Recently I had to crop an image in a circular shape for a composition. Since I usually use Darktable and I also had to edit some details of the image, I tried to do it directly with it. Nonetheless, the crop option only let you chose rectangular shapes. I thought that if I couldn't just shape the image circular, I could try to make the outside of the circle transparent, but I couldn't find any option that would allow me to do it.
Is this possible? And if so, how is it done?
Note: I would prefer an answer for Darktable since I'm used to it, but I could use an answer for any free linux-compatible software.