A GIMP frequency separation plug-in: a simple, easy approach

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

Frequency separation is essential for photo retouching. It separates tone (light and dark) and colour information from the details of an image so you can adjust details and colours 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 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, there’s no simple out of the box frequency separation option. That’s why I created this GIMP frequency separation plug-in.

The historical GIMP wavelet decomposition approach is counter-intuitive and difficult

The built-in GIMP “wavelet decompose” filter separates the image detail into anywhere from 1 to 7 separate layers called ”scales” for some reason. As far as I can tell, it doesn’t generate a separate blurred colour/tone layer.

I found GIMP wavelet decomposition hard to understand and work with. If you’re interested, I share more detailed thoughts about this at the end of this post.

For whoever likes wavelet decomposition, they can continue to use it. For the rest of us, good news! We now have a plug-in that makes frequency separation in GIMP work like that standard approach I mentioned above. It’s easy to install and use.

How this frequency separation plug-in works

This “Frequency separation group” plug-in for GIMP creates a new group labelled “Frequency separation”. That group contains two layers, “Texture” and “Colour and tone”. 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 any tools to work on each layer separately: clone or heal, brush, whatever. Any changes to the texture layer will only affect texture, and changes to the colour/tone layer will 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.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 seeThe GIMP dodge and burn plug-in creates a single layer filled with 50% grey and set to soft light mode. a subfolder named “frequency-separation-group”. 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. You should be able to right click on the .py file and set the permission to allow it as an executable (see below for screenshot). Or you could set it in your terminal app with the command chmod +x frequency-separation-group-v3.py. (See below for a screenshot.)
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

The GIMP instructions I found online conflicted with my reality. Everywhere I looked, the instructions for adding scripts to GIMP said to open the Preferences dialog and look for the relevant folder(s) where GIMP was supposed to be configured to look for plug-ins, and install a new plug-in .py file in that folder. But either those folders didn’t exist, or GIMP ignored them.

Furthermore in some versions of GIMP (eg Windows, 2.99+) you have to 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.

This appears to be a relatively recent change to how GIMP works, and it’s not obvious. Since that’s the future of GIMP, we’ll just take it as a requirement from now going forward.

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.

4 reasons why I abandoned wavelet decomposition in GIMP

It doesn’t make sense

The user-unfriendly terminology (“decompose”, “scales” etc) doesn’t explain what it does or how to use it.

It’s hard to work with

Multiple layers with different degrees of detail make it puzzling to figure out which layer you need to work on to remove or enhance specific details.

I’m a reasonably intelligent person, and I have a lot of experience with IT and software development. But I couldn’t get my head around wavelet decomposition. I gave up trying to make it work since I know there’s an easier way to do the job.

It causes file bloat

Every time you add a layer in GIMP, you increase the file size. Greg Benz explains why that matters.

It’s over-complicated

There’s no need for all those extra layers and file bloat, when a simpler approach works better. Having all your detail information in a single layer allows you to see details in context and edit them more easily.

Minor note about how this plug-in’s dialogs look and feel

I used the GIMP built-in scripting dialogs to ensure that the plug-in should install and work properly in any modern version of GIMP, on any OS. GIMP’s built-in dialog options are ugly, but they get the job done without any fuss.

I wanted to avoid the need to install additional packages, libraries, or anything that GIMP doesn’t install by default. Some operating systems make extra programming options available but others don’t. Those other options might deliver a prettier look and feel, but we can’t be 100% sure they’re readily available on everyone’s devices. Better safe than sorry!

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?

Download the frequency separation plug-in: frequency-separation-group.zip. Copy the downloaded file to where GIMP looks for plug-ins and extract the contents into a separate subfolder. Register the plug-in with GIMP: close and re-open GIMP. Check to make sure the Layer > Frequency separation group menu item is visible.