So ive been making backups of my movies by ripping my blu ray collection and filling in the gaps with torrenting. I have noticed that a 1080p movie will take up 20-40 gb if i rip it from a disc, but if i just find a torrent online its usually 5 gb or less. Both are in the MKV format
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Video encoding is a super complex topic. MKV is just *part* of how the file is designed.
Think about a company that makes furniture similar to IKEA. They might have a model of bookshelf that can come in 4 or 5 sizes, with 7 or 8 different options for accessories. If they’re clever, they might be able to make one size of box that can fit all of them. To make sure they send you the right thing, they have to put details like a model number on a label attached to the box.
The MKV format is the box. It’s just a thing the video player knows has a video inside. It includes data in a special structure that works just like the label: that tells your video player what encoding the video uses, its resolution, its bitrate, its framerate, and a ton of other features.
The thing you’re using to rip BluRay discs is probably focusing on very high quality, very fast encoding. That takes up a lot of space. The MKV format can handle it, it just notes on the label the file is using a very high-quality encoding.
When people are making movie rips for torrents, they know people don’t want to download 40GB. So they use fancier encodings and more aggressive settings. Those might not represent a large quality loss, but can take a lot longer for their computers to process. The MKV format notes on the label those settings.
Encodings are pretty complicated, but here’s a good metaphor for why smaller file sizes might not have a noticeably different quality. Imagine if I said I’d give you $100,000 if you could reorganize your room to my standards in 1 hour. That’d be a challenge for most people. You might not get your money. Now imagine if I gave you 48 hours. That’s a lot easier, right? Heck, you’d probably consider driving to a store to buy some bins and other organizational objects since you know you’re going to get a lot of money.
That’s kind of the difference in encoding. Your Blu-Ray ripper wants to be fast, and to be fast it needs to be kind of sloppy with how it puts the movie’s frames together. The torrent people want small file sizes, so they have to give their computer a lot more time to be very meticulous about exactly which pixels need to be saved.
You always can compress a video more by making the quality worse. In general a smart compression system can take a lot away before you notice. If you looked pixel by pixel at the original and the compressed version there would be lots of little “two pixels that were almost the same shade of yellow are now the same shade” in ways that good compression is designed about you not seeing much difference
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