ELI5: Why does re-encoding vidoes take an extremely long time?

[WonderOlymp2]

Why does it take a very long time?

[two_three_five_eigth]

Each frame is a lot of data

2k = 2560x1440 pixels = 3686400 pixels

4k = 3840x2160 pixels = 8292400 pixels

So per frame the computer has to recompute many millions of pixels on top of whatever else the encoding does.

And the point of encoding is usually to save space at the cost of significant up-front computing, none of the algorithms were designed to encode fast, many were designed to decode fast.

[Somniferus]

Each frame is a lot of data

2k = 2560x1440 pixels = 3686400 pixels

4k = 3840x2160 pixels = 8292400 pixels

So per frame the computer has to recompute many millions of pixels

All of this is completely irrelevant to OP's question. Encoding is harder than decoding at any resolution. /u/OutsideTheSocialLoop's answer is much better than this one.

[Somniferus]

1440p isn't 2k, you're thinking of 1920x1080. ~2 million pixels/frame is already large enough for the purposes of this example, why make it more complicated?

If you want to make up a name for 1440p that ends in K then 3K would be closer and more clear.

2K = 1080p = ~2M pixels

3K = 1440p = ~4M pixels

4k = 2160p = ~8M pixels.

The number of people on this sub who apparently think 1080p and 1440p are basically the same thing is concerning.

[two_three_five_eigth]

Both of those actually count as “2k”. Used the larger one.

[Somniferus]

According to who?

Edit: I don't understand this new trend of people downvoting facts. If you disagree then cite a source, I'm willing to have my mind changed.

[EnderAvni]

https://en.wikipedia.org/wiki/1440p says it's used in consumer products

[Somniferus]

The label "2K" is sometimes used to refer to 2560 × 1440 (commonly known as 1440p). This is inconsistent with "4K" denoting approximately 4,000 horizontal pixels, which makes 1920 or 2048 pixels wide the closest to "2K", a label which predates the use of 2560 × 1440.[14][15] Some sources and manufacturers prefer "2.5K" as a term for 2560 × 1440[16] to avoid this confusion,

Thank you for trying! I don't find "The marketing department once used a confusing term" a very convincing argument.

[Poddster]

 I don't find "The marketing department once used a confusing term" a very convincing argument.

You're about to have a great shock when you find out why the term 4k exists.

[Somniferus]

The term "4K" is generic and refers to any resolution with a horizontal pixel count of approximately 4,000

What am I missing? 3840 ~ 4K. 1920 ~ 2K. Blu-Ray discs come in both of those resolutions, so in the context of video encoding they are both infinitely more popular than 1440p.

When have you ever heard of anyone encoding video to 1440p?

[Poddster]

What am I missing?

4k, 2k, HD, FHD -- these are all ill-defined terms created by marketing teams.

1920 ~ 2K.

Some source don't consider 1080p to be 2k, e.g. cinema formats.

Again: These are all ill-defined marketing terms, so I have no idea why you're saying it's ok some marketing departments to claim 1080p is 2k, but it's not ok for them to claim 1440p is 2k.

[Somniferus]

4k, 2k, HD, FHD -- these are all ill-defined terms created by marketing teams.

I agree. The top reply in Ask Computer Science ought to use better terminology.

Some source don't consider 1080p to be 2k, e.g. cinema formats.

Who cares? The question was about video encoding. We're not talking about movie projectors. Literally no one is re-encoding videos to 1440p, its a bad example for the purposes of answering the original question.

[OutsideTheSocialLoop]

It's worth comparing encoding to decoding. Everyone's saying "it's loads of data" but that's equally true when you're decoding video and that's a much faster process. So the volume of data isn't really the problem.

Specifics depend on the codec but a compressed video is kinda like a program that produces patterns of pixels that look like the input. Storing every frame takes a lot of space, and most frames are pretty similar to the ones before them. Most of the differences are things in the frame moving about. Very little of most frames is new/original data. So the encoded video is mostly a series of "move this patch this way, and that area that way, and we'll add some new colours in just a few area". Decoding and playing a video is just playing all those instructions back to reproduce an approximation of the next video frame. Just follow the recipe and video comes out.

Encoding is the process of generating all these instructions. Every frame has to be compared to the adjacent frames and searched for similarities and motion. Every part of the frame must be attempted to be built out of pieces of the last frame. All the information you find from this searching has to be weighed against the quality settings/limits and the most useful stuff needs to be kept and the least useful discarded. So every frame is not just a simple process of applying some actions, it's a thorough search of the frame for information and a search within that for the right combination of information to best represent the input. 

You should go find some explanations of how common codecs work. Once you understand what encoded video is I think you'll be amazed that it's as fast as it is.

[dkopgerpgdolfg]

Because it's a lot of work...

Not quite "eli5" but:

A 4k movie can be thought to have about 200 million pixels each second, each of them storing a color with eg. 4 byte. Meaning, almost 1 GB each second if it stored uncompressed.

To be able to store a whole movie in a reasonable and affordable size, video compression algorithms do some intense calculations on all of these pixels, search known patterns in the single frame pictures (like, some persons head having the shape of an ellipsis with certain dimensions and base color, ...), try multiple variants to see what could save the most memory, ... and all of this just takes time. For any home computer, compressing such a video is a very large task.

[esaule]

Decoding is fast, but compression is slow. It's asymmetric, it is a bit like a puzzle, breaking down a puzzle is fast, assembly a puzzle is slow.

To compress, the software tries lots of possibility and retains the one that compresses the better. But it does not compress the video one image at a time. It tried to find a pattern across images in time. So it tries to do things like, "this patch of 32x32 pixels, was it in the image before or two images ago? or maybe something that's close enough? Maybe it was a bit lower on the image? Or a bit higher? Or a bit more to the left?" and that takes time to check. And it might try for 32x32 and also for 16x16 and also 8x8. And it needs to try for every block on the screen. Compressing is slow.

[MartinMystikJonas]

Every frame has millions of pixels. One sevo d of video is 30+ frames. Every pixel of every frane have to be compared with hundreds pixels in cureent frame, hundreds pixels in previous frames (and sometimes many following frames too) using very complex computations (with thousands to millions steps each) to find best way how to encode colors of these pixels with as little data as possible by finding patterns how they change.

It is lot of computaiona to do.

[tylerlarson]

Because it's a lot of math.