Terrain (T_)¶

The terrain engine — how a .T2 heightfield becomes drawable geometry each frame: a view-adaptive quadtree cell tessellation with distance LOD, plus the decoration-scatter system that places trees/objects across the ground. 0x4A7310–0x4ABBE2 (+ a small tail at 0x4C5D30–0x4C60E8).

Provenance: Ghidra static analysis of the game executable with FA.SMS symbols applied; recorded in the symbol database and applied to the Ghidra project. Progress: reconstruction matrix. Markers follow spec-authoring.md: confirmed · inferred · unknown. The nominal manifest range was a grab-bag; this is the true T_* cluster (the ~40 KB middle held ~15 other subsystems).

Quadtree cells with distance LOD¶

Each frame the visible ground is built as a grid of quad cells in _cellArray: T_BuildQuadCell samples the .T2 leaf heights at a cell's four corners (T_GetLeaf), computes a representative altitude (T_QuadAltitude) and a flat-vs-sloped flag (T_QuadSetFlags). T_ViewBounds rotates the view box by heading to bound the working set; T_SubdivideCells is the LOD pass — cells nearer the camera subdivide finer; T_CompactCells drops off-screen cells; and T_EmitCells walks the survivors, looks up each cell's texture (T_CellTmapLookup), computes normals where sloped (T_Normal), and emits the geometry via T_LeafOp into the render pipeline.

Decoration scatter¶

Ground decoration (trees, clutter, objects) is data-driven: T_RunAmbientProcs runs up to 17 decoration procs, T_ScatterGrid tiles a 2ⁿ×2ⁿ region calling T_ScatterDecorTile per tile, which applies per-band distance LOD, gates on the leaf id, samples altitude, and appends to the decoration queue (T_QueueDecor). T_AddVisibleObjs folds visible world objects into the same pass.

Terrain build: .T2 leaves -> quad cells -> view-bound + LOD subdivide + compact -> emit geometry; a parallel decoration-scatter pass.

Functions¶

Full record: db/symbols/terrain.csv.

VA	Symbol	Role
`0x4A9D00`	`T_BuildQuadCell`	build one quad cell (sample corners, altitude, flags)
`0x4A9E20`	`T_QuadAltitude`	representative altitude of a cell
`0x4AA070`	`T_SubdivideCells`	distance-LOD subdivision pass
`0x4AA2B0`	`T_CompactCells`	drop off-screen cells
`0x4AA4A0`	`T_EmitCells`	emit surviving cells' geometry (`T_LeafOp`)
`0x4A9ED0`	`T_ViewBounds`	rotate the view box by heading
`0x4A8090`	`T_ScatterGrid`	tile the decoration-scatter region
`0x4A8130`	`T_ScatterDecorTile`	place one decoration cluster per tile
`0x4A8C30`	`T_QueueDecor`	append a decoration entry to the queue
`0x4A9C20`	`T_AddVisibleObjs`	fold visible world objects into the pass

Open Questions¶

1. `.T2` sub-header bytes¶

Resolved statically (2026-07-05, #262). The "sub-header class constants" decode as the .T2 header field map read by the loader path, which was in the analyzed code all along: T_Load (0x4C5D70) loads the theater file through RMAccess and relocates two file offsets into pointers — the tile-summary array (+0x85) and the leaf array (+0x91) — and T_GetLeaf (0x4C6040) indexes both arrays row-major using the grid fields (+0x79 leaf step, +0x7D/+0x81 tile grid, +0x89/+0x8D leaf grid). The payload is two flat arrays, not per-tile records; full field map and the superseded readings in T2.md.

Status: resolved — see T2.md § Engine Notes.

formats/T2.md — the .T2 terrain heightfield format, and its § Terrain Texturing (per-leaf texture_variant → <theater><N>.PIC tile), the model the fxs terrain viewer draws.
render-core.md — the 3D pipeline T_EmitCells feeds geometry into.
collision.md — the terrain grid (T_GetLeaf/T_Normal) collision uses.
renderer.md — the rasterizer that draws the emitted cells.