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Has anyone come across list of which file formats generally compress well (in general algorithms)?

Generally, text files .txt compress well, and media-files (.mp3, .avi, etc) do not compress well.

I'm looking for a list (or database) of file formats so I can answer the question, "Will a general, lossless algorithm compress a <.xxx> file?".

EDIT: Of course by nature, random data, media files, and compressed data do not compress well. That is not my question.

Question: Where can I look up descriptions of file-formats, specifically to discover if the format already contains compression?

Examples:
AutoCAD Drawings, Various Microsoft Office formats, Oracle Database .DBF files, Adobe PDF files, Outlook PST files, etc

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Right now, your question is asking for anecdotes, which makes it unfit for Super User. Of the most common compression formats, LZMA generally achieves the best compression, but it's also the slowest algorithm. Pseudo-random and already compressed data cannot be compressed by any of these algorithms. –  Dennis Sep 27 '12 at 17:27
    
in general all data with lots of common patterns compress very well, data with noise does not. files which are compressed already fall into the latter category of files. –  akira Sep 27 '12 at 17:54
    
In general, files that are already compressed to some degree will not compress well. Most standard audio and image formats are compressed. Those that aren't are not generally used for internet operations because they take so much longer to transfer. –  Daniel R Hicks Sep 27 '12 at 17:55
    
See edit to question. –  Steven Sep 27 '12 at 18:37
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closed as not constructive by Dennis, Nifle, KronoS, DragonLord, 8088 Sep 27 '12 at 21:55

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1 Answer

From what I can understand of your question, it sounds like you have tried to compress an already compressed file. File formats such as .png, .jpg, .avi, pdf, .mp4 and many others may already have their data in a compressed format. Attempting to compress such files will not significantly reduce their size.

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See edited question. –  Steven Sep 27 '12 at 18:38
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