[Gimp-developer] CMYK editing (Was: GIMP PDF export plugin)

[Andrew A. Gill]

On Mon, 23 Mar 2009, Sven Neumann wrote:
>
> On Mon, 2009-03-23 at 17:51 -0400, Andrew A. Gill wrote:
>
>> I do work in the printing industry, and I can tell you that
>> output is still CMYK, and will remain CMYK for at least the next
>> few years.
>
> Output, yes, of course. But where in this process do you actually edit
> an image in CMYK? I don't mean converting it to CMYK to get it printed.
> I mean actual editing after the conversion. Could you give us some
> examples of where that is needed?

Oh, sure.

Like I said, I mainly work with vectors and spot jobs, but I 
have, in the past, had to deal with some of these issues.  Take 
the following image, for example:

<http://www.ets.ru/images/pk000075.jpg>

To properly print this image, it should be trapped--that is, 
either the red plate or the black plate should be altered so that 
the red and black overlap.  That way, a slight misregistration 
won't result in a white gap along the border.  Trapping is 
usually pretty small, around .25 pt, but here's an exaggerated 
example of what will happen if you don't trap and the plates are 
misregistered:

<http://i158.photobucket.com/albums/t102/superluser/whywetrap.png>

Some trapping can be done in vector programs and page layout 
programs, but images with non-geometric edges like the one above 
cannot.  I would have to do it in GIMP, but I cannot do it in 
GIMP, because that would require having some of the pixels at 
100% red and whatever shade of black it is at that point, and 
GIMP cannot do that because it does not have CMYK support.

Likewise rich black.  In cases where you are printing black on a 
multicolored border, it's useful to print in rich black, usually 
60%C, 100%K.  This makes the effects of trapping less noticeable.

You can find an example of rich black here:

<http://www.graphic-design-employment.com/over-printing.html>

Again, it is not possible to do this in GIMP without CMYK 
support.

Also, color correction.  If I print a proof and it turns out that 
it is too cyan, I cannot simply turn up the red, because that 
will also adversely affect both the cyan and magenta plates.

And finally, I agree with Sven that I don't know why anyone would 
want to have multipage PDF output for GIMP.  I'd much rather see 
built-in DjVu support.

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