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Is there a way to add custom matting/framing/borders preset instead of using the default rectangular ones?
I mean something like smooth corners or old fashion styles.
Update: The framing settings in darktable can add only rectangular borders to the photos. I have made few examples from Snapseed of custom setups.
You can add such frames with the watermark module, as already mentioned in the comments. Use the alpha channel in the SVG file as well as the blurring features for shadows or other transparency effects. The only drawback is that the template has to match the size of the image, meaning that you need a template for every aspect ratio you want to work with. Here is an example, similar to your third example:
