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62696Re: [ccd-newastro] Full Write-up on Embedding the ICC Color Profile into our Jpgs

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  • Paul Beskeen
    Jul 2, 2008
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      Hi Neil,

      Neil Fleming wrote:
      > --- Paul Beskeen <yahoo@...> wrote:
      >> Hi Neil,
      >>
      >> 1) I don't understand what benefit you would get if
      >> you continue to save
      >> in sRGB colour space (as suggested by your workflow)
      >> but with "color
      >> profile" ticked. The default for all browsers is
      >> sRGB.
      >
      > Think of it this way. The sRGB color space represents
      > the range of values that you *can* work with. Like
      > the range of choices on a bingo card.
      >
      > Unless you are viewing an image with the color profile
      > embedded, *and* your program will accommodate that
      > mapping, you will see "uncontrolled" results rather
      > than that the author intended. "B5" on your bingo
      > card could be "B7" on another's.
      >
      > Just take a look at the ICC example in the original
      > e-mail. The authors picked (on purpose) an extreme
      > example, but it illustrates the point well.

      I don't mean to be argumentative here, and I'm certainly no expert in
      colour management, but I guess it comes down to the question "do ICC
      aware applications by default gamut map to sRGB whether or not the image
      is tagged as sRGB?". [The default for browsers, monitors, HDTV's, etc is
      to use sRGB colour space. Most don't succeed very well so calibration is
      advisable for accurate display]

      If the person viewing your image has profiled their monitor, then the
      graphic card LUT tables will be set at startup to adjust to the correct
      gamma and white point. If a program is ICC enabled then in addition
      gamut mapping will be used to render the image's colours correctly based
      on its tagged colour profile, or (and this may be were we differ) if no
      profile information exists in the image, as sRGB.

      Ahh, this "Web Browser Color Management Tutorial" page is *very*
      informative:
      http://www.gballard.net/psd/go_live_page_profile/embeddedJPEGprofiles.html#

      In amongst all the other goodies it has a roll over graphic of an sRGB
      tagged and untagged image. I can discern no difference between the two -
      so Firefox3 defaults to assuming an sRGB profile. However... when I try
      the same with Safari I can see a subtle difference! So Safari doesn't do
      sRGB gamut mapping unless it is tagged as sRGB! From comments on the
      page it appears that Vista's new colour management system also
      automatically defaults to sRGB if an image is not tagged. Given that
      sRGB is the standard for the web I'm surprised that Safari doesn't sRGB
      map unless the image is tagged as such.

      Bottom line: Tagging as sRGB certainly does no harm and in some
      circumstances can lead to a more accurate rendering. Your advice is
      completely correct :)

      >> 2) If you save in an alternative colour space - such
      >> as Adobe RGB, then
      >> won't people without ICC enabled browsers see some
      >> very strange effects?
      >
      > I wouldn't necessarily call it "extreme", but there
      > would be minor differences. Adobe RGB is better
      > adapted to printing, with its wider gamut than sRGB.
      > Some of the colors would be shifted to something you
      > can see. What you see on your monitor would not what
      > would be printed, unless you have a high-end monitor
      > like an Eizo, one that is capable of displaying the
      > full Adobe 1998 gamut.

      Tagging images for web display with anything other than sRGB should
      definitely be avoided. The vast majority of web browsers are not colour
      managed & displays not profiled - an image that uses AdobeRGB or any
      profile other than sRGB will definitely render incorrectly.

      To demonstrate this issue go to the section "ADOBE RGB (1998) 2.2
      gamma", and try the rollover graphic.
      http://www.gballard.net/psd/go_live_page_profile/embeddedJPEGprofiles.html#

      Cheers, Paul.

      > ...Neil
      >
      > www.flemingastrophotography.com
      > Direct from Boston - brilliant diamonds in pea soup
      > Also check out the astro_narrowband Yahoo group!
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