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.
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