Re: RGB vs CMYK: gamut and some important notes for CMYK users

From: Katharine Thayer ^lt;[email protected]>
Date: 12/04/04-07:19:02 PM Z
Message-id: <41B26200.1510@pacifier.com>

Keith,
I'm having some trouble parsing this post. At first blush it kind of
looks in a way like Michael's first post that started this thread.

You're not suggesting that CMY separations output from the RGB file may
have holes in them where there were out of gamut colors that the inkjet
printer can't print because it didn't go through a CMYK profile to get
approximate colors? Separations are greyscale; they go to the printer as
greyscale and the printer does whatever it does with greyscale images.
I've never seen a hole in my RGB separations, and I don't quite see how
this could happen, although of course I am willing to be enlightened.

But you're right, gamut is something to consider. The gamut of pigments
is considerably larger than the gamut of process inks, but not of course
as large as the gamut of RGB. This, again, is an issue where IMO there
is no right answer, whether to try to determine the gamut and clip to
it, or just let the separations run free. Here's the way I look at it:
in printing separations, we don't need to worry about gamut because the
separations are in greyscale. So theoretically we can make separations
that could make all the colors in the RGB gamut if we had pigments that
could combine to create those colors. I myself prefer to give the
pigments the straight CMY separations with all the values that were in
the file, and let the pigments decide whether they can do it or not,
rather than limiting their reach beforehand by making separations that
will only print the range of colors that process inks will print. But
please believe me when I say that I'm not trying to impose that view on
anyone, it's just what *I* think. Okay? Okay.
Katharine

Keith Gerling wrote:
>
> Has anybody brought up the issue of "out of gamut"? Reflecting upon the
> examples provided here, I remembered that I had read that Adobe (and the
> revered Blatner and Fraser) have always recommended that RGB should never be
> used for anything but video monitor output. Digital printers, film
> recorders, and printing inks cannot reproduce the entire RGB spectrum.
> Photoshop's "separation engine" makes every effort to fill in out-of-gamut
> pixels with the closest colors available given the specified ink and
> separation settings. Output in RGB will result in "gamut-clipping": gaps
> in the image that contain will contain no information in a non-video
> output. (However, this may be a non-issue. If, as it was pointed out,
> printers automatically convert to CMYK, are those gaps filled in? I don't
> know, because I don't output to a printer.)
>
> Some relevant observations:
>
> 1) Some people, recognizing that the output will be CMYK make the mistake of
> using CMYK scanner output settings, or immediately convert to CMYK in
> Photoshop. Don't do that. Information that can be crucial will be lost
> immediately. Do all of your contrast correction, etc., in RGB and only
> convert to CMYK only at the time of output. Save your file in RGB.
>
> 2) The "default" CMYK settings, which apparently are so precious to some
> users, are not the best to use for capturing all data. This can be
> demonstrated by performing Jason's experiment using different CMYKs
> settings.
>
> 3) Comparing RGB and CMYK versions of an image is a pointless endeavor,
> because it unfairly hobbles CMYK while making an unreal assumption the RGB
> image can be realized outside of a video space. One conclusion that CAN be
> drawn? Don't use CMYK for video display (but who would do that?)
>
> Keith
>
> -----Original Message-----
> From: Jason DeFontes [mailto:jason@defontes.com]
> Sent: Friday, December 03, 2004 3:33 PM
> To: alt-photo-process-l@sask.usask.ca
> Subject: RE: RGB vs CMYK: some experiments
>
> Katharine,
>
> Those screen shots of the settings dialogs are from Photoshop 7, sorry I
> forgot to mention that. All the settings you describe on your page are
> there, they're just buried in different places that pop-up when you select
> things from the drop-downs.
>
> It's clear to me now that you are right about the CMYK settings affecting
> the results in my RGB channel example. I realized it while I was sitting on
> the metro today. I only did what I did to prove to myself that you could get
> anything at all from RGB separations, and in that regard I am both surprised
> and satisfied. I am in total agreement with you that RGB and CMYK are both
> models that are different ways of describing the same thing, and either is
> capable of producing equivalent results. Had I not done it for myself (in my
> admittedly limited way) I would not have believed it.
>
> My personal preference would be to work in CMYK because that is what "makes
> sense" to me, and I believe the ink controls and other settings in Photoshop
> provide the necessary control to make that workflow, well, work. But I've
> hit the limit of what I can stand to do in the theoretical world, and the
> real test will be making some prints.
>
> By the way, thanks for the level of effort you've made to describe and
> illustrate your process. I've been trying my best to keep up with the
> discussion and catch up on the archives, and it's the conviction with which
> you've described your experience that got me interested in the first place.
>
> -Jason
>
> > -----Original Message-----
> > From: Katharine Thayer [mailto:kthayer@pacifier.com]
> >
> > Hi Jason,
> > Thanks for the great visual, which shows how the default CMYK
> > alters the
> > CMY values from the true values.
> >
> > If you had inverted the RGB channels, turning them into negatives for
> > CMY, and had an expert gum printer print them on gum, I would expect a
> > closer approximation to the original than you got by importing the
> > channels to CMYK, which of course altered the color values to
> > match the
> > SWOP profile, which was my whole point, and thanks for the
> > illustration.
> >
> > It looks like the settings dialogue box has changed since my
> > version of
> > Photoshop (I kept up from versions 1 through 5 and then stopped
> > upgrading) although I did check to make sure that the CMYK default
> > settings haven't changed in the newer versions (they
> > haven't). It's too
> > bad that the new color settings dialogue just names the CMYK profile
> > rather than describing the settings. I go through all the settings on
> > the page that I would guess you didn't take the time to read,
> > since your
> > page seems to show no knowledge of the information contained therein,
> > such as how an RGB file turns CMY when you invert it, and how the
> > default CMYK alters the true CMY values to match a profile that's not
> > terribly relevant to gum printing. I'll give it here again
> > just in case,
> >
> > http://www.pacifier.com/~kthayer/html/CMYK.html
> >
> > Katharine Thayer
Received on Sun Dec 5 03:15:18 2004

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