If you are already familiar with shooting the stars, then you’ll know that most of the imaging magic happens in post processing. You’ll also likely know that the vast majority of popular online tutorials will use Adobe Photoshop which is both expensive and subscription only, with monthly photography plans starting from $14.29AUD upwards. Being neither popular nor prepared to spend that sort of cash on subscription based software, I find myself free to explore the alternatives. As far as open source software goes, Gnu Image Manipulation Program (GIMP) is enormously powerful and supports layers which is critical for common processing tasks such as star trails and light painting. Unfortunately though, GIMP has no functionality via the GUI as of the time of writing this article which allows you to modify layer attributes in bulk or as a selection of layers. In this article we’ll take a look at one way in which the extensibility of GIMP can assist with this specific issue.
GIMP is a beast of a program. Not only in functions accessible via the GUI as those of us who have used photo editing tools would be used to, but also in its extensibility. We won’t go any further on the topic as GIMP scripting is an art form in itself. All you need to know is that as soon as you have downloaded and installed GIMP, you should head over to Ofnuts’ Gimp Tools repository to get these freely available scripts. The one we are specifically interested in for this article is ofn-layer-modes.zip. We’ll be installing this script using GIMP on Windows 10 in this article.
Install Ofnuts’ Layer Modes script
With the ofn-layer-modes script downloaded, extract it somewhere handy and launch GIMP. Head to the Edit drop down menu, then Preferences and then expand the Folders node and select Plug-ins. In the right pane, select the folder for your users AppData and then click on the file cabinet icon in the far right to open this location in Windows Explorer. Simply drop in the extracted ofn-layer-modes.py Python script extracted earlier and restart GIMP.
Import you images as layers
With GIMP restarted, head to the File menu and select Open as Layers as shown in the image to the right. Simply browse to and then select the images you wish to use in your composition. In this example I will be using 99 images taken as part of a full nights worth of time star trails.
It may take some time to import all your images, depending on number and format. Once they have been imported you will see each image listed in the layers Dockable Dialog.
This is where, in my opinion, GIMP comes a little bit unstuck. If you select a layer and press Control A as you normally would to select all, you’ll notice nothing happens. Likewise if you hold down Shift or Ctrl then try and make your multi selection with the mouse. By default all layers will be imported using the Normal mode.
In the preview window the only image to be visible when using Normal mode will be the one at the top of your layer stack, such as the image shown below.
Bulk editing the layer mode
For star trails we don’t want Normal mode, we’ll want something like the Lighten Only mode. The process from here is pretty straight forward, simply make sure all the layers you wish to set to lighten are visible (have the eye icon next to the layer).
Now head to the Image drop down and right down the bottom you should see a menu item called Change mode of visible layers. This menu item represents the ofn-layer-mode script we loaded earlier. If you do not see this then something has gone wrong and you’ll need to do some troubleshooting. This will be beyond the scope of this article however.
Once you select this menu item you should be presented with the following interface, which while basic, does the job.
Simply hit the arrow on the Mode drop down menu and change the mode from Normal to Lighten Only, as shown above. With this done, hit the OK button and you are done. Sit back and watch GIMP re-render your preview into something like this.
This is still only a starting point, you’ll still need to go through the arduous task of sifting down through each layer and creating layer masks to remove jet trails, hot spots or any other blemish that impacts the over all image quality and continuity. This is true of Photoshop as well. Still, no doubt in my mind that this ability to bulk edit layers in GIMP hugely accelerates the processing speed.