Name: 有序抖動 - Computerphile (Ordered Dithering - Computerphile)
Uploaded: 2021-01-14T10:13:30.000Z
Duration: 10 min 35 s
Description: 【看影片學英語】數萬部 YouTube 影片，搭配英漢字典即點即查，輕鬆掌握單字發音與用法，長久累積看電影不必再看字幕。

情態助動詞 A2

You can reduce the file size of an image by

動名詞

Removing the number of bits you need to represent

Each the colors and if you remove that number of bits

You have less colors, you can display so the image looks worse

If you've got eight bits, you've got 256 levels of gray or levels of each color if you've got four bits

You've only got sixteen if you've got one bit you've either got black or white

and so what did ring is it's doing is an attempt to

Simulate those extra gray scales. Let's keep it to a gray scales for the purpose of this video

Just using black and white pixels or whatever colors you've got on the end for this video

We'll just keep it to either having a black or a white pixel

So quite often computers certainly early screens or even these days with printing they can either display a

Pixel or they can't display it they can either be black or it can be white

Let's take this to the extreme because it basically is the same even if you have got more colors

Let's assume that we've got a display that can either display a black pixel or a white pixel. It might be a printer

It might be a sort of very very simple video display of some sort particularly thought you find on sale

an early 80s computer something although might be black and green then but let's go black and white and what we want to do is

So they they don't just have black or white we go from black through grays to white and again

You could extrapolate this out to different colors. And so you need some way of mapping the grays

Into black and white a naive way we could do this. We've got an image. Let's say it's a photograph

I'm sure I'll apply the special effects to make it look this and we need to convert it to black and white as we can

Below a certain level will make black anything above a certain level will make white so we can do that and if you do that

You can pick the level of which you make that switch you get it with an effect like this

realistically, you've got lots of white and lots of black that call for a special effect, but not

Useful for watching or printing things. So that's a naive approach

It's actually a problem that's existed for

Years newspapers are printing in general as had this problems pretty much since photography was invented in the 8th

Hundreds and the way they get round it is that they convert the image into a series of dots actually this was originally done

optically the image that they wanted to print was put in front of a camera and a screen was put in front of it of

Lots and lots of small things and it was rephotographed with very high contrast

Holes in the screen were you've got very small pin pricks of light that depending on how bright they were made bigger or smaller

Photographic plate and that's what was printed. So we need to do the same thing

On our computer one way to look at it if we wanted to say make this square

Gray, if you wanted to make it black, then we could just color in all the pixels

So can you have it as white or black but we can also make it

Appear grey just like the newspaper did by not coloring all the squares inside we can make it appear

gray by coloring in this one and then coloring in

This one like this and if you do that and you do that for enough of them next to each other

If you zoomed out from that then it would look about 50% gray

you could do a lighter gray by just setting one pixel of those four in each one and

Just to finish off we could create a darker gray

But one of the pixels we can create just with a 2x2 grid of pixels a white one a black one

50% gray probably around 25% gray and around 75% greater and get 1 2 3 4 5

Different levels of grey just by using a 2x2 grid and selectively coloring in the pixels

what's actually happening is that your eye is at acting like a sort of optical low-pass filter and

When it's slightly fine of a way you stop seeing the individual pixels and you just see the sort of average gray from the number

Of things that are in there you sort of converting the high frequency off the norms of the image into a grayscale

Because you're looking at from a distance and so on

So that's what did the ring effectively is. It's choosing a pattern of white and black dots to create

The image to create the grey or to simulate the gray that was a better way of saying it now

How do we do that in a computer now if we think about an image the image if we've sampled it?

It's broken up into pixels and then they have a number associated with it to say how bright the artless is. Zero is black

255 is fully white. So what we could do is say well, let's say they're 50% gray. They said they're all 50% gray

This is 128. That's 128. That's 128 and so on

So how do I programmatically convert that grid which is 50% gray

Where I say? Okay, I want that one on I want that one off

I want that one on I want that one off so we could do it by patter matter

We could look at this and say okay these four pixels here or 128 therefore

we need to draw that pattern in it and these for

64 every 25% gray so we'll draw that pattern the pixels in it

You could do like that, but it's actually a clever way. You can actually do this

so what I'm going to do is I'm going to say if this

pixel is greater than 0 I will set it if the value in the pixel is greater than

64 I will set that pixel if it is greater

I will set that pixel and if it is greater than 1 9 2 I will set that pixel

Okay, so these are each of our indifferent your pixels. So go back to my pixels here and I start

With this one is 128 greater than 64. Yes, right so we color that in we're at my level again here in math. Yes

We now move on to the next pixel is 128 greater than hundred 28

Greater than that form all there isn't a next one. So we go back to the beginning

Yes, we set that again. And now we're going across here is 128 greater than hundred twenty-eight. No, so we leave that blank

So now we come down onto line two and we do the same thing with the second line here is 128 great at 192

Twenty-eight greater than zero. Yes. Okay, so we get the program to set this pixel here

Is it greater than hundred ninety-two? Nope, is it greater than zero? Yes

and so we set that one there like that and

We're back here. And we do the same thing as a greater than twenty eight and

No, and what happens is that you end up creating the pattern we'd come up here before?

Thing there. Okay. Let's try that again and we'll do it a bit quicker this time. Let's say that these are all 24

So what happens? Well, we do exactly the same thing. We apply this is 24 great in 64

No is 24 greater than hundred twenty-eight. No is 24 great in 64. No, it's 24 greater than hundred twenty-eight

No is 24 greater than 192? No is 24 greater than zero. Yes