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  • 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 you're using a bit of red and blue in there as well,

  • there just aren't 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 therebut normally, it's invisible.

  • And that's 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 you're filming a real-life scene, then that'll 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 'encoder' 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 it'll work on slower and slower connections,

  • and it'll 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 that's 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 it's dark, or out of focus,

  • chances are the director and the viewer doesn't care about it.

  • So the encoder doesn't 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.

The dark doesn't look very good these days.

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暗い動画がひどいブロッキーな理由 (Why Dark Video Is A Terrible Blocky Mess)

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    林宜悉 に公開 2021 年 01 月 14 日
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