This image has also made me realise that my game 'engine' does not currently handle objects that have 'moved' very well; it only assumes things can disappear, or change in-place... in the case of the arm in the drawing in the previous toot, the clickable area would only be in one relative location; i.e. if I make the clickable region closely around the arm on the left one, you'd have to click on the empty air on the right one. So I'd either have to put a bounding box over the shared area (the arm and the empty air), which is my current approach, or I'd have to add a second region and some how link both together.
anyway, the contour detection is meant to be an aid for adding the #SVG paths; I can always tweak, fix and add paths myself in #Inkscape and directly in the XML source whenever the tool doesn't suffice. I guess I shouldn't expect it to be a magic bullet.
#FindTheDifferences #SpotTheDifferences (I really should come up with a unique name for this project)