06-01-2022, 08:32 PM
If your initial layer has some transparency you will need some overlap. The pixels at the edge your initial object are partially transparent. If you create a smooth outline, its edge pixels will also be partially transparent. And partially transparent over partially transparent is not opaque but still somewhat partially transparent, so you will have a thin gap between the object and its outline.
Otherwise, Edit > Stroke selection doesn't give nice results (line if often a bit jerky), it is usually better to use Select > To Path and then Edit > Stroke path. In that case the path is drawn straddling the selection boundary, so if you keep the selection, you get one half of the path. If your selection is on your object, you get an inner outline and if the selection is outside the object, you get an outer outline. But with the restrictions above for the gap with the outer outline and an unwanted color blend on the edge for the inner outline.
My ofn-outline-layer script attempts to do things correctly
Otherwise, Edit > Stroke selection doesn't give nice results (line if often a bit jerky), it is usually better to use Select > To Path and then Edit > Stroke path. In that case the path is drawn straddling the selection boundary, so if you keep the selection, you get one half of the path. If your selection is on your object, you get an inner outline and if the selection is outside the object, you get an outer outline. But with the restrictions above for the gap with the outer outline and an unwanted color blend on the edge for the inner outline.
My ofn-outline-layer script attempts to do things correctly