Entry 27

Collision: the COL height field and its query, pinned to the original

2026-10-157mengineui

Work toward a playable port (the user's goal). Gameplay needs the floor under the player. LEVELnn.COL holds it, and was unread until now.

What I found:

• The COL is a height field on 1024-unit tiles: bounds, two normal arrays, 512 x columns, and per column a sorted z-cell list into stacks of 8-byte tiles.
• A tile has a floor and a ceiling height (max y - byte * 128), each optionally sloped through a plane with one of the normals.
• A tile can be halved along a diagonal.
• 0x8002b240 answers, for a point: outside (2), in the open between a tile's ceiling and floor (0), or not (1). It also returns both heights and the tile.

The port:

• rugrats_data::col::Col reads the file.
• rugrats::collide::query mirrors 0x8002b240, 0x8002af20 and 0x80068510 in integer arithmetic.
• tests/collide.rs loads each COL into rrt-emu, runs the original's setup (0x8002a65c), and compares answers at 4 million lattice points across DB00, DB01, DB02, DB05 and DB16. Every point matches: kind, both heights and the tile.

Also in this stretch:

• The title's Start Game menu (0x8003cf80) is ported. The buttons map with the pseudo-code's pad index one more than the port's constant.
• Main Game returns game mode 1, Training returns 16, and Start chooses Main Game at once.
• Main logs the mode until gameplay exists.