[Scribus] CMYK - one last try

[David Purton]

Thankyou for your response. I do appreciate that people are busy and not
everyone has time to fiddle with colours, esepcially when a program
already does what they want.

But.. :) can I clarify a couple of things below?


On Mon, Dec 06, 2004 at 07:00:55PM +1100, Owen wrote:
> On Sat, 4 Dec 2004 14:35:02 +1030
> David Purton <dcpurton at chariot.net.au> wrote:
> 
> > One last effort to try and understand what Scribus is doing, since I'm
> > not convinced so far that people on this list have followed me :(
> 
> > 
> > Now, convert it to pdf using this ghostscript command:
> > 
> >         ps2pdf -dEPSCrop -dProcessColorModel=/DeviceCMYK CMYK.eps
> 
> 
> I then imported CMYK.eps into Scribus and produced a pdf

Did you also use the ImageMagick convert program to rasterise the eps
into both CMYK colour space and RGB colour space (as outlined in my
first mail)?

And did you also then use tifficc to proof these two to screen? You can
also if you wish use a monitor profile with tifficc just specify that as
your output profile with a -o option. It defaults to sRGB if you leave
it out.

1. When you imported the eps into Scribus which proof from tifficc 
   matched to the on screen display in Scribus? The one from the RGB tif
   or the one from the CMYK tif?

   maybe you don't have ImageMagick installed. You can use these files if
   you want:
        http://marshwiggle.net/CMYK.tif
        http://marshwiggle.net/RGB.tif

        Remember that CMYK.tif has the same "numbers" in it as the eps
        file has - no colour profiles need to be involved in converting
        from CMYK to CMYK.
        The RGB.tif file on the other hand was converted from the
        original eps without the use of colour management. the colours
        it contains are #00ffff, #ff00ff, #ffff00, #000000. These are
        quite different to the four process colours. We no longer
        are using the same colours as specified in the original eps.

2. What settings did you use to create your pdf from in Scribus? Make
   sure you have it set to create the pdf using settings for print, not
   screen/web. Did you have iccprofiles turned on?

> 
> I read both in Acroread5, took a snapshot and measured the colours.
> 
> 		ps2pdf			Scribus
> 
> cyan 		C 100% M27%		C 100%
> magenta	M 100  Y 41 B 7		M 100%
> yellow	Y 100  M 5		Y 100%
> black		K 100			K 100%
> 

By snapshot, and measured... I assume you mean you used say the gimp to
take a snapshot of the screen and then used the eyedropper tool to check
what colours are being display? Right? (Or something analagous)

Now, I notice you have quoted your screenshot measurements in CMYK. You
can't really do this. Remember your screen is displaying colours in RGB,
so when you measure colours on the screen like this - especially from a
colour management aware application like acroread - you are not
necessarily measuring the colours in the pdf file - only the proof to
RGB.

The ps2pdf command I gave you creates a pdf that is in the CMYK colour
space. ie. We stated with a CMYK eps and we converted to a CMYK pdf.
There is no change of colour space, no profiles need to be involved and
the "numbers" are the same in both the eps and the pdf.

Now acroread uses colour management. It knows that you are looking at
this pdf on a monitor that uses RGB, so it uses it's own inbuilt colour
profiles to soft proof the CMYK colours to RGB. It's exactly analagous to
the first of the tifficc commands in my first post.


Now possibly the pdf you have created in Scribus is an RGB pdf (see my
comment at the top - Scribus can create pdfs for either screen (RGB) or
print (CMYK), the colours you have measured are actually #00ffff
#ff00ff, #ffff00 and #000000.

In this case, because the pdf is in RGB and your monitor is also using
RGB, acroread does no fudging and spits the colours to your screen
exactly as they are in the pdf.

Of course nothing is that simple. There is a second possiblity for your
pdf from Scribus. You may have produced it for print and used embeded
iccprofiles. I'll have to check this myself at work tomorrow using
Pitstop (prepress tool for pdfs). If this is the case then I do not think
acroread is proofing things right, but if you sent it to a print bureau,
then you might get an output similar to what you see on screen in
Scribus. I will have to look more closely at these sorts of pdfs at work
before commenting further.

> 
> I think if I was to give a printer a pdf, I would give him the one
> from Scribus
>
> If I had to produce my own separations, I woulld be happy with Scribus
> output.
> 

You mean the "Print Separations" from Scribus? Did you get one black
square on each plate? 

I would certainly be happy with these results, but you may not be if you
are expecting the output to look like what you see on your screen in
Scribus. If you sent those separations to a printer, I know from
experience you would get something like the output you see in acroread
of the ps2pdf created pdf.

Indeed if you import either the ps2pdf pdf or the original eps file into
Photoshop or CorelDRAW or... you will also see something very similar to
what you are seeing in acroread.

It's at this point that I begin to see problems with what Scribus is
doing in relation to it's on screen display, and it's separations
output. Problems compound even more if you import a CMYK bitmap into
Scribus - but we wont go there yet.

> 
> Limit of knowledge.
> 

Hehe - no worries :)

Actually I've have no formal background in prepress and all I've learnt
in colour management has been through my experience in prepress work
publishing maths text books over the last 5 years. I find the whole
thing really interesting and the possibility of being able to get a good
correlation from screen to print under linux with the super nifty little
cms library is really great.


cheers

dc


-- 
David Purton
dcpurton at chariot.net.au
 
For the eyes of the LORD range throughout the earth to
strengthen those whose hearts are fully committed to him.
                                 2 Chronicles 16:9a
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