Even if you have installed the right version of java it can happen that the GroIMP installer is picking a wrong version of Java from your system. In this case you have to edit the shortcut to enter the right path: 1. Find java 21 on your computer (most likely somewhere in “C:\Program Files” 2. Right click on the the GroIMP desktop shortcut and select Properties (see image on the right) 3. In the Properties find the filled target (see image below) 4. Change the first path to the javaw.exe of java 21.

On modern screens Java swing applications sometimes struggle with the scaling making the font and the buttons way too small. To fix this you can just manually scale it with the following java parameter:

-Dsun.java2d.uiScale=2.5

Instead of 2.5 you can use whatever scale fits for your screen, you might have to try this out a few times.

To apply this on Windows please refer to ( Windows: GroIMP doesn't start and to apply this on Debian/Ubuntu to Increasing heap space on Linux.