One other example of the "bitmask" type of platformer (although you probably wouldn't call it a "platformer", at least not a "jump'n'run" game, as far as the genre is concerned) is Lemmings. The levels of the original Lemmings games are definitely not saved as bitmaps (at 120 levels, that would have far exceeded the capacity of an Amiga floppy), and they're not tile-based either. AFAIK the levels (https://www.mobygames.com/game/amiga/lemmings/screenshots/ga...) were created by repeating certain graphical elements (e.g. the columns in the screenshot are vertical repetitions of one element, the capitals of the columns are another element), and the Lemmings would simply walk on the top side of the image that resulted when composing all these elements (which could be placed freely). Slopes were tackled by looking left and right if the neighbouring pixel was > X pixels higher than the one the Lemming was standing on. If yes, the Lemming would turn around, if no it would walk on, following the slope. This way it was also relatively easy to make the environment destructable (by digging or exploding) and constructable (by building stairs).