The miracle of green screen
Green screen was hailed a ‘miracle of technology’ on the latest edition of The Oprah Conversation, enabling an in-person interview between Oprah and Barack Obama, who were actually filmed from opposite coasts.
How does this miracle work, what’s the science behind it and who discovered it in the first place?
Quite literally, a green screen is a background “screen” (which can be made out of fabric, paper, or even paint) that is a specific colour, usually green.
Green screen technology works by ‘keying out’ the solid colour, also referred to as ‘chroma keying’ it’s easy to separate the background from the subject in front of it, render it transparent and let another image show through. Once green screens are identified and digitally removed, just about anything can be added in, while the parts of the original image that aren’t green remain unaffected.
Where did it all start?
One could argue that green screen technology originates from the matte effect. Georges Méliès created this visual effect when he needed to find a way to have an actor surreally remove his own head. To do this, he took multiple exposures of multiple frames and combined them. By separately blocking parts of the lens of the camera to leave certain areas blank, he was able to superimpose the blank frames on top of alternate frames that were filmed with a clear lens.
The history of green screen evolved with the invention of chroma key technology in the 1930s. Larry Butler, who used the “blue screen travelling matte” technique to depict a genie being let out of a bottle in the 1940’s The Thief of Bagdad, realised that using a single colour as a backdrop for filming could help filmmakers isolate the actors from the background and make special effects easier to create. The colour he selected for this process was blue, because it was sufficiently different from the actors’ skin colours, so foreground and background could be separated more easily.
So how did film makers discover that green was a more suitable colour?
It wasn’t until the development of digital camera technology that people began to switch from blue to green screens as digital cameras are more sensitive to green than blue. Since the quality of chroma keying is very much affected by how accurately you can record the edge between the foreground subjects and the background screen, the increased resolution of the green channel allows for more accurate cut outs of the background.
The Bayer filter mosaic, 3CCDs, chroma subsampling, 4:4:4, 4:2:0 are a complex sounding array of ‘miracle’ technology that we as professionals deal with on our clients’ behalf. Attention to detail is also paramount. Indeed it wasn’t a surprise to find that Oprah and Obama conducted their interview in two rooms with identical furniture (and mugs), arranged with extremely precise measurements. Production teams used the same cameras, lenses, lighting and audio equipment to achieve such a high standard.
There is a huge difference between what we can achieve at home compared to a proper studio set up, but there are some things that we can do to make it look as good as we can.
Here are a few other helpful hints to make the miracle happen!
For green screen to work effectively, the screen should be flat and smooth and as clean as possible. There shouldn’t be any wrinkles.
The area must be evenly lit to avoid a patchy background, the more lighting the better. The subject and screen should be lit separately to avoid the subject casting shadows on the background and to avoid a green hue bouncing off the subject which will cause problems when removing the background.
Equally, having as much space between the subject and green screen as possible will help. The subject’s clothing choice is important. Colours that have a hint of green can make people look semi translucent. Reflective materials are also not a good idea. Shiny objects will pick up the green from the screen.
Ideally the green screen will continue down the wall and onto the floor if you wish to show the actor from head to foot. A cove is a curved corner where the wall meets the floor, this will smooth out the transition, avoiding shadows and hard lines in the background.
The camera should be kept completely still otherwise the subject may look as though they are bouncing against the background, this can happen even when the camera moves slightly. Zooms should be avoided as it will look as though the subject is shrinking or growing in relation to the background.
Remember ‘Who Framed Roger Rabbit’?
Film makers used green screen to combine live action with animation and its use of technology marked a new era, enabling actors to interact with invisible cartoon co-stars.
Perhaps the Oprah Obama fireside chat marks a similar moment in the world of TV production, panel discussions and broadcast interviews, enabling a sense of togetherness in a socially distanced world.