[Scribus] cmyk vs rgb

[jon]

Hi Farid -

I'ld like to throw something else into this - may be something visual  
can make this a bit
more clear what happens with rgb=>cmyk conversion.

There are three images:

http://www.jonhagen.de/transfer/srgb.png
This one is srgb (a very limited rgb colorspace), say the colors we  
like or want.

http://www.jonhagen.de/transfer/gammut_warning.png
This is the same image with "gammut warning" on. Greying out the  
colors wich are not printable in cmyk color space (here ISO FOGRA27).
Here you can see that most of the rgb color space isn't printable  
with standard cmyk colors. All the greyed out colors will be converted
to it's nearest neighbours (clipping).

http://www.jonhagen.de/transfer/cmykISO.png
This is the same image softproofed with the ISO FOGRA27 profile, all  
colors converted to this cmyk space.

One point here is that additive or light colors (rgb, r+g+b => white)  
aren't really comparable to substractive or body colors (reflective,  
cmyk, c+m+y=>(sth. like) black)

If you are able to avoid non mappable colors in your rgb images you  
will get fine result when converting, if not clipping will always!  
occur.

Most of the intensive and bright blues and greens of any rgb space  
aren't printable. Reds and yellows, if not too bright, are not so  
critical.

One way around this is using only, or adding, spot colors or luminant  
printing colors, to cmyk.
  Jon


Am 22.04.2007 um 22:39 schrieb Farid Elyahyaoui:

> 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?
>
> 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?
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