CMYK Colour Separation using the Gimp

Selby Markham writes a step-by-step to CMYK colour separation for gum bichromate printing – but not with Photoshop – he used the freeware Gimp.

Writer and illustrations / Selby Markham

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CMYK colour separation gives Gum Print workers an extremely high level of control over image output. Through Gimp, the free image manipulation application, you can quite easily do colour separation.

The shortcoming is that you need a plug-in for Gimp to be able to do this.

The challenge of a CMYK separation plug-in was taken up in the Gimp community and a plug-in called Separate and then its big brother Separate+ were developed. The plug-in gives you your four negatives through an extremely simple 3-click sequence.

The Gum print shown here, and the one later in the article, used the negatives created by Gimp’s Separate+ And they are my first ever CMYK prints – but actually CMY as I did not print the K layer.

But Gimp being a non-commercial, ‘Geek’ application, the plug-in installation requires some effort. The purpose of this article is to help you work through the installation process.

In what follows I am assuming that you are using Windows. But the software can be implemented for Mac and Unix machines. The main README file for the software tells you how.

What follows also assumes a level of expertise in using Windows. This includes:

1. Downloading files
2. Extracting ‘zip’ files
3. Finding paths through the folders to copy and to paste files
4. Some understanding of opening different file types
5. Right clicking the mouse to have actions take place
6. Patience with computer processes

Installing Separate+

First, you will need Gimp 2.4 or greater.

If you have Gimp installed, at start up it will display the version number. Otherwise you can check the Help -> About menus when it is running to see what version you have.

The current version of Gimp can be downloaded from
http://www.gimp.org/downloads/.

The download option to select is:

Download GIMP 2.6.11 – Installer for Windows XP SP2 or later

Installation is quite easy with this installer.

Separate+ is downloaded from:

https://osdn.net/projects/separate-plus/

What you can download from this page is:

1
Under the Documents heading you can download from
Readme files
Separate and separate import plug-in (This file is an untyped file so you will have to tell the system to open this with WordPad or Word.)

I suggest you print it out. Have a look at it and see if it makes sense for you. Don’t worry if it doesn’t make sense. Simply follow the process I am describing. There is some information that is general but most is relatively technical. The materials in the first 5 sections are for the more general user.

Please understand that most of the developers of Separate+ are not native English speakers so some of the language can be a bit difficult.

2Under the Downloads heading you download only the following file:

separate+-0.5.8+lcms2_win32_bin.zip

This Zip file should automatically download from SourceForge (a download site).

Extracting the files

You may need a zip file extraction program, depending on which version of Windows you are using.

Extract the files from the Zip archive. They are likely to be saved in a folder structure like the picture to the right.

When you open the bottom-most folder there will be three files, like on the image to the right.

Keep this section of the Explorer folders list open. You will need to come back here shortly.

Finding the plug-in folder

Now you have to find out where Gimp is storing its Plug-Ins. Open Gimp and from the main menu bar do the following choices:

Edit –> Preferences

This will give you the Preferences dialog. In this, you choose:

Folders –> Plug-Ins

The right hand panel (Plug-In Folders) shows where the plug-ins are held.

• If there is more than one folder (as is the case here) click on any one of them. Which one you use does not matter.
• Then click on the document icon. This is shown here with a red box around it. This will cause the address of the plug-in folder to be highlighted in the box that is surrounded by the red ellipse.
• Right click on the highlighted text in that box and select Copy from the drop-down menu.
• Now go to Windows Explorer and click on the Address box. The current address will go blue.
• Right click over the Address box and select Paste. The plug-in address will be copied to the Explorer Address box.
• Press Enter. You are now taken to the plug-ins folder.
• Keep this section of the Explorer folders open.

Go back to the folder that has the three Separate+ files that were extracted from the Download. Copy these three files to the clipboard:

• Hold the Shift key down and left-click on each file name.
• While the three files are highlighted, right click over them.
• Select Copy from the drop-down menu.

Go back to the Plug-In folder you found for Gimp.

• Right click on that folder.
• Select the Paste item from the drop-down menu.

The Separate+ files are now in the plug-in folder!

