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It wouldn't be so bad if not for the fact that the game data itself is often times only a fraction of the actual disc size - for instance, the Super Mario 25th Anniversary Wii disc itself is a 4.7GB, when really the actual game data is only a single SNES ROM (12 MB of useful data, to be precise) and nothing else. Naturally, one would want to trim this extra "fat" as much as possible, which is what this page aims to help to achieve. Most of the information here is based partially on this guide.
There are many ways, some methods alter the data forever while others can be converted back and forth with generally no loss. Some conversions are only playable on specific emulators and may not work on real hardware depending on the console and the method used. It's important to take all this into consideration before attempting as most of these are console-specific.
Archive-quality dumps are ones that when converted back to its original state, will have the same checksum as the official uncompressed release. Compressions that can't be reversed, or those that can be but will have missing or altered content whether it interferes with functionality (rebuilt table of content) or not, are not archive-quality. For example, the WBFS format, used for shrinking Wii discs, is not archive-quality since it may be missing padding content and upgrade partitions (which have their uses in 3DS/Wii modding) compared to an intact, uncompressed dump.
PCs, Sega-CD, PC-Engine, PlayStation, Sega Saturn... what do these systems all have in common? They all use a regular CD format! Game developers often stored music and other sounds using the Audio-CD format, but it was terribly inefficient when it comes to disc storage as it also had to store the actual game along with the sound files (commonly known as a "mixed CD"), and these sound files are never compressed because the Audio-CD standard simply doesn't allow for sound compression. To put it in perspective, a 700 MB CD containing nothing but Audio-CD data can hold at most around 80 minutes worth of sound data, meaning games that used a lot of sounds were limited in size.
Developers have long since stopped using the Audio-CD format, and instead prefer custom audio formats that come included in the "game data" part of the disc. By the launch of the fifth generation CD-based consoles, i.e. the PS1 & the Saturn, it was more common than not to just use the Audio-CD part for messages like "Don't put this in a CD player!" and little else. That being said, there were still quite a few fifth-gen games that used Audio-CD data for their soundtracks, such as Vib-Ribbon and the Wipeout trilogy on the PS1, and Battle Garegga and Daytona USA on the Saturn.
Devs often have their games much, much bigger than they need to be by putting in accessible garbage data in the disc. Garbage data isn't useful game data and is just used to bloat the disc size. It's either a sequence of 00/FF (you know what's inside a file if you open it with a hex editor), random data, unused/cut content left during development, and in rare cases, data that is completely unrelated to the game itself. An example of this is Shrek SuperSlam on the PS2 which has a working copy of Tony Hawk's Underground 2 for the PSP hidden in the root of the disc.
The purpose of doing this can be to fill in spots on the disc so that specific parts of the game's data are in certain areas of the disc. This is done to increase the drive's reading speed so that it's quick enough in certain spots for the game to work properly. It's in the your best interest not to mess with this data arrangement (referred to as LBA and TOC in the case of GC/Wii/PS2/PSP) as it might break the game in some cases.
Another reason for having garbage data can be to screw with pirates, who download/upload these games online, by making the ISO bigger and harder to store. Some go a step further and scramble the garbage data, instead of just being a sequence of 00/FF, to make the ISO much harder to compress using regular archive formats like zip, 7zip, rar, etc. You might be overjoyed to learn this has become the industry standard nowadays.
MAME uses the CHD format for disc images in general and includes tools to convert back and forth. Before MAME v145, CHD was in version 4 and it bumped to version 5 from MAME v146 and further. The CHDv5 uses 7zip's LZMA compression on the game data and lossless FLAC compression for the audio data to optimize compression even further than using BIN+CUE+MP3/WAV data separation alone. CHDv5 is lossy until all the bugs are resolved.
InstructionsPlace the chdman executable and extractcd in the same directory as the dumps you want to compress (dumps must be in BIN+CUE format or GDI). Open Command Prompt or a terminal emulator and navigate to the directory where you placed the chdman executable and input one of the following:
If you have one of the European PSX games that features LibCrypt copy protection, then you will have a .sbi file in addition to the .bin/cue file. You will still need to have the .sbi file in the same directory as the game file (in this case, the newly created CHD file) in order to run.
