Colours Design Movies

The Modern Green Screen: A Walkthrough About Chroma Keying Throughout the Years

Banner for Green Screen History

I previously discussed how film was colourized frame-by-frame by painted each cell and the advancements of modern CGI, there is the modern classic green screen effect that is still used and still gets the job done. This video effect is the division of the background film image from the live-action subject in a film so that a background image can be inserted later in post-production.

The actors will have to use a lot of imagination to focus on a scene. It’s difficult for some people since the person would be in front of a green screen acting out a scene with some sensory deprivation involved. This is a common way to integrate a real-time person in the digital world.

This is called chroma keying which uses chroma colour background (most likely green or blue) to key out the subject to make it transparent. Then a new digital background is inserted behind the subject. These matte shots aka process shots are completed in post-production. Green and blue are used because they are colours that are nowhere near flesh tones. When the editor starts to key out the background by picking up the one colour the person would (hopefully) not be affected by the colour selection being clipped out of the scene. The software would know which colour to choose from because the colour is usually set at a high chroma colour value; which is the degree of illumination calculated by the brightness and intensity for the most purest of the colour.

A quick chart of a calibrated RGB gamut for a shade of green.

Most video editing software can clip out a background for another one with a keyer tool built with the software with various steps. Ultimatte is a commonly used one with plug-ins that can work on most applications for professional use. It’s best to remember that not all systems are created equally meaning some might be more forgiving than others for irregularities in lighting or keying out colours.

The green screenshot has to be lit up correctly and framed well. There are several ways to have an actual green screen background by a painted background, a drop-down cloth that is one colour or pops up panels. Make sure that the cloth is completely flat with no creases or wrinkles that shadows will not form. A long camera lens with low ISO settings is recommended for maximum coverage. This will ensure that everything is filmed and the colours won’t have too much noise. The lighting has to be evenly lit with no dark shadows, especially with a bright rim on the subject to help with keying and separating the subject from the background. The background lighting should be even and vibrantly lit, not underexposed.

HEX Code number of Green Screen #00FF00
A YouTube video of Keanu Reeve’s in a green screen room filming the bullet-time scene for The Matrix (1999).

A green screenshot would typically use a tungsten lit background. They are small bright bulbs that can burn 20,000 watts. These bulbs can become very hot and are shielded in heavy-duty fixtures for maximum safety and control. They are also prone to exploding, therefore, safety practices must be done to avoid injury. Green screens are easier to use with a digital camera since the green colour can be easily picked up and the bright green is not a colour that is commonly used. People have blue eyes and some could be wearing blue jeans which can be difficult later on.

HEX Code number of Blue Screen #0000FF
A Disney featurette/trailer about The Jungle Book (2013). This film was completely filmed in a blue screen studio, all indoors.

A blue screenshot would typically use filters or bulbs background. A blue screen is helpful to use when your subject is green or with green hues.

Use a keyer tool to clip out the chroma colour from the subject. Anything that makes the silhouette of the person will remain while any of the colour will be clipped out. Other problems would be fine details being lost when the subject is being processed. Avoid diffusing or blurring the edges, that will soften the edges. Furthermore, turn off motion blur while filming and film in a RAW camera format when possible.

If the lighting is not correct, a green shadow can occur around the subject. This will turn any part of the subject object transparent accidentally. Anywhere the key colour is can turn the subject sections of their body appears missing due to their bodies being clipping out of the scene. This is called spill which occurs when the green background reflects on the subject. This could be due to a shiny surface that can be dulled down with pressing powder, dulling spray or milled soap.

Besides movies, it’s used in newscasts, weather updates with a meteorologist standing in front of a blue screen, YouTube videos and anytime someone is standing in front of a screen with graphics moving in the background. HD is great with keying colour and there have been good successes with SD screens. It is best to pre-plan the shot, do test screening with the subject still and moving to use visible markers on the ground and bring a laptop with the software downloaded to the set to test out keying.

This technique is a surprisingly old fashion dating back to the 1920s when it was first used in the films The Great Train Robbery (1903), The Invisible Man (1933) and Thief of Baghdad (1940). The film Thief of Baghdad was the first film that used blue screen technology with chroma keys, Technicolor and mind-boggling special effects for a film that used camera and photography tricks without a computer in the 1940s. It won an Academy Award for Special Effects in 1940. There were a lot of techniques used before computers by manipulating photographic technology at the time. The film used a blue screen travelling matte technique to achieve the majority of “magic” effects with a single colour as a backdrop for filming. It kept the actors isolated from the background and made special effects easier to create. Some mattes had double exposures. This was done by covering up a small section of film up not to expose it to the process light using a piece of glass only to re-expose the same film again replacing it with the effect.

This was a principle that was built upon from George Méliès who directed films like Four heads are better than one (1898), A Trip to the Moon (1902) and The Impossible Voyage (1904). And from Norman Dawn who directed films like Missions of California (1907) and Call of the Yukon (1938).

There were different and multiple types of chroma key effects that used a control optical printer that can do basic video effects like mattes, fades, motion control and dissolves. This was a staple film production machine for years that accompanied the camera when doing effects like chroma-keying. There was something called the sodium vapour illumination system which is also known as a yellow screen. The subject is filmed in front of a white background lit with sodium vapour lights causing the glow of the yellow lamps to bounce off in the background. Then a yellow key plate is created of the background and subject plate to be infused together in one film. This is was used in combination with the blue screen process. It was more like a blue screen process 2.0 with more technical components. Most of these films were also using the Technicolor process to make full-colour films for big cinema screens making the process into three colour channels of red, blue and green then a yellow key channel. This was a process that was very popular in the 1960s to 1980s until computer software took over. It was originally developed in the British Film Industry then exclusively the Walt Disney Studio Company. Notable films that used this technique were Ben Hur (1956), Bedknobs and Broomsticks (1971), The Birds (1963), Dick Tracy (1990) and Mary Poppins (1964) which won an Oscar for special effects by mixing live-action people, animation and matte painted background together. The inventor of this process was Petro Vlahos. He not only invented the more refined blue screen system but founded the Ulimatte company that specializes in green screen technology and won numerous awards for his technological achievements. Sadly, he died in 2013 at the age of 96 years old.


Resources:
Ascher, S., & Pincus, E. (2012). The filmmakers handbook (4th ed.). New York City, NY: Plume.
Camberwell Studios Ltd – The History of Green Screen
BBC – Blue and green-screen effects pioneer Petro Vlahos dies
Visual Effects Society – Petro Vlahos
Filmmaker IQ – The History of Greenscreen

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