skate-3-rust-engine/docs/hails-additions/05-collision-volumes.md

5. Invisible collision volumes

Problem

On SkateSchool the player hit an invisible wall in the middle of the spawn area and could not skate around. Logs showed normal physics states (no failures), so the obstruction was in the collision data.

Root cause

Retail collision archives contain a small number of clustered meshes in which no unit carries a surface ID (unit flag 0x80 unset; units read as flags 0x21 instead of the usual 0xa1). Every one of them, in every shipped district, is a 12-triangle axis-aligned box: zone, trigger or level-bounds volumes. The game's loader (skate_world::retail_collision_world, via skate_data::retail_collision::visit_clusters) turned every triangle into solid collision, so these volumes became invisible walls, floors and ceilings.

SkateSchool has seven, all in its global stream (cSim_Global.xsf). Names are those of the matching retail trigger-volume records (see "What the boxes are" below):

Box (X / Y / Z, metres)	Name	Effect in game
−79.1…16.5 / −1.53…6.65 / −43.3…52.3	tut_sksc_inthehub_vol_02	covers the whole school area; the authored start (y 0.11) is inside it
−62.1…−0.5 / −1.53…6.65 / −3.6…12.7	tut_sksc_inthehub_vol_01	the invisible wall in the spawn area
−11.0…2.5 / 0…4.17 / −2.6…1.6 (three identical)	coach_frank_sksc / ws_sksc_coachfrank_instance_01	invisible block near the origin
−212…578 / −73…79 / −483…492	tut_sksc_reset_vol01	level bounds: floor under the whole map
104.1…199.7 / −4.1…4.1 / 151.5…247.1	tut_sksc_wrongway_vol_01	—

Evidence that these are not solid in the original game: every authored SkateSchool locator (Coach Frank, the hub, all tutorial stations) is at y 0.0–0.4 on the real floor (surface 258), i.e. inside the big box; and in DownTown, a surfaceless box (X 259–333, Y 48.9–57.7, Z −152…−71) encloses the Kube Tower landing zone (Z_DT_CubeTower, y 55.9) and four authored challenge start points.

Before the spawn change in doc 4, SkateSchool's spawn (y 7.63) stood on top of the big box, which hid the problem.

Survey of all districts

Classified per mesh by surface-ID presence:

District	Fully surfaceless meshes	Notes
SkateSchool	7 (all 12-triangle boxes)	global stream
MaloofMoneyCup	2 (12-triangle boxes)	global stream
DownTown	2 (12-triangle boxes)	city cell streams
MegaPark	1 (12-triangle box)	global stream
others	0

Mixed meshes (some units with surface IDs, some without) are common in real geometry: University cells (up to 31k triangles), Industrial, DownTown (138 meshes), BlackBoxPark, and the main geometry of several parks. A per-triangle filter would therefore remove real floors; the rule must be per mesh.

Change

crates/skate-data/src/retail_collision.rs:

• RetailTriangle gains has_surface (unit flag 0x80).
• mesh_clusters decodes and validates a whole mesh (including the triangle count check) before visiting its clusters, and skips the mesh when no unit has a surface ID. visit_clusters returns the number of triangles visited. Memory: one decoded mesh at a time instead of one cluster.

The fix is in the game, so existing installations benefit without an asset refresh.

Verification

• New test skips_meshes_without_any_surface_ids (fixture mesh with its surface ID stripped is skipped; the others are visited intact). All skate-data library and integration tests pass, as do the game's collision/world tests.
• Collision triangles loaded per map (SKATE_RWCM_READY), before → after: SkateSchool 91,790 → 91,706 (−84 = 7 boxes), DownTown −24, MaloofMoneyCup −24, MegaPark −12, all other maps unchanged. Every map passes --check-assets.
• In play: SkateSchool (spawn area free to skate) and DownTown tested thoroughly; the Maloof and MegaPark box locations (below) were visited afterwards and are also clear.

Further evidence the volumes are triggers: MegaPark's only box (tele_stadium_to_world_volume_a, X 61.4–73.2, Y −4.1…4.1, Z −140.8…−132.6) surrounds the stadium's arrival point from the world (tele_world_to_stadium_dest_locator_01, 2 m from its centre), and one of Maloof's two boxes lies 14 m from its arrival point from DownTown (tele_dwtn_to_mmcp_dest_locator_01). The other Maloof box (2.8 × 3.4 × 2.0 m) is 17 m from a street challenge start.

Notes for upstream

• cargo test -p skate-data currently fails to compile three examples (apt_data, hud_data, scoring_flow_data) that reference game-only modules; this predates this change. --lib --tests runs the test suite.
• The mesh-level rule is empirical (all 12 such meshes in the shipped data are boxes, and authored player positions lie inside some). If retail code that consumes these volumes (zones/triggers) is ported later, they should be routed there instead of dropped.

What the boxes are (research, 2026-10-02)

Every surfaceless box is the shape of a named trigger volume: a volume-set record (type 0x00EB0019: matrix, bounding box, link, instance id, name) in the same stream, whose bounding box matches the box to the millimetre. Retail registers them as triggers when the stream loads and posts enter/exit events; they are never solid. All 12 boxes:

District	Volumes
SkateSchool	coach_frank_sksc, ws_sksc_coachfrank_instance_01 (same box, 3 meshes), tut_sksc_inthehub_vol_01, tut_sksc_inthehub_vol_02, tut_sksc_reset_vol01, tut_sksc_wrongway_vol_01
MegaPark	tele_stadium_to_world_volume_a
MaloofMoneyCup	tele_mega_ramp_up_volume_01 (X −9.3…−6.5, Z 54.5…56.5), tele_mmcp_to_dwtn_volume_01 (X 36.2…42.1, Z 12.8…26.6)
DownTown	dwtn_sessionspot_01_kubetower_volume_01 (X 259–333), dwtn_sessionspot_02_spillway_volume_02 (X 66.2…105.9, Y 50.2…59.0, Z −244.7…−212.2)

The tele_* volumes are where retail offers a teleport (a yes/no challenge prompt). Exporting them as data the game can use (a "which volumes contain this point" query) is planned follow-up work; this change only stops them being solid. Research source: the TU3 code via the skate3recomp static recompilation (reference only).

Correction (2026-10-02)

An earlier version of this document gave the SkateSchool spawn-wall box as X −66…−62. That came from an analysis script using the vendored Python collision decoder, which reads the 16-bit compressed vertex deltas as signed; vertices in clusters spanning more than 32.8 m then wrap by 65.536 m. The game's Rust decoder (retail_collision.rs) reads them unsigned, as retail does, so the game and this change were never affected: the triangle counts and play tests above are unchanged. Re-run with unsigned deltas, every box now matches its retail trigger-volume record exactly. The real box is X −62.1…−0.5 (tut_sksc_inthehub_vol_01).