Digital BW, The Print

Paul,

Very helpful information on RGB separation curve creation. I have posted the body of your message in the Files section under:

Files > Inksets, reviews and techniques > MIS Variable Mix 

Those interested will also find your earlier and longer discussion of this topic at the same location.

Thanks,
Martin Wesley

----- Original Message ----- 
From: "Paul Roark" <paul.roark@...>
To: "DigitalB&WPrint" <DigitalBlackandWhiteThePrint@yahoogroups.com>
Sent: Wednesday, February 13, 2002 9:07 AM
Subject: Re: [Digital BW] MIS Full Spectrum Partitioned RGB Quad Workflow


> Alessandro Pardi wrote:
> 
> > ...I first tried to modify the global RGB curve,
> >but that caused banding, whereas changing the Red
> >curve gave no banding
> 
> Jeff Randall wrote:
> 
> >Yes, the Red curve controls the Cyan position ink (which is the dark
> >gray in the FS and Piezo systems) so adjusting it is the best way
> >change the dark end.  You can't directly control the Black ink -- I
> >think the driver automatically turns it on when each RGB values is
> >below about 50%.  Paul Roark may want to step in to correct or add to
> >this.
> 
> I haven't been following all this thread, so my comments may not be
> responsive to the issues you were originally addressing.
> 
> >Using any combined RGB transfer function ... -- a meat axe ...
> 
> I agree with this.
> 
> 
> 
> A couple general thoughts I might add -- too radical slopes on curves
> sometimes adds artifacts.  The magenta position ink is sufficiently light
> that it might be run up into the highlights a little more, allowing the
> slopes to be decreased a bit.
> 
> The black ink is turned on by the driver looking at the combined R, G, B
> values.  The black ink is a potential source of dots as it starts to be
> turned on.  And, when I measure the roughness of test strips, in general,
> the worst areas are never the highlights, but the shadows.  The
> concentration on highlight dots is a bit misplaced, in my view.
> 
> When the blue (yellow-position, lightest ink) and green (magenta-position,
> next lightest ink) have both been run all the way down -- that is, turned on
> to 100% flow -- the red curve (cyan-position, dark gray) ink becomes the
> "switch" for the black.  If you look at the red curve of the PS_FS1160a
> curve, you see a reduction of slope of the curve at about 55% (115 input on
> the 256 scale).  That might be where the black ink starts to come on.
> 
> If there are any dots are roughness in the tests strips at that point (I
> don't have one in front of me) it might be because the black ink was turned
> on before the cyan ink was on full enough to hide the black dots.  What I
> have done on some curves is pull the lightest ink back from being on 100%.
> It is light enough that even a reverse slope can easily be overcome by the
> cyan ink.  If the lightest ink curve is raised enough, then the cyan ink can
> be more fully on and the lightest ink curve can be a "switch" for the black
> ink.  It might be that this approach can reduce roughness in the shadows as
> the black ink starts to be poured in.
> 
> Often in my VM curves all three curves are close to the bottom (full-on)
> position, but not totally on.  Then all three can be used to control the
> black.  The lightest gray will give the finer control than the cyan because
> a one-unit increase in the light ink doesn't add as much density as a
> one-unit increase in the cyan ink, the black ink, indirect increase being
> equal for both.
> 
> Also, if curves have waviness down in the shadows (and my curves do
> occasionally -- things I have to get back to as time permits), if you have 2
> or 3 curves that are not totally on and able to  adjust the black, you can
> stagger the control points and get finer control.  That is, we are limited
> to 15 points per curve.  If there is a  bump between, for example, the "85%"
> and "80%" points, keep the points of one curves on those points, but move
> the points of another curve to midpoints, for example, the "82%" position.
> 
> Overall, I'm not sure that having the lighter ink curves full on early is an
> advantage.  Their ability to add to the accuracy of control in the shadows
> might be an advantage.
> 
> I run into this problem with the "warm" vm curves.  Because I'm holding back
> the toner as much as possible, that curve becomes my sole control in the
> shadows.  The result is sometimes persistent humps in the ramp that are
> tough to control.  And, when I have them controlled on one printer, someone
> else with the same printer type might report a problem there just due to the
> normal production variations in these machines.  The more radical the curve
> slope is, the more these production differences seem to surface.
> 
> And so it goes.  None of these things is perfect, and the ability of the
> individual to adjust the curves to his/her own machine is a real advantage.
> 
> Paul
> http://www.PaulRoark.com



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