Stitches screenshot splits from this Factorio mod.
Tested on Windows 10 Home x64 with Java 18.0.2.1 2022-08-18.
When you run this executable jar, it will first display a folder selection window that is asking you to select the folder containing the split images. The default location of these images on Windows is %AppData%\Factorio\script-output\screenshots\<WORLD SEED>\auto_split_nauvis. After selecting this folder, the program will start as many threads as you have CPU cores (as reported by Runtime.getRuntime().availableProcessors()). These threads will stitch the images from the specified folder and store the full images inside a new folder next to the executable.
If you see some threads change their status to "Out of memory," don't panic! As long as one thread remains, all images will be stitched. To avoid this issue, run the program from the command line using java -Xmx<RAM>G -jar Stitcher.jar, but replace <RAM> with how many gigabytes of system memory your computer has.