A GIMP frequency separation plug-in: simple, easy, UPDATED

I’ve updated my GIMP frequency separation plug-in for GIMP version 3

GIMP 3.x brings a lot of new goodness to the party: non-destructive editing, multi-layer organisation and editing, improved colour management and file handling. Under the surface, GIMP gives plug-in developers entirely new technologies to work with. That means GIMP plug-ins need to be completely rewritten.

I’ve taken that opportunity to update my frequency separation plug-in for GIMP 3.x. Of course, the previous version for GIMP 2.x still works just fine if you want to continue using GIMP 2.x.

In case you’re new to the concept of frequency separation, here’s a quick overview:

Why do we need a new approach to GIMP frequency separation?

Frequency separation is essential for photo retouching. It separates texture (high frequency) and colour/tone (low frequency) information from the details of an image so you can adjust detail and colour independently of one another.

Here’s a brief tutorial on how to use my frequency separation plug-in, and general tips about the process:

I learned a lot from Aaron Nace at PHLEARN and his Photoshop-oriented how-to introduction to frequency separation. The technique isn’t just for Photoshop, it works in GIMP too. Just the specific steps and settings are different in GIMP.

The industry standard two-layer approach to frequency separation works great

The most common approach to frequency separation is to create two layers dedicated to frequency separation in an image that you’re editing, where one layer contains the detail and texture information, and the other contains a blurred version of the image where you work only on colour and tone.

This standard approach is easy to understand and work with. Hundreds of thousands (millions?) of creative people use it every day in multiple different image editing environments, Photoshop among the most common.

But in GIMP, the out of the box frequency separation (“wavelet decomposition”) is overly complicated and hard to understand. That’s why I created this GIMP frequency separation plug-in. (If you want to read more about why I found wavelet decomposition impossible to use, click here: 4 reasons why I abandoned wavelet decomposition in GIMP.)

How this frequency separation plug-in works

This frequency separation plug-in for GIMP creates a new group labelled “Frequency separation”. That group contains two layers, “Texture” and “Colour and tone”.

You’ll find the plugin in GIMP’s Layer menu:

It prompts you to add a Gaussian blur to the colour and tone layer, then ends with a message explaining next steps to edit your image using those layers.

Use different tools to work on each layer separately: clone or heal on the texture layer, and a brush or other colour/tone tool on the colour and tone layer. Changes to the texture layer only affect texture, and changes to the colour/tone layer only affect colour and tone.

How to install this GIMP frequency separation plug-in

(If you run into any problems with any of the steps below, please let me know in the comments below.)

1. Download the plug-in

Download the GIMP frequency separation plug-in as a ZIP file. You’ll extract it as described below in the next section:

frequency-separation-group-v3.zip

2. Copy the downloaded file to where GIMP looks for plug-ins

This is something you have to do for every plug-in you add to GIMP. I discovered that the only reliable way is as follows.

Find where GIMP looks for plug-ins:

