Is Chroma Key just for the pro's ?



  • What is Chroma Keying ?
  • What are the benefits ?
  • Why Green or Blue ?
  • How we have used it.
  • Tips for filming GreenScreen shots

Overview

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.


Benefits

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 added later.

Tips for filming green screen shots

  • The goal for setting up a shoot with a green screen is to ensure that the screen is as close to chroma green as possible, and that it is lit as evenly as possible. For films on a tight budget there will always be imperfections in the screen, but you need to minimise the variations as much as possible. Remember the stupid computer? We would look at a wall of green as “green”. A computer would look at it as a wall with lots of different shades of colours (different shades of green).
  • The best way to light a green screen is from in front (not from the sides) so as to limit the number of shadows created by creases in the material. You can also light from the ground pointing up. It is important not to overlight the screen as you will get white hotspots. Actors must be lit separately and ideally between 12-14ft in front of the screen to avoid shadows being cast and green light spilling on to the actors. If necessary, backlight the actors with a magenta filter to minimise spill.
  • Ensure that the luminance of both the green screen and actor are matched as closely as possible so that exposure can be balanced.
  • DON’T introduce any camera movement when filming against the screen (unless the software you have has the ability to track; advanced topic not covered in this article) otherwise your actors will seem to float around the background unnaturally (this might be the look you want!!).
  • Make sure none of your foreground subjects are the same colour as your screen (remember the green apples earlier?)
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!

 

 

Conclusion

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

Declan Smith
May 2006
info@bristolvideo.org.uk

   
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