I noticed that a certain video had several ‘versions’ and all of them were the same 1080p resolution, but they all differed in sizes. Some versions differed by a few MB, but it could range from around 300 MB to more than 1.5 GB! All of them have the same length and resolution and are exactly the same video. Also, a 720p version of that video was twice the size of some of the 1080p versions despite it being the same video.
I noticed something similar in some other videos as well. They all have the same length and resolution, and are even the same video, but their sizes differ by several times. Why is that the case?
In: Technology
Imagine a blindfold person trying to draw a picture based on your verbal instructions alone.
Resolution is merely the size of the canvas that they’re drawing the picture on at the end. What they draw on it with (a paint roller or a tiny paintbrush), and how much care you take to draw it correctly is determined by how much information you’re giving them (“draw a cat”, versus “draw a cat, facing right, it’s tail tucked down, the colour is between orange and brown but alternating every 20 pixels between dark and light, starting with dark….” etc,)
The way the video is stored is highly compressed – those are the “instructions” that you are giving the painting (I can explain how it’s done, but I’ve done that in a previous ELI5 – look up Fourier and Discrete Cosine Transforms… basically it finds regular patterns of regular patterns of regular patterns and then just reproduces those NEARLY perfectly by cleverly choosing only a small part of the pattern information). How it was compressed is determined by who did the compression. Sometimes that’s a broadcaster (e.g. iPlayer), sometimes that’s a guy on the Internet.
Depending on the compression settings used, even though the picture is still recreated on the same size canvas at the end it might have been painted carefully and accurately with a tiny fine brush, or a huge blob of a paint roller in a rough fashion. It all depends what settings the person compressing it used, how many instructions they decided to give the painter. High compression = lower image quality but smaller file size, low compression = higher image quality but larger file size.
It also matters what codec they used as well. A codec is the way that you find those regular patterns. Modern codecs do a better job than older codecs but take far more processing power to do so. So if you’re targeting old or slow devices, you’d compress with an older compression (e.g. MPEG, H264, etc.) but if you’re targeting powerful new phones, desktop GPUs, etc. and are going for perfect quality you’d compress with a newer compression (H265+, etc.). This is like being able to only use a few basic instructions and so painting a complicated image is rather tedious and long-winded and takes up a lot of paper, or only a few instructions that are more complex but which the painter has to be able to understand and spend a long time interpreting and getting exactly right. One takes up less paper (storage) to describe than the others, but the more complex one contains more detail even though it will take longer to “decode”.
So you can easily have a 4K movie that’s compressed badly that’s actually smaller than a 720p movie that’s been compressed really well, and yet the latter will look far better. And you can have a 720p movie that’s been compressed with slow devices in mind that makes a far larger movie movie than anything you have in 4K. Video editing software lets you literally tweak those settings, even on a frame-by-frame basis.
It’s all about trade-offs… in processing power required, in bandwidth/storage required, in where you need detail or won’t notice it among the fast-paced action, in the time it takes to compress the movie, and what the individual settings and standards available were when that movie was made.
I know if I download an episode from BBC iPlayer, the quality is amazing but the file size is often huge. I can also go online, or record from my TV card, that same video in the same “resolution” and get smaller file sizes for the same quality (e.g. by using a better codec than iPlayer uses), larger filesizes with worst quality (e.g. when quickly encoding it on my laptop from recording the TV), and any combination thereof.
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