Is 100% JPG, really 100%?

Discussion in 'Digital Photography' started by Bri., Feb 16, 2004.

  1. Bri.

    Bri. Guest

    In other words, will 100% jpg quality compress the data in the way that
    a ".zip" file would, allowing full recovery of the saved file? Or is
    "100%" a misnomer here?

    For example, I open BitMap1.bmp in Irfanview, then save it off as
    Pic.jpg with a quality of 100%.
    Later, I open Pic.jpg and save it as BitMap2.bmp.
    Would the two BitMap files be identical?
    Bri., Feb 16, 2004
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  2. Bri.

    Tom Thackrey Guest

    Tom Thackrey, Feb 16, 2004
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  3. Bri.

    Bri. Guest

    In Tom Thackrey typed...
    Thanks, Tom.
    Bri., Feb 16, 2004
  4. To further make things complicated.

    There do exist a lossless JPEG compression mode,
    but it is not commonly implemented. And it is not
    called 100%.

    To even further complicate things.

    The 100% you are talking about is just something
    that your specific JPEG compressor (in some
    photo editing program I assume) has choosen as
    a name on the best quality they support. Another
    compressor might call them coarse, medium, fine
    and super fine. Or, as Photoshop do, use a 12
    grade scale. Jet another compressor might mean
    something completely different with 100% or
    with another 12 grade scale, or 10 grade scale,
    or ...

    Just confusing.

    Roland Karlsson, Feb 16, 2004
  5. Bri.

    AArDvarK Guest

    Didn't you try it before asking?

    AArDvarK, Feb 16, 2004
  6. It's not a percentage, it's just an arbitrary scale to adjust some
    parameters inside the JPEG encoder. In fact, there are several things
    that can be adjusted, and different programs use different "quality"
    scales to adjust the parameters.

    But no matter what you set the number to, the encoding is not lossless
    (with usual jpeg encoders).

    Dave Martindale, Feb 17, 2004
  7. Bri.

    Bri. Guest

    In Dave Martindale typed...
    Thanks, Dave.
    Now you've pointed it out to me, it's obvious.

    Off topic, your client has picked up my "Reply-To" instead of the usual
    "From" address, from my headers. Please don't take that as a criticism,
    it's just unusual.
    Bri., Feb 17, 2004
  8. Bri.

    Bri. Guest

    In Roland Karlsson typed...
    Thanks for the explanation and the additional information, Roland.
    Bri., Feb 17, 2004
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