And why there are some 4-5 GB size blu ray movies available for download? If both the 50GB and 5GB versions are 1080p, what has been lost due to compression? Will there be a huge difference to visual & audio?
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EDIT: Thanks everyone, now I have a new found perspective on quality.
In: Technology
The size of blu-ray movies is driven by the size of the medium. There is no advantage of just using half the space of the disk when it cost just as much to produce a full disk. So the producers make sure to use compression settings which will make the movie take up the entire space of the disk.
Compression settings is quite complex. The number of pixels is just one setting and relatively insignificant to the quality at that. In reality the compression algorithms split the image into blocks and those blocks into smaller blocks and so on. The more bitrate the more details it can use per block making the result more and more like the original image. It is possible to make a low bitrate high resolution video but it will not look as good as the original. So when people compress a 50GB movie into 5GB they are losing a lot of detail. But the compression algorithm is quite good and you just lose the harder to spot details. You might still see lots of compression artifacts in some scenes though but the movie is perfectly watchable for the most part.
The other top comment has done a good job of explaining how two different sized movies can have the same number of pixels in the frame, but not touched on why the publisher might choose to make the movie file smaller.
There are a few different reasons this might be the case:
* If you have a lot of special features (interviews, making of’s, different audio tracks, etc) on the disc in addition to the movie, then the movie itself needs to be smaller. Lots of blu-ray releases have two different cuts of the movie (theatrical and director’s), which basically halves the potential size of each one.
* Depending on the source of the movie itself, the publisher might be limited in terms of what’s even available. For example, if you want to release a newly restored version of a classic movie that you didn’t restore yourself, then you might find that the company that did the restoration has (for whatever reasons) only produced a copy at a certain size/resolution.
* Some publishers are just lazy. You often see complaints about blu-ray releases on specialist sites (such as DVD Beaver) that they didn’t bother to fill up the disc, and that the picture quality suffers as a result.
No doubt there are other reasons, but I suspect these are three of the biggest.
Some time ago I compared Star Trek: Into Darkness (1080p) at 3 different bitrates (sizes).
Here are the screenshots:
[3GB (Low bitrate)](https://i.imgur.com/R58qeRq.png)
[12GB (Medium bitrate)](https://i.imgur.com/9P7Ir8g.png)
[35GB (High bitrate)](https://i.imgur.com/Xw5dSsr.png)
All of these are technically 1080p but the the smallest one looks awful because there’s just not enough data to encode a clean image.
The middle one is pretty close to how streaming services look (nowadays they’re smaller filesizes but use newer more efficient compression algorithms – I took the screenshots at least 5 years ago).
Still the middle one is not as detailed as the biggest one which should be nearly identical to true blu-ray quality. The difference might not be as noticeable on smaller screes, especially with movement, but the bigger the screen the more apparent it will be.
So to answer the question – blu-ray movies are big to give the viewer the highest possible quality, it wouldn’t make sense to make the movie smaller and more compressed if the disk can fit more anyway.
But some (actually most, judging by the supply and demand) people prefer convenience over quality so they go for the awful looking versions just so they can see the movie – so that’s why the lower quality versions exist.
Edit: just a heads up, if you’re trying to compare these on a phone you might not see much of a difference – that was actually a big problem people had last time I posted this comparison.
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