Re: Gum Tri-Color Yellow

Keith Gerling wrote:
>
> Kate,
>
> I routinely use RGB, CMYK, and RGBK. I actually think RGB and RGBK (the K
> in this case is a skeletal black only) provides more of a success ratio,
> i.e., the color balance in the print is more "true", and requires less
> additional-coat-tweaking than CMYK. But it is difficult for me to look at a
> set of RGB negs and visualize what the resultant print will be, whereas a
> CMYK seems more "natural". So I tend to lean more toward CMYK.

Interesting.
>
> But, and this is a HUGE BUT - all talk of CMYK as it relates to making gum
> prints is totally useless unless you consider the color space used. Your
> talk of contrast as it relates to the "K" channel is impossible to address
> without knowing more. In Photoshop, for instance, the variations are
> extreme between UCR and GCR. Within either of these, you have an seemingly
> infinite number of variations. Dot gain is somewhat important, but
> especially critical are the Black Generation setting under GCR and the ink
> percentages. These settings are critical! I change them all around all of
> the time in order to get the effect I want. Perfectly color-true prints can
> be had with any of them, but the path to get there will be quite different.
>
> Keith

Keith is right. How much changing RGB to CMYK alters the color
information in a file depends to a large extent, as Keith said, on
whether the separations are GCR or UCR. It's important to be aware that
Photoshop's default is GCR, which alters the color information much more
than UCR.

I chose a pixel in an RGB file and watched the info numbers when
converting to CMYK using both algorithms. Under GCR a pixel that reads
R=110, G=81, B=69, 65% density, when converted to CMYK became 40% C,
56%M, 57%Y, and 40% K, 65% density. The same pixel, starting with the
same RGB values, converted to CMYK under OCR, became 58% C, 69% M, 73%Y,
and 18% K, and 66% density.

I've always done my color prints using the RGB file. I get excellent
color balance using this method, and see no reason to change.
Katharine

>
> -----Original Message-----
> From: Kate M [mailto:kateb@paradise.net.nz]
> Sent: Wednesday, July 07, 2004 8:53 PM
> To: alt-photo-process-l@sask.usask.ca
> Subject: RE: Gum Tri-Color Yellow
>
> I stand corrected for sending such a muddled message :$
>
> What I really meant to say was if you print CMY pigments with RGB negs,
> in other words, C with the R channel neg, M with the G and Y with the B,
> is the result (in the opinion of others) better, worse or equal to
> printing the CMYK set of negs (all three colours plus black)? I do this
> regularly and it works for colour balance and appropriate tone, although
> I'm not happy with the overall contrast. I'm not sure if this is due to
> my curves not being right or my printing processes.
>
> Supposedly ( and this is what I've read) using the RGB set of negatives
> means that the contrast appropriate to the image is carried in the
> colour layers, and when you print with CMYK the contrast is mostly
> carried in the K (Key) layer. So you shouldn't print with CMY negs
> without the K. Or am I just completely wrong??????
>
> -----Original Message-----
> From: wrleigh@att.net [mailto:wrleigh@att.net]
> Sent: Thursday, 8 July 2004 8:46 a.m.
> To: alt-photo-process-l@sask.usask.ca
> Subject: RE: Gum Tri-Color Yellow
>
> A minor terminology point: RGB is used for additive coloring and CMY[K}
> is used for subtractive. Monitors are RGB, but printed matter is CMY[K}.
> Printed things look black becasue the printing primaries of CMY
> "subtract" colors of the other frequencies from the reflected light by
> absorbing the other colors. When you see red printed matter, you are
> seeing what is left over after the printed matter has absorbed the other
> frequencies. If you are doing tri-color printing, you are using CMY, and
> CMYK is CMY with a optional K to give a true black over the combination
> of CMY.
>
> --
> Bill Leigh
> wrleigh@att.net
>
> >
> > How many of you gum printers use four separations? I have read that
> the
> > K carries much of the contrast and therefore screws up colour balance.
> I
> > was just wondering who prefers RGB, who prints with CMYK??????
> >
> > Kate
> >
> > -----Original Message-----
> > From: Katharine Thayer [mailto:kthayer@pacifier.com]
> > Sent: Monday, 5 July 2004 1:25 a.m.
> > To: alt-photo-process-l@sask.usask.ca
> > Subject: Re: Gum Tri-Color Yellow
> >
> > Katharine Thayer wrote:
> > >
> > > Ender100@aol.com wrote:
> > > >
> > > > I wonder if the muddy brown is an issue not so much of the colors
> > used, but perhaps poor color seperation in the negatives, or too fine
> a
> > screen in the seperate negatives. Larger dots of each color might give
> > better color rendition, just as it does with inkjet printing and matte
> > papers.
> > > >
> > > > Mark Nelson
> > >
> > > Not likely in my opinion. I haven't printed tricolor from continuous
> > > tone negatives, although Dave has (sorry, I can't think of Dave's
> last
> > > name at the moment, but you know, Cowboy Dave) quite successfully; I
> > > don't remember seeing any brown tones in his tricolor gums. I've
> > > printed tricolors from many different types and resolutions of
> digital
> > > negatives, and I've never seen this brown tone in my own tricolor
> work
> > > from beginning to end. So I'm inclined to say it has little to do
> with
> > > the negative and much to do with the pigment. The fact that it
> didn't
> > > improve for Tom with different curves but it did improve when he
> > > switched from PV19 to PR209 should be an indication right there (I'm
> > > assuming he was using the same negatives).
> > >
> >
> > This is not to say that I don't believe it's possible for someone to
> > make really eccentric color separations that create weird color
> > combinations, but why would anyone do that? Even if their intent was
> to
> > create weird color combinations; it seems to me that weird color
> > combinations can be better (and more cheaply both in time and money
> for
> > negative materials) effected by choosing pigments that would create
> > weird colors in combination with eeach other than by trying different
> > weird color separations to see what happens, unless it's the
> > unpredictability that interested one.
> >
> > Also want to make it clear that I don't think this brownish or
> > brownish-purply cast has anything to do with how dilute or
> concentrated
> > the pigment is. I've printed with pigment concentrations ranging from
> > totally saturated color to just a whisper of color for each of the
> > primaries, and have never seen these brownish tones with the pigments
> I
> > use; the color combinations are clear and not brown no matter what the
> > pigment concentration.
> > Katharine
> >
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Received on Sun Jul 11 23:12:48 2004