The dark doesn't look very good these days. And I'm really sorry if you're one of the
lucky ones who's never noticed this, because now I point it out, now I artificially increase the brightness
on the dark parts in this video so you can see what the shadows behind me
actually look like... now you'll notice. In movie trailers, in dark scenes in even prestige television, in YouTube videos where there are
nice, calm gradient backgrounds: you see this dreadful colour banding. There are three reasons for it. And the first is that
there aren't enough colours. ...all right, I'll turn the lights on. In most modern digital video, there is a grand total of about
16.7 million possible colours. And that number comes from how your screen works. When you watch a video,
your phone, computer or TV takes that compressed digital signal
that's being sent to you, and it converts it into instructions. Those instructions go to the screen, which changes the brightness
of each of the millions of red, green and blue lights
that make up your screen, up to 60 times a second, perfectly. It's incredible technology that
we just take for granted. But those little lights can't be
adjusted to any brightness. The simplest digital signal
would just tell the screen whether to turn each light on or off. Each instruction would take a single bit,
a single one or zero. So for each pixel,
each combination of red green and blue, that gives you eight possible colours: two times two times two. Which doesn't look great. So, okay, let's add another bit for each colour. Two bits gives us four options for each light:
off, dark, sorta-bright, and completely on. Now we've got four times four times four,
64 colours. Still doesn't look great. For a modern screen,
you need eight bits for each pixel, so you've got 256 shades each
of red, green, and blue, which gives you those 16 million
colours that you're used to. That ought to be enough, right? It seemed to be, when the standards were written. And to be fair, every bit you add means more data to transmit
and more expensive equipment. It seemed like a pretty good compromise
at the time. Professionals, like people working
in digital cinema, they go further,
they might use ten bits for each pixel, giving them about a billion
total possible colours instead. And you may well have a fancy screen that
can show those more precise billion colours, but the picture quality is determined by the
weakest link in the chain. A ludicrously expensive HDR monitor isn't
going to fix the colour banding on that regular YouTube video. Anyway, it was decided that 256 shades
each of red, green, and blue, combined however you like,
that should be enough for most people. And for almost everything, it is. But at full resolution, this video is 1,920
pixels wide. And there are only 256 shades of green. So if I have a green gradient across
the whole video, even if you use every single shade of pure
green that's possible on this format, then you're still going to have a colour band
every seven or eight pixels. And if everything's much darker than that.. suddenly you only might have half a dozen
different shades of green available across the whole picture. Even if youāre using a bit of red and blue
in there as well, there just arenāt that many
available dark colours. But if you've got
plain, bright background, why doesn't that have
colour-banding all over it? Well, the second reason is
how human eyesight works. There is colour banding all over this video.
Right now. If we take a sample of the bright gradient
behind me and massively increase the contrast, you can see it's thereā¦
but normally, it's invisible. And thatās because while the absolute difference
between two bands is still the same, the relative difference is tiny. It's the same reason that this chart looks
a lot closer than this chart. Going from 201 to 202
feels like a tiny change but going from 1 to 2 is a doubling. Even though in both cases, the absolute change is the same: one, we perceive one as being
bigger than the other. But even if you don't have enough colours, you can break up gradients another way,
you can use dithering. Instead of going from one colour to another
at a line, you can steadily transition from
one to the other, you can make the boundary fuzzy, so that from a distance
the colours appear to blend. And if youāre filming a real-life scene,
then thatāll happen in-camera, the natural light and the noise in the signal
and the sensor will do that for you. And it actually works really well,
it more or less solves the problem... but it won't look like that here. And that brings me to the third reason:
compression. A raw HD video, uncompressed, needs somewhere around
a gigabit per second of data. Even if you have an internet connection that
can somehow support streaming all that, it's incredibly wasteful and expensive to
use all that data. So every streaming service,
YouTube, Netflix, everyone, uses lossy compression: a program called an āencoderā takes a
massive high-quality video file, and then throws away fine detail to save data. The more the video is compressed,
the worse it looks, but itāll work on slower and slower connections, and itāll be cheaper and cheaper
to run your streaming service. But at some point, the viewers are going to notice that the picture
doesn't look great. But the encoder, the compression software,
is written by very clever people: and it works out which bits of the scene the viewer is probably
going to be interested in, and puts most of its effort into making
that part look good. And thatās usually a bright, sharp part
of the scene, something well lit and in focus, and probably the bits that are moving. Now there are exceptions, where there's some slow, menacing threat
out of focus in the background, but for almost every scene,
if itās dark, or out of focus, chances are the director and the viewer
doesn't care about it. So the encoder doesnāt care about it either: if there are only fifty kilobits, fifty thousand ones and zeros available to
describe each frame of video, then the encoder will make sure
that most of those are spent on where the viewer's
likely to be looking. All that dithering, all the fancy stuff
in the background? Too expensive. Smear it out into just big blocks
of solid colour, no-one'll notice. And we won't. Unless it's also very dark. At which point, there will be something nasty
lurking in the shadows.
It's pretty amazing to me how I never noticed stuff like this growing up. But once you see it, you can't unsee it.. Kind of like some games lacking anisotropic filtering or anti aliasing.