How Green Screen Worked Before Computers
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How Green Screen Worked Before Computers

These days, green screen is easy. You get
your lighting right, you point a camera at someone, and then you click a few buttons
and tweak a few settings in your graphics software, and there you go. Perfect. But how did they do this sort of compositing
before computers? Well, there are two answers to that. One for
film, and one for video. And before anyone gets pedantic: I’m summarising here, and skipping
over some early efforts and dead ends, but first, let’s talk about film. Actual, bulky
reels of film. You’ve got no pixels to deal with there: it’s all chemical processes, all
light hitting bits of photosensitive real stuff twenty-four times a second. If you go
wrong, you can’t erase and restart, that film’s used up. Gone. Go buy some more. Anyway, the earliest method for this sort
of effect involved filming an actor against black, and then making copies of that film with
higher and higher and higher contrast, until you had a simple matte. With that, you could pull
off a double exposure, using chemical trickery and something called an optical printer to
leave a hole in the background picture where the actor would then be slotted in. Or part
of the actor, if we’re talking about Claude Rains as the Invisible Man. Right. Variations on that high-contrast technique
were used for decades. The next major breakthrough was the travelling matte. The cleverest version
of this was bought up by Disney, who owned the only camera in the world that could pull
it off. They lit the actors normally, but put them in front of a white background lit
with sodium vapor lights — orange streetlights. Sodium light is monochromatic at one very
specific wavelength — so with some fancy optics, you could simultaneously record onto
one piece of film that would mostly ignore that light and another bit of film that only
reacted to that light. Presto: a near-perfect travelling matte, even for semi-transparent
objects like Mary Poppins’ hat. Bluescreen and greenscreen on analogue film
work much the same way. They use filters to block out particular colours and matte them,
and then they’re carefully combined with an optical printer. Then, of course, computers
came along and made the whole thing easier. How about television? Computers arrived earlier
than you might think here; the BBC used a bit of kit called the Quantel Paintbox for
Doctor Who as early as 1980. But before that, video was entirely about electrical signals
from a scanning camera sensor — if it was recorded, those signals would just be put
straight on magnetic tape. There’s no film to play about with there. But what you could do with electronic signals
is combine them and build circuits that basically do analogue maths on them. Adding two signals
together, or just fading between them, was mastered early in the days of black and white
TV. Now, when colour came along, you had red, green, and blue signals — although they weren’t
transmitted over the air like that, for backwards compatibility, it’s a long and overly-complicated
story — but you could make a system that would watch the signal, and when it hit a
particular colour — the colour you wanted — it’d switch out that signal with your background
one. And generally, this was done live on set. There were even attempts to do moving
shots by rigging up really complicated gearing arrangements to sync the movements of two cameras — one
pointing at actors on a bluescreen, one pointing at a model. It even worked sometimes. No pixels. No memory or hard drives, no ability
to just tweak your settings and try something else: if it didn’t work, well you did the best
you could, because every day in the studio cost an incredible amount of money and the
time of at least half a dozen technicians. And then, of course, computers came along
and made all of that obsolete, to the point where some jerk with a laptop and a copy of
After Effects can make this video in an afternoon. [Translating this video? Add your name here if you’d like credit!]


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