What manipulations are adversely affected by 8bit images?

Discussion in 'Digital Photography' started by Mueen Nawaz, Oct 9, 2006.

  1. Mueen Nawaz

    Mueen Nawaz Guest


    I use Linux for pretty much everything, and am in the process of
    getting a new camera after all these years. I'll be using RAW fairly
    frequently, and want to ensure I take advantage of the increased bit depth.

    However, the main software I use, Gimp, currently does all its
    operations in 8 bits. Looking around, I often see people saying, "Do the
    relevant operations where 16 bits matter (e.g. Levels adjustment) in
    another program (e.g. Bibble, Cinepaint), and then use GIMP for the rest".

    The question is, "For what digital manipulations does it matter if you
    have 8 or 16 bits?" I know of Levels, and Curves. What else? How would I
    know - is there a rule of thumb to follow?


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    Mueen Nawaz, Oct 9, 2006
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  2. Mueen Nawaz

    Rod Williams Guest

    Photoshop is really missing the boat not having a Linux version.
    To my untrained but fairly critical eye, I can not see much difference
    between 8 and 16 bits, using a Canon i960 printer. I don't use 16 bits.
    It also slows things down a lot with my aging computer.
    Rod Williams, Oct 10, 2006
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  3. Mueen Nawaz

    Mueen Nawaz Guest

    Photoshop is really missing the boat not having a Linux version.
    I think generally, few can tell the difference. That's not the reason
    people use 16 bits. When does some level adjustments - say to stretch
    the histogram to get better contrast, a lot of interpolation goes on,
    and the results aren't as accurate in 8 bit mode. Supposedly, if done in
    16 bits, you get better (and probably more noticeable) results.

    Cartoon Law: Any violent rearrangement of feline matter is impermanent.

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    Mueen Nawaz, Oct 10, 2006
  4. Mueen Nawaz

    Mueen Nawaz Guest

    I know - the problem is that other than levels and curves, I don't know
    what else creates such gaps (I suppose I could just "do" it and compare
    before and after histograms...). Would sharpening do that, for example?

    Laugh alone and the world thinks you're an idiot.

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    Mueen Nawaz, Oct 10, 2006
  5. Mueen Nawaz

    jpc Guest

    Low light shots where you want to 'push process' to compensate for
    several stops of under exposure end up less posterized if you use 16
    bits instead of 8 bits. My rule of thumb would be that any software
    manipulation that makes a large number of gaps in the histogram should
    be done using 16 bits.

    jpc, Oct 10, 2006
  6. SNIP
    With 16-bits/channel, you can avoid cumulating rounding errors at each
    subsequent correction/adjustment you make. Then finally when it comes
    to outputting the image, you'll have less chance on posterization
    artifacts showing up.
    Bart van der Wolf, Oct 10, 2006
  7. Mueen Nawaz

    bugbear Guest

    Indeed - for this reason I requested that gimp have
    an option to

    1) save a level/curve/colour adjustment to disc, as floating point
    2) apply a sequence of saved adjustement in one go.

    The curves and maps could be combined ("composed", technically)
    very efficiently in floating point, producing one "net" mapping,
    which is then applied.

    This operation is quick, and only introduces one stage of rounding,
    thus avoiding the cumulative rounding problem you mention.

    And withou the CPU costs of doing everything 16 bit.

    Gimp have not implemented my suggestion :-(

    bugbear, Oct 10, 2006
  8. Mueen Nawaz

    jpc Guest

    I doubt 16 bit sharpening would be that much different than 8 bit

    jpc, Oct 11, 2006
  9. Mueen Nawaz

    rowan194 Guest

    One thing that doesn't seem to have been mentioned yet - RAW images may
    contain more bits per channel (eg if it's a Canon it would probably be
    12 bits). If you work in 8 bits then you're throwing away a fair amount
    of data right at the very start.

    However, if you can do the majority of your adjustments (contrast etc)
    in your RAW converter then this will not be so much of an issue, as the
    adjustments are performed before sampling down to 8 bits.
    rowan194, Oct 11, 2006
  10. Mueen Nawaz

    Lionel Guest

    In general, the more 8-bit operations you do to your starting image,
    the more damage you'll get, because it's due to clipping & rounding
    errors at each operation, which accumulate with each operation. It
    shows up in the form of posterisation in one or more colour channels,
    most visibly in highlights & shadow areas.

    I run Linux myself for many things, but unfortunately, Gimp just isn't
    capable (yet!) of doing high quality photo work.
    Lionel, Oct 12, 2006
  11. Mueen Nawaz

    Lionel Guest

    Yes, absolutely. Really, almost anything you're likely to want to do
    to a photo (other than clipping) will introduce errors if done in 8
    bits. This is why professional image or cine editing systems all
    operate in 16 or 32 bits per channel, per pixel.
    Lionel, Oct 12, 2006
  12. Mueen Nawaz

    Mueen Nawaz Guest

    So what do you use?

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    Mueen Nawaz, Oct 12, 2006
  13. Mueen Nawaz

    Lionel Guest

    Photoshop, or one of several RAW image processing packages.
    Lionel, Oct 13, 2006
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