[Scribus] Colour separations and line screen

[Steve Jacobs]

On Tue, 28 Feb 2006 10:03:54 -0600
Frank Cox <melville.theatre at gmail.com> wrote:

> On Tue, 28 Feb 2006 12:26:53 +0100 (MET)
> Peter Nermander <m8130 at abc.se> wrote:
> 
> > The printer should have a RIP that is suitable for the press, just find out what
> > input file format that RIP takes.
> 
> "The printer" is us.  We own the printing press.  (Actually three, but only
> one colour machine.)  As far as I'm aware, we don't have a RIP for it, though,
> it's just a big machine in the corner.
> 
> > You can not use the driver for your desktop printer to create suitable files.
> > Your desktop printer most likely has its own built in "RIP", so the computer
> > sends it postscript and the printer itself turns it into rasterized bitmaps
> 
> Our workflow is as follows:
> 1) Create the document with Scribus.
> 2) Print the document from Scribus on press plates using the HP5100 printer
> that's plugged into the computer that we run Scribus on.
> 3) Put those plates in the press and start the print run.
> 
> This works great for doing black-and-white and spot colour on the other two
> presses.  But now we are trying to get this third press that is supposed to do
> full colour working and are getting quite lost in the process.
> 
> > Note that the bitmaps output from a RIP becomes HUGE so unless you really have a
> > very good reson it is best to let this part be handled by the printer.
> 
> We have a very good reason to handle it ourselves, as you can see above.
> 
> What exactly is a RIP?  Software?  A special printer driver, perhaps?  Or
> something else altogether?
> 

I work with a little country print shop (single-color presses) and make
dozens of laser-direct plates on an lj5100 every week, so I understand
your desire to make this process work for cmyk.

In theory, ghostscript can do what you need. Ghostscript is, for
practical purposes, a software RIP (although I don't often see it
described as such), and you can use it to dump postscript files to the
5100 with controled halftoning.

I use ghostscript for this a lot (whenever I need halftone dots other
than the 5100's default), but haven't had yet needed to try cmyk
separations. So what follows is not something I'm sure works, but
rather what I'll try first when the need arises, which sooner or later
it will.

Ghostsrcipt (again, this is theory) generates proper cmyk screen angles
and dots with the "-dCOLORSCREEN" argument. This would need to be
accompanied with "-dDITHERPPI=150", because ghostscript will not
calculate screen angles if the output is coarser than 150 lpi, and the
5100 (so far as I can tell with a linen tester) uses a built-in default
of around 108 lpi.

You may also want to experiment with different ghostscript devices.
I've had good luck on the 5100 with lj5mono, specifying 1200 dpi
resolution ("-r1200") in the command. Do "gs -h" to see what devices
your ghostscript has to work with.

In terms of workflow, tell Scribus to make separations, then print to
file. Then use ghostscript to dump the resulting postscript to the
5100. The command will look something like:

gs -dNOPAUSE -sDEVICE=lj5mono -r1200 -sPAPERSIZE=11x17 -dCOLORSCREEN
-dDITHERPPI=150 -sOUTPUTFILE=\|lpr separation.ps -c quit

Replace "separation.ps" with the name of the postscript file Scribus
created. Crafting ghostscript command lines is a minor art in itself,
which you can learn more about at
http://www.ghostscript.com/doc/AFPL/8.50/Use.htm. If this works,
install gsview, whose print dialog will allow you to easily specify
these arguments and save them for future use.

Digressing OT a bit, when you get this to the press I would very
interested in hearing how it turns out. My main concerns would be: 1.
The 5100 may not position the images on the plates consistently enough
for color work. You pressman may go crazy trying to get the plates in
register with each other. 2. The 5100's 1200 dpi might not be enough to
render good 150 lpi halftones. You may get banded gradients and such.

Good luck and best regards,

Steve

-- 
Steve Jacobs
Steve Jacobs & Associates
Trinidad, CO US