u/Willsxyz

To make my question concrete, suppose I have a 100x100 pixel image that has a single circle, no fill, 4 pixel wide black stroke. Apart from that, the image is transparent.

What I would like is to change this to an image in which the transparent parts of the original image, both inside and outside the original circle are black, while the areas that used to be colored black are now transparent. So, as if I had a transparent circle on a black background.

I can subtract the circle from a 100x100 pixel square easily enough, but that removes the interior of the circle and not just the stroke.

Is there an easy way to do this? Thanks.