Re: cmyk negatives?
Actually, the CMY channels won't be the same as the CMY values you'd
get by inverting the RGB channels, because even though you've set the
black generation to 0, you've still altered the color channels
through the separation option and through the specification of SWOP
inks; the color values are altered to adapt to the limitations of
commercial process inks, which limitations aren't shared by the
pigments we use. I won't go through the details here, since it's all
well spelled out on the page I pointed to, but it's much more
desirable, IMO, and much more straightforward photographically, to
arrive at CMY through inverting RG.
kt
On Aug 25, 2007, at 6:00 PM, Rodolpho Pajuaba wrote:
It's very simple to convert from RGB to a blackless CMY(K). In PS,
go to Edti>Convert to Profile, then Destination Space pick up
Custom CMYK. On the dialog, select SWOP, Euroscale, whatever; write
0 (zero) on Dot gain, Separation Options choose GCR, and on Black
Generation choose None. Give a describable name and click OK, and
you're done. The only problem is that you're gonna miss the great
advantage of the K channel (because the CMY channels will very
likely be similar to their RGB correspondents, only muddier), which
is to enhance the details.
Regards,
Rodolpho Pajuaba
Katharine Thayer wrote:
Hi Charles,
The short answer: leave your image as a RGB image; invert,
separate the channels, and print.
For more detail about why that's the best way to do it (IMO, of
course) and how RGB inverts to negatives that print CMY, if
you're interested, see this page:
http://www.pacifier.com/~kthayer/html/RGBseps.html
For more detail on why using the default CMYK space may not be
the best option, especially if you discard the K layer, see:
http://www.pacifier.com/~kthayer/html/CMYK.html
The short answer is that the default Photoshop CMYK is designed
for printing SWOP process inks on coated paper on a web-offset
printing press, and when you convert the RGB image to CMYK, the
color information in the RGB channels is altered, in some cases
drastically, to accommodate the addition of black without
changing the brightness value of the pixel. That's why the CMYK
image looks flat without the black, because the color information
isn't the same as the color information in the RGB space.
Katharine
On Aug 25, 2007, at 5:03 PM, ryberg wrote:
Folks, I've finally worked up nerve to try a three color gum.
I have made curves for yellow, cyan and magenta which look like
they might work. My problem is how to make the negatives. I
thought it would be simple--convert image from RGB to CMYK,
separate out the channels, invert, print each one and that's
it. Well, first off, without the K channel the image on the
screen is really flat, but I sort of remember that few of you
actually print a black layer--am I wrong? Any advice is
appreciated.
Charles Portland OR
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