Preping non-CYMK risograph prints without Adobe
August 6, 2022 8:14 AM   Subscribe

I'm working on a zine which I intend to get printed on a risograph. I'm not planning on using CYMK colors, and I don't use Adobe products (mostly I've been using Scribus and GIMP thus far). Does anyone have any suggestions for how to do color separation and previewing?

I have the color swatches as a ASE file, as well as hex codes (which are obviously just a approximation, but probably good enough?)

I want to be able to do two things:
  • Decompose a RGB image into N different layers of non-CYMK color. This will obviously be lossy, but that's OK.
  • Take N different greyscale PDFs or images, and preview what they will look like printed with different colors. I understand that this will always be a little inaccurate, I'm just hoping for something better than using my imagination to check that I got alignment/etc right.
Does anyone have suggestions of workflows here? Ideally that could run on Linux, but that's not a hard requirement. Not willing to use Adobe products, though.
posted by wesleyac to Computers & Internet (8 answers total) 5 users marked this as a favorite
Decomposing an image into an arbitrary color space is not what scientist call "a trivial problem"

Maybe some crazy person has made a plugin or script for GIMP that does this, but if they have, I haven't come across it.

The next part might be a little easier, though I suspect working with an RGB color model is not going to go very well. An RGB color model is designed to work with computer monitors where you are adding different colors of light together to form the spectrum of your color, and when you're printing something, you are adding pigments together to subtract light out of (presumably) white light to form the spectrum of your color.

So you need to make sure your program is doing the right sort of mixing approximation. Which probably means you'll want to be working with a program that's using CMYK model. (Though if there's a color space that the image is going to be scanning your image into to tell the risograph what to print, there might be support for that particular color model somewhere! I am not familiar with how risographs work, except that I'm assuming it prints with multiple colors of non-CMYK ink.)

I haven't used GIMP in a while, and last I saw it didn't have native CMYK support, but I do use Krita, a nice open source art program that I run on Linux natively, and it does have native CMYK color space support.

What can be done in Krita is to separate an image into the components of CMYK colorspace, recolor those layers, and see what they look when combined with CMYK color mixing. While still probably not very accurate to how the pigments you actually use will end up mixing, will at least give something in the right direction.

My process would be.
- Open the RGB image as a single layer
- Separate out the CMYK colors into layers usingImage->Separate Image
- Convert the white in each layer to an alpha channel using Filter->Colors->Color To Alpha
- Set all the layers to additive color mixing and add a new layer of pure white underneath
- Lock transparency on the "CMYK" layers
And now you play around using flood fill with fill entire selection checked to try out different colors of "ink" for each of the channels and see how they might look.

Now pigment mixing is complicated, so this isn't going to be the most accurate, especially if your inks are thick enough that it strongly matters what order they get applied in, but it might help with getting some good ideas to try.
posted by Zalzidrax at 9:43 AM on August 6, 2022 [2 favorites]

I'm a bit confused about what you mean by decomposing an RGB image into non-CMYK colour. Risographs are extremely limited in the colours they offer, they only have access to about 12-20 ink colours max, which you then apply in various percentages. This is the same as CMYK but with that you specifically have cyan, magenta, yellow, and black. With riso you could choose yellow, teal, and orange, for example. Typically riso printers charge by the ink and there's a limit to how many inks you can put on a single area before it gets saturated. If you have an area with two or three inks in various percentages you can mix other colours, but again, very limited. RGB (ie hex codes) has 256 colours, and can't be directly converted to risograph inks at all UNLESS the printer has said they will handle the conversion from RGB > Riso inks for you. In which case, just send them your files as-is and ask for them to make you a preview. But if you want to do it yourself, here's some more information from a risograph printing service. Download that PDF called "Risotto print bible" and read it thoroughly. You're doing an extremely complicated and somewhat obscure thing here.

If the printer won't do it for you you'll need to supply each layer for each ink as a separate file, using black from 0-100% on each. See page 17 in that PDF. You'll have to make a preview file then prep the print files differently. I am not familiar with Scribus or GIMP so I'm not sure how you'd go about that in these programs. I do know that GIMP is a raster image editor, which means you've already made it way harder for yourself than if you'd used vector images (equivalent is Inkscape I think). For 4 riso inks use the process above, but for any other number — for example, 2 or 3 inks — you will have to cut out each area of different colour, copy it onto the various corresponding ink layers based on what inks will need to be there, and then fill it with the percentage of transparency you want that ink to have, but in black (ie a 20% transparency on the teal layer and a 100% transparency on the yellow layer to make a green). Even with Adobe products there's no automatic way to do this, risograph is a somewhat obscure process and it would involve a bunch of fiddling no matter what program you use. I'd use layer masks to do this in Photoshop, hopefully GIMP has something equivalent. I'd google "screenprinting color separations GIMP" to see if you can find a tutorial on it. Screenprinting file prep is a very similar process and less obscure than riso.

