I have a PNG image which has two figures, bounded by black lines, against a purely white background. My figures have also some white "pixels" (dots, points) inside, not contiguous with background "pixels". Use of the word pixel is not meant to be technically strict. I mean to get an image with transparent instead of white background, same as if it were generated from the get-go, by removing all contiguous white pixels. This seems something attainable. Preferably with Inkscape.
I tried this, but the result was not good.
Later I found