Using green screen technology to its full potential
Green screen has transformed the film and TV production business. Here we provide a guide to its use and provide some pointers to help make full use of the technology’s potential.
What is green screen technology?
The technical term for green screen is chroma-keying. This is a special effects technique that involves digitally stripping out the background from the subject or star of a video or film production. Once the background has been removed, another one can be put in its place during post-production.
The name green screen derives from the fact that the background the subject is filmed against is usually green (though blue is also sometimes used). As an example, an action hero could be filmed running away from a green screen towards the camera. The backdrop could then be removed and replaced with an explosion, or a dinosaur or a villain in a car.
The reason green and blue are chosen is because they differ most from human skin colours and are easily being picked up by the camera. For the technology to work effectively, the subject cannot wear the same colour as the screen, otherwise that will also be stripped out when the background is removed during post. [Interesting footnote: the very earliest pioneers of this technique were producers at Walt Disney who reportedly used yellow].
Why is green screen used?
Traditionally, green screen has been used to provide a graphic uplift to content such as news bulletins and weather reports. But these days, green screen has become a highly-significant part of the film and TV business. With the quality of computer animation now so high, green screen production allows producers to place their actors in virtually any environment, from fantasy landscapes to detailed historical backdrops. Another advantage of green screen is its versatility. If you are shooting multiple scenarios one after another with the same presenter it is easier and cheaper to drop them into a virtual green screen set than to keep breaking down and building sets.
What kind of productions have used green screen?
Baz Luhrmann’s The Great Gatsby was heavy reliant on green screen as can be seen in this excellent trailer. So was the movie Fast and Furious 6. Green screen isn’t just for Hollywood movies, though. It has also revolutionised the TV drama business. If you want to get some sense of just how many dramas use green screen go to this website from the Stargate Studios and take a look at the showreel (The Walking Dead, Pan Am, 24, Heroes and many more are featured).
In the UK, green screen is also playing a bigger part in the BBC’s production activities. A good example is Da Vinci’s Demons, a historical/fantasy series that was shot in Swansea using a mix of green screen and real sets.
It’s important to stress that green screen effects can also be achieved on very low budgets (eg for online productions), but tend not to look as slick. They are also still used in the live arena (news, sport etc) though this requires a different production process to the movies/dramas mentioned above.
What are the business implications of green screen?
Clearly there’s a saving in terms of set design. There’s also potential for reduced travel costs. For example, there’s no need for a Hollywood production to go to Egypt to put its actors in front of a pyramid. These savings are significant but need to be offset against the extra time spent in post-production. While it’s inevitable that green screen is thought of in terms of budget savings, the real advantage for big-budget theatrical productions is being able to create backdrops that simply wouldn’t be possible via a more traditional set-build.
What are the key technical issues?
There’s no question that green screen can look pretty awful if it isn’t done properly. Wrinkles in the green backdrop, poorly-designed lighting and cameras with low resolution can all break the illusion you’re trying to create. Often, the most noticeable thing about cheap green screen is that the edges between the backdrop and the character are blurred. Jonas Hummelstrand provides an excellent summary of the key technical issues, which you can see here.
One of his key pieces of advice is to “compare a roughly keyed-out foreground against the background that it will be composited against. Not only is immediate feedback important for the talent, it is also invaluable when it comes to matching the lighting and perspectives. If you can’t use a real-time keyer with a feed from the camera, at least bring a laptop and a digital still camera and do a quick key until the lighting matches perfectly”.
More generally, Hummelstrand says: “Avoid the temptation to think that problems on set can be fixed in post. Everything that can be done in front of the camera should be done on set. Make sure the time allocated for post-production is used to enhance the final outcome instead of fixing mistakes done when shooting.” Key to success with green screen is a good director of photography and a talented team of editors and compositors in the post-house.
Crews, cast and green screen
One of the biggest issues that comes up with green screen production is how to get people to react with or understand something that isn’t there. It’s important, for example, to make sure that the crew has seen a good storyboard so everyone knows what they are trying to achieve. It’s also important to talk them through exactly what is required in order to achieve the right end result. In addition, it’s crucial that the actors have some visual aids to help them immerse themselves in the scene.
Even greats like Ian McKellen admit that it is hard to raise their game when no-one is around. After filming scenes of The Hobbit: An Unexpected Journey with only pictures of the cast, he said: “Pretending you're with 13 other people when you're on your own stretches your technical ability to the absolute limits.”
Critics believe that this disconnect is one reason why big budget green screen movies can sometimes lack emotion. It’s also worth noting that green screen probably encourages producers to make up for inadequacies in the script by piling up the number of sfx – another recipe for disaster. Disney’s John Carter is a classic example of the downside risk.
The BBC has a few interesting insights at the following address http://www.bbc.co.uk/learningzone/clips/using-green-screen-in-tv-post-production/12719.html.
And this Wiki is also a good guide for DIY green screen http://www.wikihow.com/Set-Up-a-Green-Screening-Studio.
Finally, just for laughs, you can look at this ad campaign from BBDO Berlin, which has a funny twist on the use of green screen http://adsoftheworld.com/media/tv/true_fruits_smoothies_the_green_screen_prank_surfer