whats the deal with iso compression?

first off id like to say i wasnt sure where to place this post, i thought software was slightly more appropriate than anything else.

ive heard on the web and from friends that ISO is an incompressible format, and recently my friend tried to compress an ISO of a wii game and was getting a 100% ratio with 7zip. i know that ISO can be compressed, as ive downloaded plenty of RARed ISOs, my question is why does it seem like some ISOs are incompressible and why are there people who swear up and down you cannot compress it?

as a note, im aware that all data is different and some things will compress much better than others, and also the wiki page about ISO file format says its incompressible too, ive scoured the interwebs for an answer and wound up just asking you guys.

 

I can't tell you from a true technical standpoint but you can only compress data so much. An ISO is just a container for other files that usually also include files to make the disc bootable.

Video files are already highly compressed. So if you copy a DVD to your HD and have a VIDEO_TS folder that is 6.03gb and make an ISO of it, the ISO will be very close to the original size. If you then try to 7zip the ISO it will not be able to be compressed much at all because the original video files were already highly compressed.

Similarly most program/application iso discs are already highly compressed so that they fit on 700mb CDs, so they won't zip up much smaller than the original.

If you had a folder of a thousand jpeg photos (105mb)and made an ISO of that folder the ISO would also be about the original size. If you than 7zipped that ISO you could get a much smaller zip file because jpeg data can still be compressed.

I think rar may be used on iso files for p2p as a way of putting in a hash check to make sure the file is transferred completely. Winrar won't unrar a file that doesn't pass its integrity check.

Hope that helps some.

 

If you have an iso file on your system try compressing it with WinRAR. You can't.

WinRAR treats an iso file as a compressed container and offers to extract the files for you. It does not allow you to try to compress it.

 

I just created an ISO from an a photo album before posting above and then compressed with winrar. The original folder 117mb, the ISO 105mb the RARed ISO 73mb. So some compression is possible. I would guess that even greater compression ratio would be available on an folder of documents or bitmaps.

 

Must depend on what type of file the iso contains then. I tested a VistaPE iso, no compression possible.

 

Yes, program/installer files are usually highly compressed in order that they are smaller files to download or fit on a CD (when you click on an exe installer it usually starts by uncompressing the files to a temp directory before installation). Spanktasktic couldn't compress a Wii game any further but I was just saying that technically you could compress an ISO if the files were of a type that weren't already compressed. It is just that most ISOs are program files and thus already compressed as much as possible.

 

the RARed ISOs i download are smaller in the rar format though, even if they are not split into volumes.