13. Scrolling up
This was much harder than scrolling right. If I had chosen to scroll downward, it might have been easier, because scrolling up is like going backwards in the code.
Skipping over the y values of 0xf0-ff wasn’t that bad as long as you use my functions add_scroll_y() and sub_scroll_y() that handles this for you. But, I also had to add them to the BG collision code, since you can be half in one nametable and half in another. And, the code assumes that Y position is between 0 and $ef.
Since scrolling up is going backwards, I’m starting at the maximum scroll position, and going backwards to 0.
I had to build the array of room arrays backwards too.
const unsigned char * const Rooms[]= { Room5, Room4, Room3, Room2, Room1 };
Anyway, I made 5 more rooms, in Tiled, and exported 5 more csv files, converted to C arrays, and imported those into the code.
And after a few days of debugging (yikes) it finally works. I have to give some due respect to the guys who made games like Kid Icarus, or Metroid, or Ice Climber, because scrolling upward is not easy.
https://github.com/nesdoug/16_Scroll_Up/blob/master/scroll_up.c
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