3/4 walkable tile issues [b984]

Walking onto tiles that are 3/4 walkable doesn't operate in an NES consistent way, and it actually creates a real annoyance in the official quest overworld when you walk onto the partially walkable tiles from the east on screen 4C (that is, transition from screen 4D to 4C, ending on the partially walkable combo). Currently, Zelda Classic never allows Link to be partially on unwalkable ground and, if when he is fully on those tiles on 4C, he is only allowed to walk east. I can swear I've seen this discussed before, but I have actual research into NES mechanics here. Screens 79 and 7A are also useful for experimentation.

NES consistent behavior is as follows (investigated with the ROM).

xo
xx


This can be walked onto halfway from the north, putting Link's lower left body on an otherwise unwalkable area. If you attempt to walk onto it from the east directly, you can never end up on an unwalkable area (that means if you walk onto it from the north, walk east, and then go back west, you can't go back where you started). If you end up fully on this tile, you are allowed to walk north or east toward walkable ground.


xx
xo


This can be walked onto "halfway" from the south, putting Link's lower left body on an otherwise unwalkable area (you might consider this him being fully on the combo, but the upper part of his body ignores walkability across the board so it doesn't count). If you attempt to walk onto it from the east directly, you can never end up on an unwalkable area (that means if you walk onto it from the south, walk east, and then go back west, you can't go back where you started). If you end up fully on this tile, you are allowed to walk south or east toward walkable ground.

xx
ox

This tile will never allow Link to walk onto an unwalkable area from any direction. This tile never appears in such a location in the original game that you could end up fully on it.

ox
xx

This tile will never allow Link to walk onto an unwalkable area from any direction. This tile never appears in such a location in the original game that you could end up fully on it.

The side of the screen is always considered walkable as is correctly represented in Zelda Classic. That means if you have either of these combos at the edge of a screen:

xx OR xo
xo xx

The player can walk halfway onto it and then walk off the side of the screen, possibly onto a fully unwalkable combo (this happens in Zelda 1 on the transition from screen 7A to 79). Even in this case, the player is allowed to walk toward walkable ground (north or south and east to the edge of the screen); he never gets "stuck" this way.