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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.

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Every image format (JPEG, PNG, TIFF, etc.) that I know of can only represent rectangular images. This means that unless you define your own image format, this simply cannot be done.

The only thing you can do is to work with transparency. Even though the image itself will still be rectangular, if only a circular portion of it is opaque, it will look like a circular image when displayed on a website, for instance.

I do not think that Darktable can do this. However, the GNU Image Manipulation Program (GIMP) can. You can use selections and layer masks to control the alpha channel (transparency) of your image. There are various tutorials about this on the internet (e.g. https://www.gimp.org/tutorials/Layer_Masks/). When saving your image, you have to make sure to select an image format that supports transparency. For instance, PNG and TIFF do, while JPEG does not.

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