Lossless compression for movie ISO files?

Question

What is the most efficient compression format for ISO files created from DVD movies?

I wouldn't mind compressing them one at a time or a whole group of them, whatever would save me the most space on my storage device.

Solution 1

Don't bother. First of all, the movies are already compressed. (What do you think MPEG-2, h.264, etc. are?) They're not going to compress very much at all. Second of all, even if the data were compressible, the result of them being put into an ISO filesystem makes them much less compressible than they would normally be. That's not to say that ISOs don't compress very well, just normally not as well as the raw data would.

You're talking about tons of overhead to save, maybe, a few MB per ISO.

If you really want to save space, convert the movies to a more space-efficient format, like XviD (MPEG-4 ASP) or preferably h.264 (MPEG-4 AVC).

Solution 2

I'm 100% sure you could, in theory, recompress DVDs in a lossless manner with non-trivial ratio of compression, because MPEG-2 is not really optimal (read: atrocious) and was thought more for speed of decoding than for efficiency, but I can't find anything out there that does it (it would actually be very complicated and I can't see it being neither easy to sell nor terribly fun to code).

Actually, read this reply in a forum from one of the lead x264 devs.

As for practical solutions, you can re-compress it using a lossy format. This assumes you only care about the movie and not about menus or specials (you'd have to encode each extra clip as an extra file). I actually like this and don't care too much for the menus, so this is good enough for me, and you can fit an entire DVD into the size of a CD with unnoticeable difference in quality.

There are many tutorials online about how to do this, and it would be boring to just say the same things, besides they're so easy to find. My tools of choice are MeGUI, Avisynth, x264, Nero's AAC codec and MKVMerge GUI, but anything that you like is good. These tools I mentioned also let you deal with noisy sources, de-interlacing (a whole topic of its own), and various artifacts (dot-crawl, logo removal, blurry images, cartoon or anime restoration, grainy film, etc).

Solution 3

VOB files, which are the bulk of the data on a DVD are already compressed, you will gain very little space by compressing the ISO file.

I did a sample compression of just one VOB file I had,

• Original size: 555 MB (582,238,208 bytes)
• Compressed with WinRar lossless compression 539 MB (565,308,348 bytes)

Roughly a 3% size reduction.

Comments

[TheCompWiz]

640x480 @ 30fps = 27MB/sec 1080p @24 fps (minimum blue-ray 1080p resolution)= 145.8 MB/sec --- With a RAW format for blue-rays at 2 hours... that's > 1TB of raw data.

[afrazier]

There's plenty of lossless video codecs. Linky However, they're generally only used for mastering or archival, not distribution due to the size and storage space required.

[TheCompWiz]

You are right, but in the "home-production" and "general-consumption" arenas, it's highly unlikely you will ever use them. Still "Usagi" had already mentioned that they were dvd-format. I was merely trying to point out that his description of "lossless" was incorrect.

[Camilo Martin]

This answer is not very correct. If the only thing you care for is the movie (not menus, extras, etc.) you can have excellent compression by re-encoding it into another lossy format (DVDs are lossy, even their audio is lossy-compressed).

[Peter Cordes]

IDK why you wouldn't use a lossless format for tmp files while doing home video. You're not going to keep them long, so huge size doesn't matter. In 2015, lossless x264 is a good choice. So is utvideo.

[Peter Cordes]

I'd love to have something to turn a video DVD (ISO or directory) into something with the same menus, but x264 or VP9 video. I sometimes like to have the "extras" dvd, but they use a huge amount of space. Just extracting all the clips would not preserve the information contained in the menus. Also, sometimes different menu entries reference the same actual segment of video.

[Camilo Martin]

@PeterCordes Well, as I said it's some non-trivial work; but I'd also love that possibility. One problem is that there's no standalone format supporting that; MKV could in theory have menu support, but nobody has worked on it and if you take into account that MS has commited to supporting an MKV splitter in Windows 10, I think they also expect the format to be pretty much set in stone. Also note that MKVs support linking (like having consolidated opening/ending MKVs for a whole series in the same folder). Biggest problem is destination format.

[Camilo Martin]

One important thing to note about this "statistics", though; if you recompress a JPEG you'll have negligible gains (or even slight overhead-related loss), but if you extract the DCT tables and recompress that, you can get to even 30% size reduction or more over the original JPEG. What is hard is not compressing the underlying data, but undoing the awfully simple compression used; badly compressed data is hard to compress even if there's little actual data entropy.

[Camilo Martin]

(hard as in nobody cares to do it, but actually simple in terms of most libraries for the hard parts already exist)

[Peter Cordes]

"destination format"? I think mkv is the obvious choice, too, given how open-ended it is. Or did you mean that you'd need to write a bunch of support for it in a player? Yeah of course. You might need a new "codec" id for storing menu links in mkv, if there isn't already something in the standard. Or just do it as auxiliary data, and have build your own format. (you can already mux in the encode log, or a README, if you want.)

[Camilo Martin]

@PeterCordes That's the problem. Since there's no open format that supports subtitles, you'd have to come up with a custom, proprietary one (even if open, or free); and really nobody would use it. Would you, for example, convert all your movies into a format that works but is obscure, nobody uses it, and requires installing software that may not be mantained in the next years? (because players would not support it as-is)

[Peter Cordes]

Uh, you can mux VOBSUB or OCRed text subs in mkv. Did you mean menus? And the obvious way to get anyone to consider using it would be to support xcoding back to DVD Video with menus (ffmpeg has an mpeg2 encoder). That way, you can still use your file at some point in the future when the patch to vlc or xine stops working and you can't directly use your xcoded version.

[Peter Cordes]

Also, I wonder about storing h.264 video in VOB files. Possibly you could just make minor extensions to the DVD-Video standard. I know the MPEG2 transport stream container can hold h.264, since it's used on blu-ray. ffmpeg can already mux a h.264 stream from a mkv into a .vob, but then it can't read the vob.

[Camilo Martin]

@PeterCordes Yes, menus, because that's the only thing missing in open standards. Even if you hack the already-existing formats, you'll need a custom player, and that sucks. But now I wonder, maybe (software) players that would run a normal BD would also run a BD with non-standard, better-compressed x264 streams. Because albeit closed it's a standard that supports menus.

[Peter Cordes]

Oh yeah for BD it's easy, since you could just replace the h.264 streams with more-compressed ones. I was thinking that with DVD, you'd save a LOT of work designing a DVD-Menu-in-mkv standard if you just used h.264-in-VOB as your custom format. And then getting back to something that anything can play is as simple as xcoding back to MPEG2. (with another generation of lossy compression).

[Peter Cordes]

Yes, I'm sure no software BD player defaults to enforcing the BD restrictions on h.264 streams (all the stuff that x264 --bluray-compat sets, like strict B pyramid). Probably some h.264 decoder has extra code to notice and warn about streams that a BD player wouldn't be required to accept. The design mantra for dealing with standard formats is to be liberal in what you accept, and strictly compliant in what you generate.

[Moab]

We are not talking about jpeg compression?