How Green Screen Worked Before Computers

If you know anything about how films are made then you have probably heard about the “green screen” before. The technique is also known as chroma key compositing, and it’s generally used to merge two images or videos together based on color hues. Usually you see an actor filmed in front of a green background. Using video editing software, the editor can then replace that specific green color with another video clip. This makes it look like the actor is in a completely different environment.

It’s no surprise that with computers, this is a very simple task. Any basic video editing software will include a chroma key function, but have you ever wondered how this was accomplished before computers made it so simple? [Tom Scott] posted a video to explain exactly that.

In the early days of film, the studio could film the actor against an entirely black background. Then, they would copy the film over and over using higher and higher contrasts until they end up with a black background, and a white silhouette of the actor. This film could be used as a matte. Working with an optical printer, the studio could then perform a double exposure to combine film of a background with the film of the actor. You can imagine that this was a much more cumbersome process than making a few mouse clicks.

For the green screen effect, studios could actually use specialized optical filters. They could apply one filter that would ignore a specific wavelength of the color green. Then they could film the actor using that filter. The resulting matte could then be combined with the footage of the actor and the background film using the optical printer. It’s very similar to the older style with the black background.

Electronic analog video has some other interesting tricks to perform the same basic effect. [Tom] explains that the analog signal contained information about the various colors that needed to be displayed on the screen. Electronic circuits were built that could watch for a specific color (green) and replace the signal with one from the background video. Studios even went so far as to record both the actor and a model simultaneously, using two cameras that were mechanically linked together to make the same movements. The signals could then be run through this special circuit and the combined image recorded all simultaneously.

There are a few other examples in the video, and the effects that [Tom] uses to describe these old techniques go a long way to help understand the concepts. It’s crazy to think of how complicated this process can be, when nowadays we can do it in minutes with the computers we already have in our homes.

17 thoughts on “How Green Screen Worked Before Computers

    1. I was thinking the exact same thing. I remember an article in TV Guide entitled “Am I Blue” about the special effects used on David McCallum on the old “The Invisible Man” TV series.

  1. In my Seventies childhood in Britain, this was called “colour separation overlay”. I don’t know if a copy still exists of a skit done for the BBC programme “Blue Peter” in which John Noakes was recorded singing in slightly different parts of the studio a dozen or perhaps even twenty times, then the recordings overlaid repeatedly to turn him into a one-man male voice choir.

  2. For what it’s worth – the original Star Wars was all done with models and computer controlled cameras (aka “motion control”) the only CGI was the wire frame in the heads up display. 2001 A Space Odyssey was all done with optical printing and rear screen projection, truly amazing when you think about how much work went into some of the iconic scenes!

      1. Thank you for the correction, pedantry accepted. Actually I wasn’t sure of the details on that bit, just remembered it was projection, too lazy to g**gle it. Saw the original full “trip” scene edit in a real cinerama theater when it came out – blew my 12 year old mind!

  3. OK, let’s step back a few more years.
    The original technique from film was widely known as a ‘travelling matte’ which masked some part of the foreground image vs others. When this was reinterpreted for analog video, the ‘blue screen’ was dominant before the ‘green’ screen techniques (reasons in another post one day).

    The choice of blue, green or any other colour is highly dependent on what you’re shooting or trying to achieve – and the foreground and background content. Of course you remember the invisible jeans and shirts/ties from years gone by…. !

    To get the best results, the blue was a very particular hue and reflectivity, but still frequently left backlight fringing and ragged edges if not lit and setup perfectly.

    Petros Vlahos (of ‘Ultimatte’ fame) patented a few extra tricks in analog keying techniques, but it was still prone to the weaknesses of leaked colour, internal reflections of the lens and a few other factors that made analog keying relativeky difficult to do well. A good TD could get near perfect keys with the right ingredients — and could add live shadows as well!

    If you understand the fundamentals – I worked on several projects back in the late 70s, where we lit he foreground (white) and background (85) with different gel – to increase the colour (vector) separation – required for the keying process. It worked so well, that we were able to successfully key moving glass – no mean feat at the time.

    When component digital arrived (let’s ignore composite digital for now) – the absolute purity and management of colour-space allowed the operators to specify the HSV gamuts required to perform the keying much more accurately. Hence much tighter and accurate edges – which in fact are so good, that (like Ultimatte twenty years earlier) extra processing was added to soften and feather the edges slightly to blend the keyed outlines.
    BUT the issue still exists of internal colour reflection and refraction in the camera lens – which means the studio must still be keenly aware of light/lens position and distances needed to get the ‘perfect’ key. It’s possible – and modern keyers also provide even more processing to correct for ‘colour leakage’ in the finished composite shot.

    And there are still occasional reasons to use the old travelling matte techniques, and high-contrast intermediates, but modern digital keying is way ahead on most lists.

    So there.

    1. “Of course you remember the invisible jeans and shirts/ties from years gone by…. !”

      Late ’70s early ’80s, a family friend was the local news anchorman, and his blue eyes were occasionally replaced by the “over the shoulder” picture thing. It was funny as hell.

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