RE: Gum Tri-Color Yellow

Note that I said "simplistic views of the arithmetic conversions".

For much of this discussion, what I wrote is close enough.

--
Bill Leigh
wrleigh@att.net
> Bill says:
> 
> "The conversion to RGB from CMYK is simply a case of inverting the CMY
> Channels to each of the RGB channels. There is nothing magic in it."
> 
> Actually that's not correct.  Maybe in theory is should be, but in practice
> it doesn't happen.  CMYK (or CMY - and CMY is just CMYK with an empty "K"
> channel) is not simply a reversal of RGB.  Rather, it is a set of
> instructions to the printer, which include percentages of ink to use, and
> such arcane matters as "dot gain".  If one considers Photoshop, for example,
> the CMYK settings are practically infinite.  Even the type of ink is
> factored into the RGB to CMYK conversion.
> 
> "RGB only cleanly goes to CMY"
> 
> Not true for the same reasons as above.  The RGB and LAB spaces are
> lossless, but conversion to CMYK will always result in a changed file.
> Well, almost always.  I guess it would be possible to create an image that
> is only one flat field of color, by scanning a Pantone chart, for example.
> 
> 
> -----Original Message-----
> From: wrleigh@att.net [mailto:wrleigh@att.net]
> Sent: Thursday, July 08, 2004 9:17 AM
> To: alt-photo-process-l@sask.usask.ca
> Subject: RE: Gum Tri-Color Yellow
> 
> 
> I've not tried RGB negatives, but I should think that they are functionally
> equivalent to CMY negatives. You are using CMYK pigments, so in the places
> where the Cyan should be in the print, the neg is clear and place where you
> don't want Cyan the neg is opaque (and similarly for Magenta and Yellow and
> Black). If you get there by taking a CMYK image, and inverting the channels
> and making your negs from them, it or converting your CMYK image to RGB,
> then taking the separate channels and printing them, I think they should be
> effectively identical.
> 
> The conversion to RGB from CMYK is simply a case of inverting the CMY
> Channels to each of the RGB channels. There is nothing magic in it. The K
> channel is then applied to the other RGB channels.
> 
> The conversion back to CMYK, BTW, unless Photoshop simply maintains the
> color information in CMYK and only uses RGB for some manipulations and
> display, is not absolutely reversible, because you will loose information
> going from CMYK to RGB to CMYK again. RGB only cleanly goes to CMY. The K
> channel must be resynthsized out of the RGB information, and it probably
> will not be identical to the original.
> 
> Assuming the values of the colors are in a range of 0 to 1 where 0 means no
> color and 1 means full color, simplistic views of the arithmetic conversions
> are
> 
> R = 1 - C - K
> G = 1 - M - K
> B = 1 - Y - K
> 
> we subtract the K value because K makes things blacker, and when R, G, and B
> are 0, they make black. If as a result of these calculations R, G, or B go
> negative, they are set to 0.
> 
> Converting back, we get
> 
> C = 1 - R
> M = 1 - G
> Y = 1 - B
> 
> K = MINIMUM(C, M, Y)
> 
> This step is called black generation is is used to synthesize black out of
> the CMY.
> 
> C' = C - K
> M' = M - K
> Y' = Y - K
> 
> This step is called undercolor removal to avoid saturating the paper with
> excess ink and overprinting too much. This comes from printing presses to
> prevent soaking the paper, but the calculations and process still hold in
> modern computer printers.
> 
> If you take these calculations and work thru them with some set of values,
> you will see that you don't necessarily end up with what you started with.
> 
> To answer Giovanni's question, if you wanted to be scientific about it, you
> could probably use the eyedropper in PS, determine the color you are seeing,
> and using these calculations to figure out how much more of any particular
> color you might need.
> 
> Finally, the K in CMYK stands for blacK, not Key. This is used because B was
> already taken for the Blue in RGB.
> 
> 
> If you want more detail, the Adobe PostScript Language Reference manual, 3rd
> edition, page 210, contains much more information than you probably EVER
> wanted to know about RGB, CMYK, and several other color spaces.
> 
> 
> --
> Bill Leigh
> wrleigh@att.net
> 
> 
> > 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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