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
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.
jpeg/gif/avi/mpeg/mp3 and already compressed files wont change much after compression. You may see a small decrease in filesize.
Media files don't tend to compress well. JPEG and MPEG don't compress while you may be able to compress .png files
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.
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.
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.
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?
You will probably have difficulty compressing encrypted files too as they are essentially random and will (typically) have few repeating blocks.
PDF files are already compressed. They use the following compression algorithms:
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
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.
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.
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.