[andrewingram]

I used something similar for my Super Mario Maker 2 level viewer (eg https://www.smm2-viewer.com/courses/1HH-CJ8-KYF)

In the level data, the start/goal tiles (approx 10x10 blocks of solid ground under the player and goal) and slopes aren't represented by their tile offset in the level data -- whilst all other "ground" tiles are. Instead, for slopes you're only told the start and end coordinates, and they often overlap.

So to render the slopes correctly I had to work out all the rules for which tiles were allowed next to each other, and solve some ambiguity rules -- I figured out that shallow slopes take precedence over steep ones. Eventually I cracked it, but it took quite a week or so of iteration to figure it out.