Alexandre Prokoudine a écrit :
> On Wed, Mar 25, 2009 at 6:21 PM, Alexandre Prokoudine wrote:
> 
>> 1. Client brings an image for poster in CMYK which needs color
>> correction. Urgent work, not time to ask him to redo it. Double color
>> space conversion is out of question. So he had to use Photoshop from
>> VMWare.
>>
>> 2. You have a newspaper where first page should have a two-color
>> photo: black (C=0%M=0%Y=0%K<=100%) and blue (C<=100%M=0%Y=0%K=0%).
>> separate+ however separates black to 4 channels.
>>
>> 3. Some print houses set limit to overall sum of colors, for example
>> 180%. So if you take Cyan 100% + Magenta 100% (already 200%) + a
>> little of K and Y this will result in unnatural colors in a newspaper.
>>
>> 4. Live density control for each CMYK channel is a must (Scribus/SVN
>> has that in preview dialog).
> 
> I was reminded that I actually forgot
> 
> 5. Part of an image should be b/w and the rest should be colorized
> with just one tint. E.g. Cyan + Black for sea. separate+ and exporting
> are of no help here.

Working in CMYK at one point in the workflow is a question of control. 
You need to obtain a predictable result on press and you need to work 
with what you really have, often "exactly", in terms of color 
combination. When printing in color on an offset press you have 3 
possibilities. a) Spot colors (blended or preblended inks that could be 
just any color), b) 4-color process (CMYK inks) c) Hexachrome or 6-color 
process CMKYGO.

So, pursuing with this interesting list of real-case scenario or why 
can’t we just rely on RGB throughout the creation process (while I agree 
some people can decide they work only with it):

6. In a layout,  you have a picture from which you want to pick a 
specific color and apply it to another element in the page. You will 
want to have control over that color. For instance, if this color is to 
be applied on text, you will want to make sure you don’t end up with ink 
in the four channels because on press you might (you probably will) 
encounter registration issues with such tiny elements as the hairline 
portion of a font. You will need to limit this to a 3 color 
combinations. Now, how do you do this with a RGB>CMYK converter? There 
has to be some human supervision in the process. So, it doesn’t matter 
really if you do the whole work in GIMP instead of if you split the job 
between Scribus and GIMP or Inkscape for instance: at some point you 
will need to have all color elements to speak the same language and this 
language will have to be the lower common denominator: CMYK.

7. You want side by side a dark background and a color image. Both can 
be created in GIMP but for the dark background you will really want to 
control the combination of inks that produces that specific color and 
again it’s going to be difficult to just let the converter do the job 
without you being in full knowledge of what’s going on behind the scene.

I realise that my examples are not purely "image" manipulation (which is 
the core task of GIMP) but instead "image usage and combination" but 
really, this is mostly what graphic design is all about!

8. If a user is not concerned about precision, he/she might not need 
that much control over an image but if you work for an ad agency that 
needs to produce tons of images that include, for instance, skin tone — 
then you also need to have total control over the colors and this has to 
be done in CMYK which is the very end of the workflow. In the end we 
will have to turn the image into CMYK and it does really happen often 
that we have to adjust colors at this far point in the workflow.

9. While prepress and press shops can handle pretty easily RGB data, 
it’s going to be a "best effort" made by the RIP itself according to 
curves and algoritms the designer has no control over. And I know not 
many designers who will accept that. At some point, they will need both 
a good converter and means to adjust the resulting image. This is 
fine-tuning, I agree.

10. The packaging industry makes a great use of CMYK + Spot colors. To 
convince yourself, unfold any packaging and you will notice all the 
press marks on the inner flaps and the colors used. This means that both 
pixel and vector driven applications need to be able to work in CMYK + 
Spot if we want to address the packaging industry needs. There is quite 
a lot of design there to accomplish!

I guess we could find other real life scenarios where CMYK control is 
important or even stronger, a necessity.

I humbly wish this short intervention will help understand better the 
needs from the print point of view.

Louis


> 
> Alexandre
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