Composite Star Trails in GIMP Using Lighten Only

Creating star trail photographs involves combining a sequence of individual long-exposure night sky shots into a single, striking composite image. In GIMP, the most efficient method for stacking these exposures is by utilizing the "Lighten Only" blend mode. This mode works by comparing every overlapping pixel across your image layers and displaying only the brightest value, allowing the moving points of light from stars to accumulate seamlessly across the dark sky without overexposing the static background.

Step 1: Open Your Images as Layers

  1. Launch GIMP.
  2. Go to the top menu and select File > Open as Layers...
  3. Navigate to the folder containing your star photos, select all the sequential images you wish to stack, and click Open.
  4. Allow GIMP to load all the files into the Layers panel. Depending on the file sizes and the number of images, this may take a few moments.

Step 2: Set the Base Layer

  1. In the Layers panel, scroll to the very bottom layer. This will serve as your base image (usually your first or cleanest exposure).
  2. Leave this bottom layer's blend mode set to Normal and its opacity at 100%.

Step 3: Apply the Lighten Only Blend Mode

  1. Select the layer directly above your base layer.
  2. At the top of the Layers panel, locate the Mode dropdown menu (which defaults to "Normal").
  3. Click the dropdown menu and select Lighten Only.
  4. Repeat this process for every layer above the base layer:
    • To speed this up, you can group layers by placing them into a Layer Group, or simply click each layer consecutively and switch the mode to Lighten Only.
  5. As each layer is switched to Lighten Only, the stars from that frame will appear added to the cumulative trail without affecting the darker night sky.

Step 4: Refine the Foreground (Optional)

If moving elements in the foreground (such as swaying trees, passing cars, or moving clouds) introduce unwanted artifacts, you can isolate a clean foreground:

  1. Duplicate your sharpest, best-lit foreground exposure and move it to the very top of the layer stack.
  2. Set its mode to Normal.
  3. Add a Layer Mask to this top layer and paint out the sky using a soft black brush, allowing the composite star trails beneath to show through while locking in a clean, static foreground.

Step 5: Export the Final Composite

  1. Once all trails are visible and the image looks balanced, go to Image > Flatten Image to merge all layers into one (optional, but saves memory before exporting).
  2. Go to File > Export As...
  3. Choose your desired format (such as JPEG or PNG), select your destination folder, and click Export.