The GIMP and CMYK

One big challenge I have found in the process of using only free software but at the same time engaging with people in the printing industry is how to create files in the formats and modes they require.

OpenOffice (now LibreOffice) kind of does the job for text. All I needed to do was to make sure that the fonts were embedded so instead of using the shortcut icon, I did File-Export as PDF and in the field PDF producer:
PDF/X-1a:2001.

Graphics are a different matter.

I have so far managed with The GIMP (Gnu Image Manipulation Program).

The GIMP gives most of the functionality that the average photographer needs. However, if you are dealing with industrial press, you are no longer an average user. My printers needed my book cover in CMYK mode. Most probably because this need was not felt by developers, The GIMP does not provide support by default for the CMYK colour-space.

Luckily, one of these unusual users could develop a plug-in that, at least for me, goes all the way to rectifying the situation.

The plug-in converts an RGB image to individual CMYK layers and saves that collection of layers as a CMYK TIFF. In fact it does many more things explained in the developers’ pages: blackfiveservices and yellowmagic

It is important to note that this is experimental software, and if you are trying this, you are doing so at your own risk.

The requirements included the Littlecms library, LibTIFF and ICC profiles. I did not need to download any of this separately, but then I have used Ubuntu and have been downloading a wide range of the software available in the repositories (read Synaptic), so they may have come with some of that software without me noticing.

There are detailed instructions available for other operating systems from the links mentioned above and more at the end.

Using Ubuntu 10.04 (and surely valid for most versions too), these are the steps I followed: First of all, I closed all programs that deal with graphics. Then I clicked on the main menu: System. Then: Administration – Synaptic Package Manager. I did a quick search for “CMYK”, which got me many things as well as what I needed, which was the «gimp-plugin-registry».

Just clicking on it, a whole list of what is there to download appeared below. I had to download the whole archive even though I only wanted the “CMYK Tiff 2 PDF for GIMP”. So I marked it for installation, clicked on apply, and after the computer finished the process, I opened The GIMP again, and with it, the image I needed to send to the printers, cover.xcf, which was, as is usual, in RGB mode.

When I restarted GIMP, new menu items were available.

The first thing to do is to make sure the image is 300dpi. Then, after having worked with many layers, flatten the image. Not merge, as merging will only merge one with another, one by one; to flatten them is to merge all of them together with the background.

The next step was to separate the colours, literally, into layers. To do this, you can bring up the right-button menu. I just went to the normal menu and then to “Image”, then “Separate”, then “Separate (normal)”. Yes, “separate” twice: Muenu – Image – Separate – Separate.

In the dialogue window that ensued, I chose:

Source color space: sRGB

Destination color space: froga CMYK

And I left the rest untouched, unticked, which meant:
Rendering intent: perceptual.
Use bpc algoritm? not ticked.
Options:
preserve pure black: not ticked (I ticked this initially and it did not work; it said it couldn’t handle alpha channels)

Make CMYK pseudo-composite? unticked.

Then I pressed OK.

A new image was created, separate and independent from the original RGB image, with four greyscale layers, named “C”, “M”, “Y”, and “K”. All and each of them looked negative-like. The name given by GIMP to this new file, not yet saved, was cover-CMYK.tif, and on the top bar it had the “modified flag”, which in my case is an asterisk next to the name that indicates that I have made a change on the image, so the image in the RAM memory I’m working on is different (i.e. it has the latest change) from the image in my hard drive (which it hasn’t). Every time I save, this flag goes away because I am recording the file I have on the RAM memory into the hard drive.

Then I needed to save this new, 4-negative-like-layers-image, in order to send it to the press. On this new file, cover-CMYK.tif, I selected from the menu: “Image->Separate->Save…”. I changed the name to cover.tiff as it was the extension required; it seemed to work with my printers.

This saving does not clear the image’s modified flag, because what I am saving is a modified version (tiff) of the file I have on RAM, not the file itself (xcf). When closing I was discarding an image that had not been saved. It doesn’t matter; if I want to modify the image I’d need to do it on the RGB version and do the whole process again anyway. This may change with further development of the plug-in. For now, I just saved this separated image as a .tiff file and the printers have managed to produce a fine book cover with it.

Possibly useful links:
scribus
GIMP glossary
forum
GIMP talk
Digital Photography