> Anyways, I had some conversation with two graphics designers about
CMYK
> problems and the Gimp at the Systems, and I think it might be
worthwhile
> to read the following "sometimes true" observations. Remember, they
are
> hearsay ;)
Thanks for writing this stuff up. I think I should mention that I work
in the prepress department of a highend lithographic printing company so
that you know what my biases are when reading my comments below.
> 1. "colour matching for photos is a don't care". Ok, this is a
blatant
> lie, however, exact colours are not that much of a concern for
> photos. Much more important are logo colours (most companies
have
> pretty strict definitions of these). If a photo doesn't exactly
> match the screen colours ("which screen colours, anyways") this
> is often not a reason to not use gimp. If a logo colour can't be
> reproduced, however, it keeps Gimp from being used.
You should not create a logo requiring "Logo Colours" in a program such
as photoshop or gimp. You will get superior results using a vector
based application such as illustrator.
Sometimes you will need to match a logo captured in a photograph to a
specific "logo colour" , but the first step would be to convert your
photograph to CMYK.
"Logo Colours" (aka spot colors or PMS colors) can already be used in
gimp if you are only using one ink at a time. Just save your image as a
grayscale tiff and place the image in quark using whatever ink you want.
> 2. "Logo colours are not CMYK". Yupp. Logo "colours" might not be
> representable in CMYK at all (gold etc...). Even if, you often
(but
> not always) want seperate planes for important colours.
>
> Most uses of spot colours want the concept of an "indexed
channel",
> i.e. a channel where each value represents a different palette
> colour. No bleeding with image contents.
>
> Gimp's channels can be used instead, which is not that perfect
for
> all uses, but exists and at least photoshop doesn't offer a
better
> solution ;) They also allow gradients of a single spot colour,
which
> indexed channels wouldn't allow. Wether all this makes them
easier
> or harder to use is something to explore.
>
In my experience, when you are using a spot color in a raster image you
are not usualy working with a logo. Usually you are trying to match a
color in a photograph that is not representable in the CMYK color space
such as some reds and oranges.
Any image that you would want indexed "Logo Colours" for would be better
off handled in a vector based program such as illustrator.
> 3. "You don't print from within the gimp". At least you don't print
> brochures from within the Gimp. You use gimp for artwork, often
the
> logos, but you obviously don't set text using the Gimp. You
instead
> import images into some layout program (quark xpress ;).
>
> I was told that the principal reason for bad quality of gimp
> images within quarkxpress is that quarkxpress imports gimp's rgb
> tiffs like garbage. I was told that loading the rgb data into
> photoshop, associating sRGB with it (changing _nothing_ else)
> improved the quality very much, making the results reproducable
on
> printers. Without absolute colours, they look different.
In my experience people don't use gimp (or photoshop) for logos
(I mean for print work, of course the web is a whole different story)
I am not certain, but I think that the rgb->cmyk conversion is not done
by quark, but by the postscript rip when you print your file.
Regardless of weather it is quark or the rip that convert the colour,
setting the colour profile to sRGB in gimp is the wrong fix. There
should be a setting on either quark or the rip that tells it what color
profile to use for images that have no assigned profile.
Where I work, we _never_ place rgb files in quark. If a client of ours
gives us file in RGB the first thing we will do is convert it to CMYK in
photoshop.
> 5. "Logos are done by overlays". At least one method of using spot
> colours is to create them as seperate channels. Tiff/Eps are
> reportadly able to save additional channels in a way that a
program
> can read them sensibly.
>
> The spot colour "planes" are then laid over the other graphics.
For
> this to work a mask is necessary, since channels range from
white
> (not transparent) to "channel colour", at leats in quarkxpress.
>
> It seems that traditional masks are not what's called for -
instead
> you want a path saved in the tiff/eps file (don't ask me wether
that
> is possible). This clipping path is then used for the overlay -
gimp
> can't create this kind of paths, nor can it save it.
The industry standard way of saving raster files that have spot channels
in them is the DCS file format (A very close cousin of EPS).
Clipping paths are commonly used to overlay an image over text or
another image in quark.
>
> CONCLUSIONS - THE 60% WAY TO CMYK
>
> If one were so bold as to draw some conclusions, they would
probably be very
> similar to these:
>
> 1. Enhance the tiff/eps save plug-ins to do cheap RGB->CMYK
conversion. This
> would work around conversion problems in other programs.
I would suggest the menu items file->export->{cmyk tiff,cmyk eps}
> 2. Associate sRGB or any other colourspace with the saved data in
> tiff/eps. It doesn't matter wether it's true or not, just give
> programs something to depend on.
Photoshop 4.0 had a lot of long time users very pissed off because it
blindly attached color profiles to images without asking. Lets not
repeat that mistake.
> 3. Educate users about channels and what they can be used for - on
this
> Systems I was frequently confronted with users who were unhappy
with
> the gimp because it didn't allow them to do things as easily as
under
> photoshop. Often(!) I was able to get exactly the same results,
with
> a much easier and faster sequence than the one that user used
with
> Photoshop.....
>
> This could be a start, to work around bugs in other programs. Also
> relatively cheap, unlike the following:
>
> 4. Find out wether saving paths as paths as opposed to masks is
really
> required to do overlays in common layout programs. If yes:
This is correct. quark and friends do not understand alpha channels.
The only way to overlay your graphics in a non rectangular fashion is to
use clipping paths.
> 4a. enhance the path tool to be able to work with generic paths
(holes,
> multipart etc.).
>
> 4b. enhance the tiff/eps plug-ins to be able to save these paths
together
> with channels.
We will need to be able to select which path is the clipping path (there
can be only one), and which channels are spot colors, and which channels
are just channels.
> 4b. (optional) make tiff/eps save images together with their
channels in
> the same file.
The tiff plugin (and any other plugin that supports it) should save all
channels and paths.
> 5. Implement "indexed channels", or somethign else that makes
handling spot
> colours easier. An easy way is to use one channel for each spot
colour.
> Finished.
Jay Cox
jaycox@xxxxxxxx
