Works best (as you might imagine) if there's a dramatic color difference from the background and the foreground item. I spent a lot of time trying to get an image lit with mostly browns separate from the area behind it. There's no undo funtion if one of your lines gets too close -- you must reload the image. Even when it works well, its still a quick and dirty solution: images I tested always had rough edges I'd want to hand off to GIMP to clean up. Note that this can really look interesting if you cover the image with both various red and green dots: it gives it a cut-out / broken glass look that I actually found to be a really interesting effect.