the scene”. Computers just can’t do that! The reason for using either a blue or green screen (or any other colour for that matter) is to enable a computer to make a simple decision: Please remove everything in this scene that is blue and replace it with the background I want!
Blue has been traditionally used when filming on standard film as it is the colour that film is most sensitive to and is the opposite of skin tones, making keying easier. For Digital CCD cameras, green is favoured as this is the colour the cameras are more sensitive to. Blue and red channels on digital cameras produce the most noise (like grain on film) which present problems for chroma keying. Also green reflects light more easily than blue which makes lighting the green screen easier as not so many lights are required.
How we used the technique at BFVS
Our up and coming production (Clarkson) has used the technique extensively. Our use of it has primarily been to cut down on production time. It’s always difficult to arrange crew and actors in the same place at the same time, and to shoot all the scenes at the locations (and period) that the film is set in, would have been a logistical nightmare. The other advantage is that we film in the relative comfort of indoors and have plenty of hot drinks and food (we didn’t get the food because someone bought green apples to put in front of the green screen, and they disappeared!!).
Take the scene below. This actor was filmed with some foreground elements (the books and table) against the screen. The final shot needed to be in a Georgian library.
|The background is made up from a photo of a library and that of the front
of a Georgian building. The above shot is currently for review and may
be decided that the background isn’t right. This also allows for
changes to be made relatively easily.
The next shot shows how the final picture can be built up by layering
the actor amongst other images. Again the actor was filmed completely
in front of a green screen then the foreground beams added and background
Tips for filming green screen shots
|Having said that, it is not always possible to get the right lighting conditions and distance from the screen. Our screen is homemade from material bought from Ikea, lit by two redheads with green gels to help saturate it green. Our actors were about 4ft away from the screen in some shots. The shot below was taking in my kitchen under natural daylight conditions (not part of the Clarkson film I hasten to add!). As you can see, the screen has a nice big crease in it and graduates in lightness from bottom to top. Fortunately there are ways to fix this later, but the primary purpose (like with most of filming) is to get the shot right in the first place!
So is it just for the pro’s? Absolutely not!! Most filmmakers can make use of green screen for the reasons stated, and even when working on a tight budget using formats like miniDV, good results can be achieved by ensuring good filming practices and using one of the many software packages available. Go experiment. Once you start you will realise what a good tool this is and the possibilities it gives you.
For tips on getting a good chroma key please CLICK HERE