[Scribus] Images in PDF output

[David Purton]

On Sun, Jan 09, 2005 at 08:39:02PM -0500, Kobus Wolvaardt wrote:
> 
> >the photograph in that document.  I create a PDF document from that  
> >scribus document and take it to be printed.  The photograph, however, is  
> >seriously washed out.  It appears washed out in the pdf when viewed  
> >on-screen, also.
> 
> I also experience this. Correct me if i'm wrong, but it has to do with the  
> conversion from rgb->cmyk. I think that converting a file to cmyk with  
> photoshop or gimp (I dont know how good gimp supports this) and including  
> it into scribus might improve the color.

I think you are right in thinking that the problem lies in the RGB->CMYK
conversion. Scribus does not use colour profiles to perform the
conversion from RGB to CMYK when it outputs images.

As for starting with CMYK images - looking at how Scribus handles images
- I think this will not help.  I'm only just starting to have a poke
through the code to see what can be done about this issue for my own
purposes.

So far it looks like Scribus does something like this when outputing
images to a CMYK colour space PDF (assuming you work with tiffs):

The tiff is loaded using TIFFReadRGBAImage (a libtiff function) and the
resulting RGB data dumped into a QT QImage class. In other words, no
matter what the tiff actually contains, an RGB image is loaded. I assume
that libtiff does not do anything cunning in its conversion from CMYK to
RGB.

When outputing to postscript or a PDF for print, this RGB data is then
converted back to CMYK. Unfortunately, you will probably not end up with
the same data as in your original tiff, because a maximum black pullout
is applied by Scribus in the RGB->CMYK conversion. i.e., any pixel in
your original image that has data in the C, M & Y channels will be
changed.

> A while ago there was a discussion about cmyk images being changed when  
> exported as cmyk in scribus, and there was talk of exporting all cmyk  
> files unchanged to pdf. Is this true? There are companies who has very  
> harsh demands when it comes to the color of their ads and it would help if  
> I can export cmyk images (pdf's, eps's, tiff's) without their color being  
> altered.

As far as I can gather it is currently impossible to do this in Scribus.
It will also be a *big* pain to change, because Scribus relies very
heavily on the QT QImage class to deal with image data. The QImage data
knows nothing about CMYK colour space. :(