[scribus] LSI 240% ink limit.

[Hal V. Engel]

On Saturday 08 August 2009 12:07:59 pm John Culleton wrote:
> On Saturday 08 August 2009 11:58:40 am Gregory Pittman wrote:
> > On 08/08/2009 11:57 AM, John Culleton wrote:
> > > On Friday 07 August 2009 07:19:29 pm John Culleton wrote:
> > >> LSI is enforcing a rule that total ink coverage of any CMYK
> > >> color cannot exceed 240% The usual example is rich black.
> > >>
> > >> 1. Can this be a problem in Scribus?
> > >> 2. If so, how does one detect/fix such overcolored areas?
> > >>
> > >> I presume that none of the standard colors exceed this limit.
> > >> But I worry about graphics from e.g., Gimp which are RGB
> > >> converted to CMYK when Scribus writes the file.
> > >
> > > To further illuminate the question, these are the recommended
> > > values whan using Photoshop:
> > > ------------------------
> > > Engine: Adobe (ACE)
> > > Intent: Relative Colorimetric
> > > Use Black Point Compensation: Yes
> > > Use Dither: Yes
> > > -------------------------------
> > > and
> > > -----------------------------------
> > >      Ink Colors: SWOP (Newsprint)
> > >      Dot Gain: 25%
> > >      Separation Type: GCR
> > >      Black Generation: Medium
> > >      Black Ink Limit: 100%
> > >      Total Ink Limit: 240%
> > >      UCA Amount: 0%
> > > -------------------------------
> > > for the custom CMYK settings.
> > >
> > > Perhaps there is an ICC profile that can take care of all this.
> >
> > I don't know that there are any color sets that come with Scribus
> > that will exceed this. Of course, we get some list questions from
> > some who think that they want all the CMYK saturated, but
> > obviously don't understand the printing issue.
> >
> > What it does suggest, though, is that one of the things the
> > Preflight Verifier could do would be to check for ink limits.
> > Maybe you could file an RFE.
> >
> > Not sure how they might imagine someone specifying > 100%
> > Black...seems that would be kicked out by the printing equipment.
> >
> > Greg
> >
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>
> Consider the image from Gimp that has black on it. That is RGB 0 0
> 0. What would Scribus change it to in CMYK?

There seems to be some misconceptions here.  CMYK is a subtractive color space 
so higher values mean more ink (if you are talking about a printer).  The K 
ink in a CMYK system is used as a substitute for higher levels of process gray 
(C = M = Y) to create darker tones and reduce ink load for the same darkness.  
Many printer/ink/paper combinations can handle total ink levels much higher 
than the 240% talked about above but some can not and some can not handle even 
240%.  

The darkest blacks on most printers are a combination of K plus C and M 
(sometimes called rich black).  In addition on many modern ink jet printers 
the "black" ink has a yellow tint that is intended to allow these very dark 
blacks without causing a hue shift because of the added C and M ink (IE. to 
keep the resultant black neutral). The normal operation for these printers is 
to always mix some C and M ink with the K ink to shift the hue toward neutral 
even for lighter grays although for the very lightest grays this may not be 
needed because the white point will be shifted toward blue (IE. away from 
yellow) because of the "whiteners" used in many papers.

All of this is very device specific, meaning specific to a 
printer/paper/ink/work flow, and the only way to insure that you are doing the 
right CMYK separation for your device is to use the correct ICC profile for 
the device in question.  These profiles will have an optimized GCR setup and 
also have proper ink limits so that there is no over inking and there are 
proper transitions to the K ink as well as proper use of C and M inks to keep 
grays and blacks neutral.

Hal