Inner shadow

Hello,
Try the G'mic module > Frame Painting
several options are available and interesting effects :-)

--
scorpion-phoenix; I have the gift of palingenesis...

Thank you axel_8410, it's not bad at all and the result is there. In the meantime, I discovered a very simple procedure:

For an idea with Gimp
1. Make a selection within the frame.
2. Copy/paste this selection onto a new layer and fill it with black.
3. Ensure that black is selected and reduce this selection, Select, Shrink.
4. Clear the inside, Edit, Clear.
5. Select, Invert.
6. Add blur, Filters, Blur, Gaussian Blur. (Drag the slider to see the result) There are 2 dropdown menus in this window for other options, such as changing the color to white.
If you don’t want the entire border, add a layer mask to this layer and erase the parts you don’t want.
7. Merge the layers.
8. Here, at the end, I traced the selection, Edit, Stroke Selection. Here is the result,