Katharine Thayer (kthayer@pacifier.com)
Sun, 24 Jan 1999 22:20:38 +0000
FotoDave@aol.com wrote:
>
> In a message dated 1/22/99, PM 07:01:59, kthayer@pacifier.com writes:
> <<It makes perfect sense to me to take the basic steps of this model to
> the digital world, staying in RGB space and forming CMY negatives by
> inverting the RGB layers (Remember, RGB inverted=CMY) and then using
> those negatives to gum print using CMY pigments.
>
> Katherine,
>
> You are an experienced printer using CMY, so I don't understand why you said
> RGB inverted = CMY.
Well, because RGB inverted=CMY, that's why. That's what I learned in
color photography 101. Yes, I do use CMY, but I don't use the CMYK
color space, which as I keep saying, is not the same animal but
something different altogether, developed for the printing industry.
That is not true in traditional film / paper (that is why
> we have the integral (orange) masking of negative film instead of straight
> inversion.
I did say I was being overly simplistic, I believe, in laying out the
rules of color theory.
Neither is it true in digital (that is why RGB of (128,128,128)
> would not simply invert to (128,128,128,0) of CMYK. You even talked about the
> numbers in your earlier post, so why are you saying that RGB inverted = CMY???
Well, ummm, I guess I'm saying that because it happens to be true. I
have *never* said that RGB of 128,128,128 would translate to CMYK of
128, 128, 128, 0. In fact my argument all along has been that the
conversion from RGB to CMYK is not a straightforward conversion, and
that's why I would never use the CMYK color space unless I developed an
ICC profile myself for gum printing. But if you invert an RGB of 128,
128, 128, you *will* get a CMY of 128, 128, 128. Which is my whole
point.
Look, I didn't just make this up myself. This comes out of everything
I've learned about color printing both for traditional color photography
and for gum printing. If you don't believe me, there was an excellent
article on making digital separations for color gum printing in Photo
Techniques several years ago, 1995 or 1996, and I don't remember the
guy's name, but he does his color separations exactly the way I do,
working in RGB and inverting the channels to create CMY negatives. And I
don't remember anyone writing in later to say the guy was a fool.
Besides the fact that it's the way I learned it, it has the added appeal
for me of making sense, which CMYK doesn't, and it also makes prints
that look right to me, which CMYK doesn't. But as I keep saying, each to
his own!
>
The reason I talked about
> light vs print (additive vs subtrative) was because someone asked about why
> use CMY instead of RGB.
.
Actually, his question was:
>> I've been wondering why I here so much about ***CMYK*** gum and so >>little about RGB.
> But again, Katherine, the person was NOT asking why someone
> should or should not use K. He asked something else, and so the answer and/or
> discussion would be different
You answered the question you thought he asked, and I answered the
question I thought he asked. It's for him, not for one of us, to make
the final determination of what question he was asking.
Why do you always link everything that has the
> letters CMYK to the question of whether K is to be used.
Goodness. Since I never once mentioned black in the post that seems to
have got your shorts all in a wad, I don't quite know how to respond to
that. But be that as it may, the fact remains that CMYK *is* about
black. The whole model was developed to deal with the fact that in the
commercial printing process, black ink needs to be added to the three
colors because of the properties of printing inks, and the other colors
need to be backed off to compensate, and so each CMYK profile is
developed to add black and adjust the other colors accordingly,
depending on the specs of each combination of printing press, ink, and
so forth.
Katharine Thayer
P.S. I said once, back a month or two, that we could argue about this
forever and nothing would ever be gained by it; I think that may be the
most profound statement on the subject.
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