1. Open GIMP if it’s not already open and select Edit > Preferences in the GIMP menu.
2. In the Preferences dialog, scroll down the list on the left until you find Folders, then click on the expand icon in front of the word to expand that list.
3. Scroll down the list of folders and select Plug-ins.
4. Still in the Preferences dialog, on the Plug-in Folders panel on the right, select the plug-in folder that’s listed for your user name.
5. Click on the “Show file location in the file manager” to open your computer’s file manager.
6. Copy the .ZIP file you downloaded into that folder and extract it there. You should see a subfolder named “frequency-separation-group-v3”. The plug-in is inside that folder (see the note below for why it should be in its own subfolder). Don’t move it, just leave it in that subfolder.
7. Files downloaded from the internet or transferred across systems may not retain their “executable” flag, especially on UNIX-based systems like Linux and macOS. The .py file needs the correct permission to be executed as a script. It’s a quirk of the different operating systems’ security models.
   • Linux:
     You should be able to right click on the .py file and set the permission to allow it as an executable (see below for an example screenshot). Or you could set it in your Terminal app with the command chmod +x [PATH TO WHERE YOU EXTRACTED THE PLUG-IN FOLDER]/frequency-separation-group-v3/frequency-separation-group-v3.py (GIMP 2) chmod +x [PATH TO WHERE YOU EXTRACTED THE PLUG-IN FOLDER]/frequency-separation-group-v3/frequency-separation-group-v3.py (GIMP 3). For example, with GIMP 3 I extracted the plug-in folder into a subfolder of Documents, so the command would be chmod +x "Documents/GIMP plug-ins/frequency-separation-group-v3/frequency-separation-group-v3.py".
   • macOS:
     Apparently, in macOS, there’s no visual way to mark a file as executable. You have to do it via Terminal. It’s not difficult, here’s how:
     1. Quit GIMP.
     2. Open the Terminal app.
     3. Enter this command: chmod +x "[PATH TO WHERE YOU EXTRACTED THE PLUG-IN FOLDER]/frequency-separation-group-v3/frequency-separation-group-v3.py" (the quotation marks are important if you have any spaces in your path to the .py file). For example, I extracted the plug-in folder into a subfolder of Documents, so the command would be chmod +x "Documents/GIMP plug-ins/frequency-separation-group-v3/frequency-separation-group-v3.py".
     4. Press Enter.
     5. If you don’t see any error messages, that’s good.
     6. Open GIMP.Check the Layer menu – if the dodge and burn plugin menu item is there, success!
8. Switch back to the GIMP Preferences dialog, and select the “Add a new folder” icon, then click on the icon for “Open a file selector to browse your folders” to … open a file selector to browse your folders.
9. In the file selector, navigate to the frequency-separation-group subfolder you added already, select it, and click OK. You should see something like this (a Windows screenshot but the same principle holds true for Linux and macOS):

Click OK in the Preferences dialog to close it. When you close the Preferences dialog you’ll be prompted to restart GIMP for the change to take effect.

3. Register the plug-in with GIMP

Now you have to register the plug-in with GIMP: just close and re-open GIMP.

If all has gone according to plan, you should find a menu entry called Layer > Frequency separation group…. If not, please leave a comment below.

4. Optional: create a keyboard shortcut

If you want to assign a keyboard shortcut to this GIMP frequency separation function, go right ahead.

Why it’s important to follow these steps

GIMP version 3.x requires that every plug-in must live in a folder with exactly the same name as the plug-in file. So for example, you add the subfolder where you’ve saved the plugin file to the list of folders where GIMP looks for plugins, and that folder needs to be named exactly the same as the plug-in with that subfolder.

So for this situation, when you unzip the plug-in download, you should see a folder named “frequency-separation-group-v3” and that folder should contain a single file, “frequency-separation-group-v3.py”.

This is an example of how working with free software isn’t entirely free—you have to do some of the work that other systems automate, and put your thinking cap on. Bear in mind that GIMP is developed and maintained by volunteers who donate their time and talents as and when they can. So sometimes documentation lags behind development.

Now you can take advantage of simplified frequency separation in GIMP

Please let me know if you have any questions or thoughts you want to share in the comments below.

FAQ

Why do I need the GIMP frequency separation plug-in?

The GIMP frequency separation plug-in makes it easy to do frequency separation retouching. Frequency separation allows you to edit details and colour/tone separately. It’s a simpler alternative to GIMP’s built-in “wavelet decompose” filter.

What does the GIMP frequency separation plug-in do?

The GIMP frequency separation plug-in creates two layers in a dedicated frequency separation group. One layer contains detail (or high) frequency information, the other layer contains colour and tone (or low) frequency information. You can edit details separately from colour and tone.

How do I install the frequency separation plug-in for GIMP version 3?

Download the frequency separation plug-in: frequency-separation-group-v3.zip. Copy the downloaded file to where GIMP looks for plug-ins and extract the contents into that folder. To register the plug-in with GIMP, close and re-open GIMP. Check to make sure the Layer > Frequency separation group menu item is visible.