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

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 Sat Dec 4 12:13:17 2004