问题
I have written a java program for compression. I have compressed some text file. The file size after compression reduced. But when I tried to compress PDF file. I dinot see any change in file size after compression.
So I want to know what other files will not reduce its size after compression.
Thanks Sunil Kumar Sahoo
回答1:
File compression works by removing redundancy. Therefore, files that contain little redundancy compress badly or not at all.
The kind of files with no redundancy that you're most likely to encounter is files that have already been compressed. In the case of PDF, that would specifically be PDFs that consist mainly of images which are themselves in a compressed image format like JPEG.
回答2:
jpeg/gif/avi/mpeg/mp3 and already compressed files wont change much after compression. You may see a small decrease in filesize.
回答3:
Compressed files will not reduce their size after compression.
回答4:
The only files that cannot be compressed are random ones - truly random bits, or as approximated by the output of a compressor.
However, for any algorithm in general, there are many files that cannot be compressed by it but can be compressed well by another algorithm.
回答5:
Five years later, I have at least some real statistics to show of this.
I've generated 17439 multi-page pdf-files with PrinceXML that totals 4858 Mb. A zip -r archive pdf_folder gives me an archive.zip that is 4542 Mb. That's 93.5% of the original size, so not worth it to save space.
回答6:
PDF files are already compressed. They use the following compression algorithms:
- LZW (Lempel-Ziv-Welch)
- FLATE (ZIP, in PDF 1.2)
- JPEG and JPEG2000 (PDF version 1.5 CCITT (the facsimile standard, Group 3 or 4)
- JBIG2 compression (PDF version 1.4) RLE (Run Length Encoding)
Depending on which tool created the PDF and version, different types of encryption are used. You can compress it further using a more efficient algorithm, loose some quality by converting images to low quality jpegs.
There is a great link on this here
http://www.verypdf.com/pdfinfoeditor/compression.htm
回答7:
Files encrypted with a good algorithm like IDEA or DES in CBC mode don't compress anymore regardless of their original content. That's why encryption programs first compress and only then run the encryption.
回答8:
Generally you cannot compress data that has already been compressed. You might even end up with a compressed size that is larger than the input.
回答9:
You will probably have difficulty compressing encrypted files too as they are essentially random and will (typically) have few repeating blocks.
回答10:
Media files don't tend to compress well. JPEG and MPEG don't compress while you may be able to compress .png files
回答11:
File that are already compressed usually can't be compressed any further. For example mp3, jpg, flac, and so on. You could even get files that are bigger because of the re-compressed file header.
回答12:
Really, it all depends on the algorithm that is used. An algorithm that is specifically tailored to use the frequency of letters found in common English words will do fairly poorly when the input file does not match that assumption.
In general, PDFs contain images and such that are already compressed, so it will not compress much further. Your algorithm is probably only able to eke out meagre if any savings based on the text strings contained in the PDF?
回答13:
Simple answer: compressed files (or we could reduce file sizes to 0 by compressing multiple times :). Many file formats already apply compression and you might find that the file size shrinks by less then 1% when compressing movies, mp3s, jpegs, etc.
回答14:
You can add all Office 2007 file formats to the list (of @waqasahmed):
Since the Office 2007 .docx and .xlsx (etc) are actually zipped .xml files, you also might not see a lot of size reduction in them either.
回答15:
Truly random
Approximation thereof, made by cryptographically strong hash function or cipher, e.g.:
AES-CBC(any input)
"".join(map(b2a_hex, [md5(str(i)) for i in range(...)]))
回答16:
Any lossless compression algorithm, provided it makes some inputs smaller (as the name compression suggests), will also make some other inputs larger.
Otherwise, the set of all input sequences up to a given length L could be mapped to the (much) smaller set of all sequences of length less than L, and do so without collisions (because the compression must be lossless and reversible), which possibility the pigeonhole principle excludes.
So, there are infinite files which do NOT reduce its size after compression and, moreover, it's not required for a file to be an high entropy file :)
来源：https://stackoverflow.com/questions/1136268/which-files-does-not-reduce-its-size-after-compression