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How is the compression ratio set when creating archives in File Roller? If an archive of JPEG images is compressed, can it be un-compressed so all the color information of ...
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  1. #1
    Just Joined! bezdomny's Avatar
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    [SOLVED] File Roller: how to set compression ratio? How to preserve all color info


    How is the compression ratio set when creating archives in File Roller? If an archive of JPEG images is compressed, can it be un-compressed so all the color information of the original image is intact?

    I have occasion to send sets of image files, usually JPEG format, to Windows users. They have the native ability to manipulate ".zip" archives, so that is my preferred archive format. However, File Roller's default compression ratio when creating ".zip" archives is 1:1 and I can't figure out how to increase it. At that rate, I may as well just attach individual photos to e-mails, or, in the case of a set of many images (often the case), go through the hassle and expense of burning the images to a CD and send them by surface mail.

    I don't want to compress or reduce the resolution of the JPEG's-- I want my correspondents to receive high-resolution images with the broadest color-range possible, best-suited for printing. Otherwise, I could just optimize the JPEG's for web publication and upload them to my MySpace photo album. But that isn't the best quality for printing, which usually is my goal.

    Any help increasing the compression ratio in ".zip" format with File Roller would be appreciated, and any advice as to whether all the JPEG color information can be recovered when the archive is un-compressed, would be appreciated. By the way, this is version #2.22.4 of File Roller.

    Many thanks and best regards to all.

  2. #2
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    There are "lossy" compression algorithms and there are "lossless" algorithms. ZIP is lossless - what you put in is the same as what you get out. MP3 would be an example of lossy compression - once it's compressed, you *don't* get back all of the same data that was in the original file.

    JPEG is a *compressed* file format. So what you are saying is that ZIP won't compress a *compressed* file even further. No surprise there. You will see the same result with binary, MPEG, MP3, etc. files.

    Can you get file formats that are already compressed to squeeze further? Yes, sometimes - but only from a specialized algorithm targeted at that file type. Because ZIP is a general-purpose compression tool, you will likely not see much more compression of a JPEG file. Have you "zipped" these files using something like Winzip and viewed the results for comparison? It's unlikely you will see a large difference in size between the ZIP and the sum of the files individually.

    You may want to put the zip file in a location and provide an HTTP or FTP link for them to be downloaded.

    However, File Roller's default compression ratio when creating ".zip" archives is 1:1 and I can't figure out how to increase it.
    No, the algorithm is simply unable to compress the data further and the resulting ZIP file is the same size as the sum of the files it contains. You can "zip" a large text file and see that the result is much smaller...because a text file has large amounts of repetitive data.

  3. #3
    Just Joined! bezdomny's Avatar
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    *solved* how to set compression ratio? How to preserve all color info in JPEGs?

    Dear HROAdmin26, thank you very much for clarifying my basic misconceptions about JPEG and compression. I appreciate your taking the time to educate me.

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