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03.21.2008 at 07:02PM PDT, ID: 23260989

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9.0

Why can you not compress compressed files?

Zone: File Compression Utilities

Tags: compression, zip

Why is there a limit to compressed files?

If I ZIP a file, and I ZIP it again, it doesn't get smaller... why not?

If I use different compression methods, shouldn't it still get smaller?

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Question Stats

Zone: Software

Question Asked By: rcoast

Solution Provided By: MrHusy

Participating Experts: 4

Solution Grade: A

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03.21.2008 at 07:33PM PDT, ID: 21184528

MrHusy:

Hi rcoast
To get the answer, you should first have a basic understanding of compression
http://www.2brightsparks.com/resources/articles/understanding-compression.html
And here are file type based compression percentages
http://www.bleepingcomputer.com/tutorials/tutorial107.html

Regards

03.21.2008 at 07:36PM PDT, ID: 21184536

rcoast:

Thanks... but this information doesn't answer my question.

I understand how compression schemes work independently, but why can't I apply them over and over again, to the same file?

03.21.2008 at 07:58PM PDT, ID: 21184593

windows_eXPert:

Compression works because it finds repeating sequences of data and reduces them. So, for a simplistic explanation, if I have the data 0000000100000, I could represent it as 715 (7 zeros, a 1, and 5 more zeros)

After compression, the algorithm has gotten rid of all the patterns. The same algorithm won't find anything to simplify, hence no space savings.

In theory, you could RAR-compress a ZIP file, but the compressed file is much closer to randomness (a lack of patterns) than a non-compressed file.

In short, don't bother. Find a good compression algorithm (GZIP, ZIP, RAR are pretty good), set it to the highest level compression, and be done with it.

03.21.2008 at 08:01PM PDT, ID: 21184597

rcoast:

I understand what you are saying... but there are always patterns...

For instance,

If you have your compressed data: 7' 0s and 1' 5... as in your example... that is a pattern. Why can't another ZIP tool recognize this and compress it?

03.21.2008 at 08:05PM PDT, ID: 21184607

Delphineous:

Compression tools may use various techniques to reduce the size of files, but ultimately they are only removing unneeded space/contents. Once these files are reduced to their smallest size, it simply isn't possible to reduce them to a smaller size simply by the fact that the compression software has already done this. There is nothing left to remove without damaging the contents.

03.21.2008 at 08:07PM PDT, ID: 21184608

MrHusy:

Well it actually does. Let me explain according to the example in above article.
Lets say that you have a text file containing the following text

"One swallow does not make a summer, neither does one fine day; similarly one day or brief time of happiness does not make a person entirely happy"

Each character consumes a space of 1 byte. And there are 146 characters above, therefore the file is a total of 146 bytes.

As you can see, there are words repeated like "one" (repeated for 3 times), "does" (repeated for 3 times), "not" (2 times), "day" (2 times).
Compressing creates a chart and assign these words to characters that consumes less space (while "does" consumes 4 bytes, compressing assigns this word to "1" for example. When it sees "1"
while decompressing, it reads the chart and knows that it is in fact the word "does")
Lets assign the words to chars
1=one
2=does
3=not
4=make
5=day

After the compression, text looks like following
"1 swallow 2 3 4 a summer, neither 2 1 fine 5; similarly 1 5 or brief time of happiness 2 3 4 a person entirely happy"

We are ok untill now. Lets take that compressed file and try to compress it again. Now tell me can you see any repetetive words to assign to a char? For example the "2" which is the product of previous compression is a 1 byte char and used 3 times. Can you assign this to soething which consumes less space? NO because byte is the least usable thing. Thats why you can not compress it again.

Accepted Solution

03.21.2008 at 08:11PM PDT, ID: 21184620

Delphineous:

When a file is compressed the patterns are removed, then the compression method adds information to the file to allow decompression software to understand what is necessary to return the file to its original state.

Files are not like air where you can first compress 10 psi, then compress again to 20 psi, then to 30 or 40 psi - as long as the compressor and container can handle it, you can keep compressing. Files are more like water; compression is limited.

03.21.2008 at 08:13PM PDT, ID: 21184627

Delphineous:

Excellent explanation MrHusy!

03.21.2008 at 08:16PM PDT, ID: 21184636

windows_eXPert:

Exactly what I was trying to say. We're fast approaching data theory here, but I'll continue for a second.

Let's say I look for long sequences of data and replace them. This is what normal compression does, and they tend to look for lots of unused space, padding (empty data in a file can be very well compressed), and raw text (which compresses very nicely).

Now, I spit out my compressed format. This is a lot more random and a lot less ordered and predictable than the original. It contains, basically, notations of the sequences that were removed during the compression, interspersed with data. If I were to try and compress that, there'd be a lot less order and a lot less sequences to abstract away.

So, you say, make longer and more random sequences, and try and recompress using those.

Unfortunately, you eventually take more space to identify what you removed than you are removing.

Modern compression algorithms are very good, and have evolved for nearly twenty years. You're going to save a fraction of the total file size if you compress a ZIP file using, say, WinRAR.

The theory is interesting, but in practice it's not helpful. You may save a few kilobytes off a 200 megabyte file, but that's not worth the significant time it takes.

P.S. This is why video and music files don't compress well (that is, using a general purpose compression like ZIP) - they're already compressed and have no (or few) patterns.

Assisted Solution

03.21.2008 at 08:19PM PDT, ID: 21184639

windows_eXPert:

I concur - I wrote that novel you see above in the interval... marvelous explanation MrHusy. Exactly what I was trying to say. Couldn't do better (or comparably) myself.

03.21.2008 at 08:22PM PDT, ID: 21184645

MrHusy:

Thanks m8s :)

03.23.2008 at 03:58AM PDT, ID: 21189206

Barthax:

The above excellently covers the algorithm methods but doesn't cover the mathematical impossibility of decompression. Assume that you have finally managed to figure out how to compress something down to just a single byte: the decompression algorithm then only has 256 possible ways to de-compression that byte. Any one of those 256 possibilities only has one method of de-compression or is the extracted file. It is possible to proove from this basis that most files cannot be compressed. Thankfully, as humans, we don't deal in the kind of random data required to find this out on a daily basis & the very small subset of data we do use has significant redundancy.

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