In May of this year (2018), Davies Media Design created a tutorial on how to edit photos using a luminosity mask in GIMP 2.10. This technique allows you to edit the light, lighter, and lightest parts of an image, the dark, darker, and darkest parts of an image, and the three different levels of mid-tones in an image separately. In other words, you can make image adjustments that will only affect the aforementioned portions of the image – like changing the colors of the lightest parts of the image using the Color Balance tool or increasing the shadows of the darkest parts of an image using the Shadows-Highlights tool. For more information on how Luminosity Masks work and why they are useful, I recommend checking out the tutorial.
Despite this method being useful and giving you more control over your image, it can be a bit of a time consuming process as it involves duplicating and desaturating your original image, selecting areas of certain luminance in your image and creating custom channels from those areas, and then applying each new custom channel to a new layer mask – which creates 9 duplicated versions of your original image plus 9 layer masks.
That above paragraph was not designed to actually tell you how this process is completed – just to give you an idea of the complexity of the process.
However, a developer and GIMPer (the term we use to describe subscribers to the Davies Media Design GIMP YouTube channel) named Kevin Thornton created a custom plugin using some coding (he created it using the Python coding language) to allow us GIMP users to perform adding Luminosity Masks to image compositions simply by selecting the command from the Filters Menu. This plugin is based in-part on a script that was originally created by Pat David, but Kevin’s script incorporates the task of automatically labeling each layer, adding the appropriate corresponding layer mask to each layer, and grouping the layers into Layer Groups to keep the composition more organized.
In this quick tutorial, I show you how to download Kevin’s plugin and install it in versions GIMP 2.10 and newer!
Step 1: Download the “Luminosity Mask Setup” Plugin
Kevin has made his Luminosity Mask Setup plugin available to download for free on GitHub. To download it, start by visiting this link. This will take you to Kevin’s gimp_scripts page under the tab “Code.”
You will see a bright green button labeled “Clone or Download” (denoted by the red arrow in the image above). Click on this to bring up the “Clone with HTTPS” dialogue box. From here, you will see another button labeled “Download ZIP” (denoted by the pink arrow in the image above). Click this button to download a ZIP file of the plugin to your computer.
The ZIP file comes with all of the files shown in the above screenshot (COPYING, LICENSE, README.md and setup_luminosity_mask.py – this last file is the actual plugin file).
Step 2: Extract the Plugin Files
Once you’ve downloaded the ZIP file to your computer, locate it in the folder where it was downloaded (most likely your “Downloads” folder, unless you have custom settings on your browser). The easiest way to locate the download file is to click on the arrow next to the ZIP file name after it has finished downloading and go to “Show in Folder” (as depicted in the image above).
Once you have located the ZIP file, right click on it and go to “Extract All.” Windows will ask you where you want to unpack the file in the “Extract Compressed (Zipped) Folders” dialogue window.
In the photo above, the destination where I have chosen to extract my file is on my C drive, in the “Users” folder, then in my main user folder (which I censored for privacy), then my “Downloads” folder. The final folder will be called “gimp_scripts-master.” I’ll go ahead and click the “Extract” button to decompress the folder to this location.
Step 3: Manually Add the Plugin to GIMP (Don’t Worry, It’s Easy)
Once the folder has been extracted, locate it and enter the folder (in other words, click on the “gimp_scripts-master” folder – your file manager should automatically pop up inside of this folder after the file has been downloaded). Once in this file, you will see another folder with the same name (“gimp_scripts-master”) as shown in the photo above. Enter that folder.
Inside this folder, you will see 4 files. The only file we will need is the one titled “setup_luminosity_mask.py” – this is the plugin. Copy this file by clicking on it and hitting ctrl+c on your keyboard. Or, you can right click on the file and go to “Copy” (as shown in the image above).
You’ll now need to navigate over to your GIMP Plugins folder for GIMP 2.10. Your Plugins will likely be in the same location as mine, which were in the following path: C:\Program Files\GIMP 2\lib\gimp\2.0\plug-ins (you can see this path in the image above – each backslash represents a new folder you’ll need to enter). Once you are in the Plugins folder, find a blank space, right click and go to “Paste” (as demonstrated in the photo above). This will paste the script in your Plugins folder.
You may receive a message asking for administrative permission. If you are an administrative user on your computer, you’ll just need to hit the “Continue” button.
The plugin will now be in GIMP!
Step 4: Close GIMP 2.10, Then Re-open It & Find Your New Plugin!
You’ll need to exit out of GIMP if you currently have it open in order to have the new changes take effect.
Once you’ve reopened GIMP, open up the image you would like to add the Luminosity Mask to. Then, while clicked on your main image layer, go to Filters>Generic>Luminosity Mask Setup.
After you run the plugin, you should now see all of the various layer groups for each luminosity range in the image (lights, midtones, darks) and the corresponding layers within each range. The layers will also have their own layer masks, and will be labeled based on what part of the image you will be editing by edited that layer. For example, I have a red arrow pointing at the “Lighter” layer of the image, which is within the “Lights” layer group and labeled “LL” to denote that it is the second lightest layer you can edit. The “L” layer is the “Lights,” and the “LLL” layer is the “Lightest.”
If you want to see how this technique can now be applied, I recommend checking out my GIMP 2.10 Tutorial: Using Luminosity Masks tutorial on our YouTube channel.
That’s it for this tutorial! If you liked it, you can check out my other GIMP Help Articles or GIMP Video Tutorials.