This video is sponsored by CuriosityStream, and my own Nebula Original series.
More on that later.
I’ve been using a green-screen for my recent computer-science videos.
I don’t like using green-screens, I think there’s no substitute for filming in a real physical location,
but I haven't had much choice lately.
And some folks, very kindly, said they couldn’t tell,
Now, a lot of that is down to my animator for these videos,
but a lot of that is down to some good planning.
Big-budget television shows get green-screen wrong. All the time.
This clip from 30 Rock looks… dreadful,
but it’s difficult to immediately point to why.
Well, first of all, there is the key.
That’s the term for how the computer works out what’s background and foreground.
There are all sorts of settings you can use to tweak this:
the exact colour of green it’s looking for,
how close to that green it has to be,
how it deals with colour in the transition between background and foreground,
how much it smooths out fine details like hair.
Get it wrong, and hair can appear and disappear as someone turns their head.
But let’s assume that you get a perfect key.
It can still look weird.
And that’s because if you want convincing green-screen footage,
you have to film with a camera that more-or-less matches the one that filmed the background.
Even for all the technical stuff that most people have no idea about:
because even if your viewers have no clue why it looks weird,
they’ll still be able to tell something’s wrong.
Before that, though, you have to match the lighting.
You’d think this one’s obvious, right?
But have a look at this scene from The Big Bang Theory,
who despite being one of the most popular shows in America when this episode aired,
apparently couldn’t afford to film at some rocks twenty miles outside Los Angeles.
Sure, you could argue it’s a reference to Star Trek
who used to have alien planets filmed on stages with some rocks unconvincingly strewn around, but…
I’m not convinced.
Anyway: the sun is clearly over there.
The rocks are in harsh shadow, there’s haze in the background.
And the actors are… lit like they’re on a stage.
This guy’s head casts a shadow on this guy’s chest in completely the wrong direction.
Even if you nail the lighting, you’ve got to get the colour balance right.
Daylight has more blue in it, most indoor lights have more orange in them.
Screw that up, and the different locations are clear.
Next, you’ve got to match the shutter speed.
So in this shot, filmed outside under bright sunlight,
you can see there’s almost no motion blur,
even on fast-moving vehicles that are passing by.
The shutter speed is short: in a thousandth of a second,
the camera sensor can get all the light it needs for a clear picture.
But I’m indoors right now. I can’t match how bright that sunlight is,
so the camera sensor has to sample for about ten times as long to get a decent picture,
which is enough time for me to blur when I move.
And if I’m blurred, then the background isn’t,
and then it’s going to look wrong.
And that blur is also really difficult to key out as well,
but that’s a separate problem.
Next up: framing.
If the person on screen appears to be hovering in mid-air,
or is shot straight-on while the camera is looking up or down at the background,
obviously it’s going to look wrong.
But there are more subtle things too.
Now, I was really lucky here,
I remember how the camera was set up for this original background footage in the computer museum.
It was four or five metres away, zoomed in quite a bit,
with the camera positioned just above my eyeline, so I’m looking up a little.
Means I don’t end up with too much of a double chin.
But let’s just pause for a moment --
-- okay.
The camera is now physically closer but zoomed out.
And suddenly, this looks wrong.
The same parts of my body are in the shot,
in roughly the same positions,
but I don’t fit into the background nearly as well.
This is not how I would look sitting in that chair in the real Museum.
I couldn't reach out and nearly touch the camera.
People’s faces look different, everything looks different,
depending on how close the camera is.
In this shot from Guardians of the Galaxy 2,
the camera is changing distance and zoom at the same time,
and the difference in how the actor’s face looks at the start and end is remarkable.
Okay, let’s go back.
Okay, that’s better! If you want your green screen to be convincing,
those are just some of the things you’ve got to match.
There may be others that I’ve missed, too.
But there is another approach, and I’d say it’s a much better idea.
Don’t try to be convincing.
For those of us who have the creative option,
it’s a lot easier to just acknowledge the green screen, lean into it,
and just put something interesting up there instead that isn’t meant to be real.
It’s not as good as filming in an actual location, but at least it’s honest.
I've got an original series over on Nebula. Here's the trailer.
"I invited five people to play some games."
- I trust no-one. - None of us are trustworthy.
"In an environment designed to slowly break their team apart."
- This is real money.
"But all they knew is they'd be sat round a table
"trying to win real cash. $10,000."
- The vibe's changed after that theft.
"This is a show about trust, about loyalty,
"and about money."
- Tom wants the chaos.
Along with my series, Nebula has Wendover Productions' original hour-long documentary,
The Final Years of Majuro,
and loads of other content and collaborations
from education video creators that you may well have heard of.
It's bundled at this link with CuriosityStream, a subscription streaming service with
thousands of big-budget, professional documentaries and nonfiction titles.
CuriosityStream is $2.99/month or only $19.99/year, including access to Nebula and my series,
when you sign up at curiositystream.com/tomscott.