I remember the time I get involve in the krita project, my first visible feature was the invert filter, but it took me three months to write it. Mostly because there was a lot of infrastucture work to do before But the point is that today’s filters are simple because I wrote them to test that writting filters outside krita was possible.
Here is the result of a filter that will filter to leave only the minimal channel:
Here is the result of a filter that will filter to leave only the maximal channel:
This filter remove a color from the image and transform it to alpha to make it transparent, here I have selected a tone of blue and I have nearly remove the sky with a next gradient: