Super User is a question and answer site for computer enthusiasts and power users. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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.

share|improve this question
up vote 2 down vote accepted

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).

share|improve this answer
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). – Camilo Martin Jul 29 '14 at 1:14

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.

share|improve this answer
+1 for the statistics – Kenneth L Jul 29 '14 at 1:41
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 Jan 15 '15 at 18:38
(hard as in nobody cares to do it, but actually simple in terms of most libraries for the hard parts already exist) – Camilo Martin Jan 15 '15 at 18:39
We are not talking about jpeg compression? – Moab Jan 24 '15 at 14:26

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).

share|improve this answer
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. – Peter Cordes Jan 15 '15 at 1:44
@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 Jan 15 '15 at 18:17
"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.) – Peter Cordes Jan 16 '15 at 1:07
@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) – Camilo Martin Jan 16 '15 at 2:49
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 Jan 16 '15 at 3:30

First, lossless movie formats don't really exist.

Attempting to capture 640x480 pixels at 24-bit color 30-times every second would result in ~27MB/second of raw video. It is very unrealistic to store that on most forms of portable media (DVD/portable players/etc.), especially when you stop to consider video at hidef resolutions like 1080p.

It only gets exponentially worse. Typically, for most DVD formats you are using an MPEG format (MPEG 2) which has a pretty good quality to loss ratio. It uses similar compression schemes to the JPEG image format.

A lot of new streaming media makes use of other compression schemes (they're still lossy compression) like h.264. Such formats are not typically readable by your home DVD player. They're still not "lossless" compression.

Additionally, moving from one compression scheme to another, will typically result in additional loss in quality.

share|improve this answer
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. – TheCompWiz Apr 15 '11 at 17:19
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. – afrazier Apr 15 '11 at 17:33
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. – TheCompWiz Apr 15 '11 at 18:15
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 Jan 15 '15 at 1:38

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.