Filming with green screen – the basics, how does it work?
We have all seen green screens, we know that they are used in the CGI. part of movies but how does it work? we take a sneak peak to find out the basics.
The technique actually goes back much further than you might think, with the first documented usage as far back as the early 1900s. They used blue screens at first because they found it worked better with the celluloid film of the time, this later became green with the rise of digital film production.
Well you could actually use any colour, but a bright neon green is a good choice as it is usually distinctly different from what the actor might be wearing or anything else in the scene, that is unless you happen to be filming a Ford Focus ST. or say Kermit the frog. In cases such as these the team would probably use blue instead, it just has to be clearly and distinguishably different from clothes, hair, and objects in the foreground of the shot otherwise they will disappear when the rest of the process is completed.
Actors and Actresses in a green screen scene can appear different against the green background, sometimes looking pale and a bit sickly, so having a good make-up artist on hand to correct and offset this effect using make-up is essential. Shiny jewellery or clothing is also a big no-no as the reflective properties will also interfere with the process.
The distance gap which we will talk about below must be maintained so as to avoid a green hue reflecting off the actor and making it impossible to remove the background.
This is a much more complicated process than it might first appear and is fraught with pitfalls.
Using real objects in the foreground makes it much more believable against the background, for example, Fred Weasley’s broomstick at the bottom of the page.
Actors must be set in front of the green screen but not “against” it, there needs to always be a separation gap to allow distance between the subject and the screen otherwise the depth of the scene will be compromised.
At least 6ft must be maintained as a minimum distance from an actor to the green screen. This distance must be multiplied up for a full-length shot, to usually around 25ft from the background, so often a large studio is needed for action shots, and studios have to get creative with movement often using treadmills and the like to give the appearance of movement whilst still being on the green screen.
Typically a “coved” screen which is a curved screen going under the actor’s feet is used to create seamless screen effect, this creates a smooth transition from floor to wall in the shot, avoids hard lines and allows the SFX team greater scope to seamlessly alter the background.
The next important thing is the lighting, the trick is to light softly and evenly, giving as even a texture and gradient as possible. The background must be lit independently from the subject to keep the separation in the shot.
You don’t want the actor to cast shadows on the screen or this will again change the gradient of the background in the shot and compromise the end green screen result. If the green screen is made of a fabric type then ensuring that it remains smooth and wrinkle-free is essential, if it is painted, then it must be a non-shiny paint and any scuffs, scrapes or scratches MUST be touched up immediately or this will also spoil the filming.
The technique basically works by using software to layer images so that you can put whatever background you want behind the actor. They can be doing their shoot in a studio in London but they look like they’re in the Arctic say, or a strange alien world. You’ll have seen this in the simplest form in news broadcasts, where the reader looks like they are in front of a picture, a map for example.
After the initial footage is shot, it is sent to the compositors, who do as it sounds, create a composite video made of multiple layers. The team will do a number of things, the multiple video feeds or image streams are layered together, then something called a chroma key is added. This singles out the green layer, makes it transparent and removes it from the image letting the other image show through. This is why it is essential for nothing else green to be in the shot otherwise it will also disappear along with the ‘green’ background.
Then new layers are added with sophisticated 3D effects such as smoke, fire architecture etc. these layers are blended with the rest to give realism to the scene.
The final result appears as though the subject is in front of a new background, with the addition of whatever extra layers the SFX team have added. Clever stuff, and we are very grateful for the teams who make it happen as a Marvel movie for example just couldn’t happen without these techniques! Great job SFX!
Article- Emma Murfin for Movie-Reliquary