Using compression when making a disk image?

Discussion in 'Mac Basics and Help' started by Wie Gehts, Oct 23, 2009.

  1. Wie Gehts macrumors 6502

    Mar 22, 2007
    If I remember, theres an option for compressing data when you want to make a disk image.

    How does this work? Is it lossless? IOW, is what I'm saving onto a dmg going to be exactly the same when I open it as before it was compressed?

    I want to archive QT movies to backup dvd storage. I'd like to compress the files but I'm not clear on whether or not I'm goin to lose quality by doing that. It'd help cut down on the number of discks I need to use to backup all of them, but if compression sacrifices quality then I won't do that.

    I notice that with a jpeg, for example, will show as, say, 500k on disk...but when you open it up, like in photoshop, it'll say something as 8mb for the file size.
    So where did the pixels go when its stored on disk but then how do the pixels come back when you open the image?

  2. phormality macrumors newbie

    Oct 23, 2009
    It is lossless, it just reduces redundancy in the file for the most part to make it smaller.

    Depending on the video you are trying to compress you may not see much in terms of file size decreases. For example if you are trying to compress an mp3 or divx movie you’ll see the file size remains virtually unchanged since these files are already designed in a way where they are fairly compressed
  3. Wie Gehts thread starter macrumors 6502

    Mar 22, 2007
    ahhh...thanks. so if the stuff was already compressed in the first place there won't be much more gain to be had.

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