PBP is the official format used by Sony for the PS1 Classics on PSP and PS3. Audio tracks may be compressed in ATRAC3 or as raw PCM (unsupported on PS3's ps1_netemu, supported on PSP only by the earliest versions of POPS or the newest ones with the cdda_enabler plugin)
Often times you can just open the ISO in UltraISO and find the dummy files. Sometimes they are obvious looking files, like DUMMY.BIN, DATA0.BIN (or .DAT), DUMMY.DAT, etc., or are folders with names like "PADDING" and such. You can look into the files (with a hex editor) to see if they're obviously padding data (usually the offsets will be full of 00/FF, though sometimes it's not as obvious). However, you must never mess with LBA and TOC when removing padding.
Use 7zip ("Add to archive..." then choose to gzip) or Pigz (multi-threaded, much faster compression) to generate gzip archives containing the ISO file in question. PCSX2 will build an index of each gzip compressed game it loads (as a file in the same directory as the gzip archive), so after the first time where you'll have to wait for the decompression, in all subsequent times there is no speed difference between playing an uncompressed and compressed game. Of course, you can extract the ISO back from the GZIP archive.
For a long time (before the availability of Cobra and Mamba) the only playable format for PS3 backups (relying on little more than "peek and poke" CFW-provided syscalls to mount the game's folder over the currently inserted disc) but also the least accurate one, with significant compatibility issues (varying on whether the backup is stored on an internal or external drive, whether an original game disc is currently inserted, various optional hacks such as "BDMirror" moving the files to the root of the external drive on demand, ...)
NoNpDRM is a Vita plugin that generates decrypted licenses (valid for any console) when running an original Vita title, as well as allowing the system to accept those licenses, allowing for playing the encrypted files directly copied from a game card or memory card (by extension the name is therefore also used for such files, or the combination of the game's files and a decrypted license).
People used to resort to WiiScrubber (Wii) and GCM Utility (GC) to scrub/trim games to end up with dumps that while they had no immediate size change, their randomized garbage data (like "dummy", "padding" or "znull") was still there but zeroed out making archived dumps using zip/7zip/rar formats have stunning gains (from 1.4GB uncompressed to 26MB zipped for Animal Crossing for example!). Of course, the file still needed to be uncompressed to its full size everytime you wanted to play it.
Trimming and scrubbing (in Wiiscrubber terms) aren't the same! While they both are terms for "zeroing garbage data" to make it more compression-friendly, trimming does not just that like scrubbing but takes the extra step of relocating the garbage data to the end of the file, hence altering its TOC and requiring the disc to be fakesigned, for a not-so-big compression gain. Hence why scrubbing is by far the most authentic and safe way to solve the garbage data problem.
The Dolphin team developed a new compression format based on WIA called RVZ. Usage is very similar to GCZ in Dolphin itself, but it only works on newer Dolphin versions. It can efficiently compress both the actual data and junk data, without altering the file/directory structure.
Scrubbing games zeros out garbage/dummy data in the ISO file. The resulting file will be the same size, but it will be able to compress better than unmodified ISOs. The difference can be huge depending on the game. Animal Crossing, for instance, will compress to just a 26 MB .gcz file after being scrubbed!
Trimming (also known as "trucha scrubbing", and substantially different from the definition of "trimming" used in ROM-based systems) games will also defragment the files moving them towards the start of the disc image, resulting in a smaller ISO file. The trimmed file can be used immediately at a smaller size without compression, but the game's internal structure will be wholly modified. Games relying on direct sector access (as opposed to consulting the filesystem) will break if you trim them.
A scrubbed and compressed format designed for maximum space savings without cutting corners on corruption detection, but it's not directly editable. Never really caught on and may be considered de facto deprecated by NKit.
Apart from its previously mentioned unscrubbing/untrimming features mentioned in the above formats' descriptions, NKit is able to convert any ISO (clean dump or otherwise) to and from an intermediate format applying various reversible changes that optimize the image for lossless compression, including full decryption and optionally splitting the often non-unique update partitions to separate files.
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