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From Sonic Retro

The solidity of a tile determines how it will interact with Sonic. There are four possible states:

• Not solid - Sonic will pass through the tile, without interacting with it.
• All solid - the whole tile is solid.
• Top solid - the top (or bottom) of the tile is solid, and uses the main collision array.
• L/R solid - the sides of the tile are solid, and use the rotated collision array.