Also, because you're doing a multi-page bound booklet you'll need to prep each page this way, but you'll probably also have to figure out the spreads and bleeds and alignment to get the 2-sided printing to line up based on how the spreads fit onto an A4 page. Unless the printer has said they'll do that for you! I'm not going to explain that because I would be here all day, hopefully you already know about that part or it's being done for you. Some info on that is also in the PDF I linked.
posted by 100kb at 10:29 AM on August 6, 2022 [5 favorites]

Linux colour separation is fairly terrible even for CMYK. Scribus will do spot colour, kinda, and CMYK at a level roughly where Adobe was 25 years ago. The graphics tool with an ableist slur for a name is not there yet: it still does everything internally in RGB(A), so your CMYK separations will get mashed on import. Inkscape doesn't even try to support CMYK.

Riso, however, is weird magic. While there are attempts at open source palette management for Riso workflows, none of that has made it to Linux. If you have access to a Mac, there's a free app that claims to be able to make Riso stencils/separations from input: Spectrolite
posted by scruss at 11:41 AM on August 6, 2022 [1 favorite]

I've used Krita to create and prep files for riso printing and it's worked well, it's pretty similar to Photoshop but its layer masking/grouping system is a lot more robust which is useful for riso stuff. I don't have too much experience with converting preexisting RGB images to riso separations, but I think Zalzidrax's method should work well although I usually use the "Multiply" blend mode on the ink layers to preview the final image (it's not technically physically accurate because there will be differences depending on the transparency of the ink/layering order, but in my experience it comes pretty close to the final).

Have fun though, pondering the colorspace conversions between riso inks and RGB/CMYK can be pretty mind-bending but I think the fun of riso is in the imperfections - you will probably not get a 100% accurate replication of the original RGB image but it should look cool!
posted by limnerent at 12:18 PM on August 6, 2022 [1 favorite]

Response by poster: Thanks for this advice! It's all useful. To clarify what I mean about non-CYMK — I am planning to use a combination of inks that does not include all of CYMK, but does have other colors (for instance, metallic gold, which maybe gets you a tiny bit of the space that Y does but not really).

I'm making/picking images that are primarily composed of the same color pallets as my inks (just, like, visually). Those are RGB because most stuff on the computer is RGB. I want to take that, and convert it into greyscale images for each ink (which is what the riso place wants) — I understand that there will be large sections of the RGB colorspace that are unrepresentable with the inks I've picked, and plan to work around that — I think that this sort of limitation often produces a interesting aesthetic, anyways.

In principle, it seems not that hard to make something that would work, by printing some color mixing tests, then working backwards from that to figure out a function that maps between the mix of inks and the RGB color you get from scanning the actual print. Once you have a approximation of that function, it should just be a pretty simple matter of going through each pixel in the input image and solving the function for whatever mix of inks gets the closest RGB value (or really you'd want to convert it to HSV or something first so there's a nicer perceptual meaning of colors being "closer" but you get the idea). This project seems to just use multiplicative blending on a RGB value for each ink and it works well enough, which is a little surprising to me, but definitely makes things simple.

I had hoped that there's some tool out there that would just do this — I don't think it's conceptually hard — but I haven't found one. It looks like Spectrolite is maybe that, though? I'll see if I can get it running.
posted by wesleyac at 1:01 PM on August 6, 2022

Apparently darktable will do chromakey stuff using "parametric masking" or something. Tutorials may be more helpful than the basic documentation.
posted by sebastienbailard at 6:06 PM on August 6, 2022

This is easy with Photoshop altho not obvious the first time you do it. Picking a pantone or other color and using in the image will not give you a plate with that color. It will split that color into CMYK. You make a plate in that color and only draw on that plate when you want that color. When you separate, you can turn off the CMYK colors you're not using.

You can still print on an ink-jet printer a draft that will approximate your inks with the CMYK. But if you look within Photoshop at the color plates you're not using, they will be blank.

I didn't describe that very well. It's been a long while since I did it but I did a lot of two color design from Photoshop/Illustrator printed by professional printers for about ten years.
posted by tmdonahue at 5:44 AM on August 7, 2022

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