Ok. Created another script for SMB1 (well, it's my first, but somebody already posted a script for it).
This script shows the hitboxes for you and the enemies. When the green boxes collide, so do you. Otherwise you dont collide! (Use it in level 1.2 and watch the pirana's and the elevator near the end, isn't that interesting?).
The blue box shows your hitbox for hitting solid level elements like the floor, bricks and whatever.
Will (probably) work with the fceu build of nitsujarehtona, or otherwise the current build of FCEUX.
Works on mario and duckhunt U rom, but probably any SMB1 rom will do.
http://lua.pastey.net/92520
I actually never suspected the hitbox to work like that. Quite interesting.
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I would Youtube it, but it has been bitching all day, not accepting a single video I uploaded, so I'm not going to try.