[Scribus] cmyk vs rgb

[Hal V. Engel]

On Sunday 22 April 2007 13:39, Farid Elyahyaoui wrote:
> I sent this one with attachments but It didn't get through.
> So here it is without attachment.
> ---------------------------------
> I'm still trying to grasp the color management settings in scribus
> (and color management in general). I understand that RGB and CMYK are
> completely different color spaces. What I fail to understand is the
> following though:
> If I load a RGB jpeg into scribus and output a cmyk pdf and view the
> pdf on the same machine with acrobat reader the colors look greyish. I
> understand that I need color management to get matching colors between
> screen and what gets printed on paper.
>
> BUT (!) why do I get the different colors if I only work on one
> monitor on one machine?

Because you are displaying two different color spaces on a monitor that is 
apparently not color managed.  To make this a little clearer here are a few 
questions.

1. What color space was the jpeg image in?

2. What color space did you specify as the destination color space in Scribus? 

3. Did the image have the correct profile embedded when you imported it into 
Scribus?  

4. Is your monitor calibrated?

5. Is your monitor profiled?

6. Do you have the monitor's color management settings correctly setup in 
Scribus?

If you answered no to any of the last four questions or RGB/CMYK or "I don't 
know" to either of the first two then you don't have a color manged system 
and what you are seeing is what you should have expected.  My point being 
that the number of these questions you answered "incorrectly"  is an 
indication of how good or bad your CM work flow is.

>
> One would expect that if scribus calculates RGB->CMYK and acrobat
> displays these CMYK colors on the monitor (using RGB of course) that
> the colors would be the same. that is if the calculations are the
> inverse of each other. Or am I completely wrong here?

You are completely wrong.  No system can convert from or to an unknown color 
space to or from any other color space with predictable results since the 
starting point is an unknown.  Saying it is an RGB image and it is therefore 
in the RGB color space means that the color space as this would be defined by 
a color management system is unknown.  You don't expect to end up with 
greyhound puppies if you breed two stray dogs do you?  Same thing with color 
management to get predictable results you have to start out with known 
quantities like properly color managed images and devices.  

Color management is about having everything well defined at every step of the 
work flow.  If at any point something is undefined then things start to 
break.  On the other hand if things are well defined at every step of the 
process it works wonderfully.  Your post is an example of what happens when 
parts of a color managed work flow are broken.

Color Management has a significant learning curve.  Don't expect to understand 
it in a few hours or even a few days for that matter.  I work in the field 
and I learn something new about color management every day.  There are many 
very good resources on color management on the net.  I would recommend the 
Norman Koren web site http://www.normankoren.com/color_management.html .  It 
is very detailed yet it is accessible and can be understood by those new to 
the subject.  It is somewhat Windows centric so those running on other 
systems will sometimes have to read between the lines a little. 

Hal