Google Summer of Code, CMYK Project for GIMP [Survey]

Software GIMP
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#1

Hello! I plan to apply to GSoC for GIMP this year. My goal is to add more support for a CMYK workflow - at least enough where artists could use GIMP for printing, even if it’s just the essential features for now.

As part of my proposal, I’m researching what “essential” CMYK features would be for GIMP users. If anyone here designs artwork for printing and would be willing to share your thoughts, it’d be most appreciated.

I’m assuming (at minimum) the ability to import and export popular formats with CMYK profiles (e.g. .tif, .pdf, etc) and possibly updates to the existing soft-proofing features. Thanks in advance for your feedback!

#2

Hi,

Adding basic CMYK support should not be that hard.

You need to

  • Import CMYK and convert to RGB

    • Use the ICC profile included with the image as source, else use a fallback CMYK ICC profile
    • Use the default RGB ICC profile as the destination
    • Convert using LCMS

  • Export RGB and convert to CMYK

Same as above… Convert image using LCMS with RGB ICC=>CMYK ICC.

Plug-ins that does this already exists for GIMP (can be used as a reference).

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#3

Import CMYK files (or create one) convert to RGB, edit, then convert back to CMYK all with color profiles is the minimum.
This is a useful workflow for people who are asked to send an CMYK file to an online print shop. But not for printers, that works with plates.

What is really needed CMYK+spot color. For this Gimp needs next to the RGB and Grayscale mode an CMYK mode. Then wee need CMYK in the channels and additional spot colors. All color picker should then show by default CMYK values… But I think that’s already out of scope.

This text should help to get the bigger picture:

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#4

As a G’MIC filter developer, the issue that comes up with CMYK is that it has 4 channels. RGBA also has 4. G’MIC cannot discern what each channel is for. In this sense, CMYKA is easier to manage.

Anyway, don’t forget the A that the user may or may not add and the number of channels issues. :wink: Have fun in writing and submitting your proposal.

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#5

That can be addressed easily. In some of my filters, I have cmyka_mode as option. And yes, I know CMYKA processing is rare.

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#6

@Tobias

What is really needed CMYK+spot color.

As I see it, CMYK would suffice [with some varieties, see below],
since the majority of presses are 4-color ones. 5-color presses are a
more rare breed.

Addendum: After having thought more about this, I believe that @tobias is right.

However — CMYK is not universal. There are five print (plate|film) sequencies
normally in use: YMCK, MYCK, CMYK, KYMC and KCMY, just to complicate things :slight_smile:

Have fun!
Claes in Lund, Sweden

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#7

I know but it would be great if there was a standard way of communicating this information (pass and retrieve colour space and channels) with plugins. Maybe it is just an issue with the G’MIC plugin…

My point is that there is more to be considered. E.g., what @Claes said. :point_down:

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#8

I have to admit that I am on thin ice here, since all we did Back then™
was to send all originals to a reprographic institution, with instructions
to produce a set of films for the printing process in question.

So, what I do not know is whether each individual C-M-Y-K layer freely can
be reshuffled into, say, M-Y-C-K without having to be severely re-calculated…
Probably not, because the inks for different processes are not may not be identical.

See, for instance, this “modern three-colour process” print from a Bookprinter’s Annual 1898-99, where the “C” in CMY does not have the same colour/nuance/hue as the “C” used in CMYK, as shown in the lower right corner.

modern3color
modern3color1250×999 1.44 MB

Have fun!
Claes in Lund, Sweden

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#9

Thank you all for the feedback thus far!

@rodlie Yes, I’ve been hearing several people say CMYK import + export would be a sufficient starting point. Babl has a way to convert back and forth with CMYK built on top of LCMS, so that makes it even easier! I’ll look into the existing plugins like Cyan as well to see their interfaces and features.

@Tobias No worries about out-of-scope comments - it helps me understand what future steps would be as I design this initial support. My hope is that once people start thinking it’s possible to use GIMP for CMYK work, it will encourage more users to note improvements (and more developers will build on it).

@afre, @Reptorian Good points! Babl has a CMYKA format as well, so I’ll keep that in mind. I’m curious how printers handle alpha channels though.

@Claes Huh, I didn’t know that! I was aware of other formats like CMY, but not those variations. That will likely be a “next steps” improvement, but I’ll keep in mind that I can’t assume CMY/K/A is the only color “order”.

#10

@CMYK-Student More in-depth info:
https://www.erpublication.org/published_paper/IJETR021572.pdf

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#11

I’m the author of Cyan, feel free to ask anything :slight_smile:

#12

To those who have not discovered my apology, above:
Verzeihung, @Tobias: you are right.
Spot colors are needed as well, which (at least
in my part of the world) means the Pantone color charts.

Have fun!
Claes in Lund, Sweden

#13

We published a news post detailing planned work on this

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