Almost there: the Separate+ plug-in is just about ready. But there is one more step:

Getting ICC Profiles

In order for Separate+ to decide which CMYK separation will take place you must have ICC (International Color Consortium) Profiles. These are downloaded from Adobe at:

http://www.adobe.com/support/downloads/iccprofiles/iccprofiles_win.html

Make the following selections on this Web site:

• First screen: Select the ‘ICC profile download for End Users’ option.
• Next screen: ‘Accept’ conditions at the bottom of the page – a long way down.
• On the next screen, scroll down to ‘Proceed to Download’ and click on it.
• Then, on following screen, click ‘Download Now’.

Extract the.zip file that has been downloaded. In the extracted folder, go to the CMYK folder (image to the right).

Open the CMYK folder, then:

• Hold the shift key down and left-click on each file name.
• While the three files are highlighted, right click over them.
• Select Install profile from the drop-down menu.

Note: Nothing appears to happen as there is no message issued by Windows. But the profiles should have been installed after ‘Install profile’ click.

Gimp is now ready to do CMYK Separation

** If Gimp is open then close it because Gimp has to now load Separation+.

1. Start Gimp.
2. Load a colour image.
3. Go to ‘Image’ menu bar option. The last item in the drop-down menu will be ‘Separate’. Click on this and choose ‘Separate’ from the sub-menu.
4. You will be presented with a dialog that should look like the image to the right.
5. Click on ‘OK’ – don’t worry about the options because that is for future angst.

Gimp will create a new image box and it will contain the four images as the C,M,Y and K layers.

These can be selected and saved as individual images or the whole set can be saved as a Gimp XCF image which retains the four images for future action – but only within Gimp.

Note on the Separate dialog and the Destination Color Space

The Destination Color Space in the Separate dialog is based on the ICC Profiles you downloaded. I have found that there can be a great deal of variation between the CYMK negatives for any given image when different profiles are used. There seems to be a tendency for the ‘Uncoated’ profiles to have more contrast.

The image of a wild poppy shown here was produced from the ‘Japan Color 2001 Coated’ profile. It is very low in colour contrast. This gives a lovely soft, watercolour effect but you would need to use another profile if you wanted a stronger image. But this image did not include the K element.

The way I do CMYK separation

This approach is a bit primitive and it would be nice to have a Gimp and/or gum print aficionado give a better process.

1Load your image
– note that digital printing puts the ‘emulsion’ on the top of the transparency. If you prefer to have the ‘emulsion’ in contact with the paper then you might want to do:
Image –> Mode –> Flip horizontally

2Add border

• Using Filters->Décor->Add border.
• Change the border colour to black.
• Make the width of the border about 90 pixels.
• Set Delta value in color to 1.
• Click OK.

The image file will now include a layer with a black border.

3Add crosses
Select the Pencil tool and add a white cross to the Top Left Corner. Do the same on the Bottom Right Corner. This will give you common alignment markers across all 4 negatives.

4Add CMYK
Go to the Text tool and type in C M Y K somewhere along the top within the border – make it as large as possible and in bold.

5Merge layers
Select the Text layer and do a Layers->Merge down.
Do another Layers->Merge down.
Now you have a single image with a border, markers and text identifiers to separate.

6Separate image
Click Image->Separate->Separate then click OK in the Separate+ choices dialog.
You will now have a new image box with the C,M,Y and K images but each with an off-white border. Each also has alignment markers and the C M Y K text.

7Save images

Start with the C layer. Copy it using Copy visible and then paste it as a New image.
On the new image, use the Brush tool to remove the M Y K letters.
Save image as a.jpg file: eg ‘Image C.jpg’.
Go back to the CMYK separated image and copy, paste and save the other three images. To process each image:

• Go to the Layers toolbox.
• Click off the ‘Eye’ next to the layer you have just copied and saved.
• The Visible image will now be the one you want to Copy visable.
• Do the Paste as new image, editing of text and Save as in 6.

Now you have split the four layers into four independent files. They are no longer reliant upon having Gimp on the computer. So you can put them onto your USB stick and take them to the agency where you are having transparencies printed.

Gimp might not be the most intuitive application but if you use it for a while it works well. People are out there developing plug-ins to do good things. And it is FREE.

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Selby Markham is a retired psychologist who started with a box Brownie and now uses a range of formats from 16mm to 4×5. As a member of the Melbourne Camera Club he has become involved in developing and transferring skills in Alternative processes.