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Re: [PanoToolsNG] OT: large files, RAM, 16bit and 8bit

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  • Jim Watters
    ... Internally Photoshop uses a 15bit+1 (0 to 32768), 32769 discrete values per channel system. 15bit+1 was used over 16 bit for the purpose of increased
    Message 1 of 11 , Jan 4, 2007
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      Chris Thomas wrote:
      > Carel.
      >
      > I don't save 16 bit Master files out of Photoshop because.
      > I've been told; someone correct me if I'm wrong, but "16 bit" PSD
      > files are really only 15 bit, with the lost bit sacrificed for file
      > size. 16 bit Tiffs are ok.
      >
      Internally Photoshop uses a 15bit+1 (0 to 32768), 32769 discrete values
      per channel system.
      15bit+1 was used over 16 bit for the purpose of increased speed.
      8 bit (0 to 255) has 256 discrete values per channel.

      Files saved by Photoshop are full 16bit.


      --
      Jim Watters

      jwatters @ photocreations . ca
      http://photocreations.ca
    • Rick Drew
      Thanks - I always wondered about that - I had that happen in a really nice blue sky and spent a lot of time cleaning it up. I never thought about it being the
      Message 2 of 11 , Jan 4, 2007
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        Thanks - I always wondered about that - I had that happen in a really nice
        blue sky and spent a lot of time cleaning it up. I never thought about it
        being the being the bit depth, especially since the original was 8 bits!

        Rick

        Adjustments to images containing large flat areas, blue sky, large walls in
        same color etc, will
        easy get banding if you do it in 8 bit.
        This may be visible first after you compress the final image/movie

        Hans



        [Non-text portions of this message have been removed]
      • Roger Howard
        ... There are two factors that Hans mentioned, and both can have major impacts on image quality. Bit depth - it s highly dependent on subject matter *and* the
        Message 3 of 11 , Jan 4, 2007
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          On Thu, January 4, 2007 12:37 pm, Rick Drew wrote:
          > Thanks - I always wondered about that - I had that happen in a really nice
          > blue sky and spent a lot of time cleaning it up. I never thought about it
          > being the being the bit depth, especially since the original was 8 bits!

          There are two factors that Hans mentioned, and both can have major impacts
          on image quality.

          Bit depth - it's highly dependent on subject matter *and* the extent of
          the edits you perform. For properly-exposed material it's quite possible
          to get great results working only in 8bits per channel. There's also a
          tradeoff here with color space gamut - the wider the gamut your color
          space, the more likely you are to need to work in 16 bits per channel. So
          someone working only in sRGB may do fine (with somewhat reduced gamut) and
          not see much if any banding working in 8bits, but the same image
          originating in ProPhoto or another wide-gamut color space may well cause
          banding much more easily. Banding and other negative effects of limited
          bit depth are easiest seen in areas of subtle gradations as Hans says.

          Compression - this is a toughy. if you're outputting to a compressed
          format like JPEG, in all likelihood you're outputting 8bits per channel at
          the most to begin with. But the real problem is that the nature of lossy
          compression will often make it hard to preserve things like subtle
          gradations - skies, walls, etc - without them chunking up into visible
          artifacts. This is less related to bit depth than compression strategies
          though, and there are ways to minimize it. For instance, adding a bit of
          noise in a nice sky or other gradation will actually *help* in many cases.
          Note that, inherently, minimizing these artifacts typically means
          increasing your target file size. There's no such thing as a free lunch!
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