On Fri, Jun 19, 2009 at 12:41 PM, peter sikking<pe...@mmiworks.net> wrote:
> Chris Mohler wrote:
>
>>>
>>> <http://www.mmiworks.net/eng/publications/2009/06/gimp-squaring-cmyk-circle.html
>>
>> I like this approach.
>>
>> I have a few questions:
>>
>> Will each plate have a density or opacity attribute?  (some inks are
>> more opaque than others)
>
> I guess that the complexities of ink simulation start showing here.

Yes - although transparency/opacity would be enough for me to use GIMP
for professional separation work.  I'm reasonably sure that PS uses a
variant of the blending mode Multiply for spot channels, and this
works fairly well.

>> Will it be possible to edit an individual plate in grayscale?
>
> well, as pippin said: the individual plates _are_ grayscale/monochrome
> drawables. they will be editable just like layer masks or selections.

Excellent.

>> And finally, will it be possible to perform operations on the RGB
>> portion of the image that do not take (immediate) effect on the
>> projection?  For example, if I want to go back and add a portion of my
>> RGB artwork to a plate, I might want to clone and existing RGB layer,
>> perform some modifications, then apply the contents of that new layer
>> to one of the plates.
>
>
> I am curious why you want to do something like that, because you
> are then going against the grain of the whole plan: freedom to develop
> the artistic concept further without (much) rework on the plates.

Imagine I'm designing a black t-shirt with say five spot colors,
including white.  After completing the artistic design, I enable the
'projection screen'.  This theoretically would result in my five
"plates".  However, the white plate will need special attention.

Here's my workflow for this in PS: I would use the (badly named)
'Apply Image' command to take the contents of each color plate and
combine them into the white plate using the mode 'multiply'.  I would
also manually "choke" the white plate - this means making the white
areas a point or two smaller than the colored areas, thereby
preventing the white from poking out at the edges of the colored
areas.  This process can get a bit tricky, especially if the original
artwork is very complex.  Often, create temporary layers (or plates),
perform selection/drawing functions, then combine the result back into
a plate in one of two ways - either making a selection on the temp
layer and going to the plate and filling or erasing, or using the
'Apply Image' command to take the RGB channel of the current layer and
combine it with a plate using a mode such as Multiply, Screen, or Add.

Now, I am quite interested in learning new workflows - so I am not
bound to the "how" of the method above, but I hope I have explained
the "why" well enough.  In addition to being able to interact with
each plate as a grayscale drawable, it would be useful to create
temporary areas for doing work - be they layers, channels, plates,
whatever - on which to create paths, selections, etc to in turn use to
modify the plates manually.  Icing on the cake would be a mechanism to
combine/subtract plates using the available blending modes.  During
the process, it is fairly critical to have an ink density/opacity
setting for each plate, to simulate (roughly) how the final print is
going to look.  EG, set the white plate at approx 90%, the colors at
approx 70% - and you can see which portions of the colors are falling
on the white underlay, and which portions are falling on the black
shirt.

I realize that this is just one corner case, but if you visualize each
plate being printed separately, in order, you may be able to recognize
some of the many 'gotchas' inherent in separating the artistic artwork
into something suitable to send to the press.  That's why (in my
opinion) it is important to have as much control as possible over each
plate.

Whew ;)

If I explained any of this poorly, I am sorry and will happily try to do better.

All in all, I am very pleased with the direction that this is taking
and I would certainly like to use GIMP for even more of my production
work. :)

Thanks,
Chris
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