WC3 Unit Altitude And Support Surfaces

Contract

Warcraft unit altitude is data-driven. UnitData.moveHeight (umvh) is the unit type's authored default fly height; edict_t.unitinfo.FlyHeight is the mutable per-unit current height used by SetUnitFlyHeight.

The server-side world position contract is:

unit s.origin.z = support surface Z + unitinfo.FlyHeight

Do not derive vertical placement from MDX bounds, model scale, collision size, or the movetp name. Ground units and horse units normally have a zero/near-zero moveHeight; the model's own origin/geometry places hooves and feet. Flying units use the same model-origin rule with a positive authored moveHeight.

Data Flow

Units/UnitData.slk moveHeight / umvh
        -> UnitData_t.moveHeight
        -> SP_SpawnUnit
        -> edict_t.unitinfo.FlyHeight
        -> M_CheckGround
        -> entityState_t.s.origin.z
        -> client MDX transform

GetUnitDefaultFlyHeight reads the bound UnitData_t.moveHeight; it must not return the mutable current unitinfo.FlyHeight.

SetUnitFlyHeight(unit, height, rate) updates unitinfo.FlyHeight and immediately recomputes Z through M_CheckGround. Current Warsmash also leaves rate unimplemented, so OpenRealm intentionally applies height immediately until rate interpolation is implemented as a separate feature.

Supporting Surface

M_CheckGround owns WC3 unit support-surface selection:

movetp	Base surface	Walkable destructable may replace base
foot	terrain	yes
horse	terrain	yes
fly	max(terrain, water)	yes
hover	max(terrain, water)	yes
float	max(terrain, water)	no
amph on walkable terrain	terrain	yes
amph swimming (SWIMMABLE && !WALKABLE)	max(terrain, water)	yes

Water height comes from the W3E waterlevel field using the same bilinear interpolation and W3_WaterSurfaceHeight decode as the renderer: the W3E level plus the tileset's Water.slk height (G_ApplyTilesetWaterHeight on map load; Outland -1.5 tiles). Amphibious swimming classification uses the immutable terrain WPM, not baked/static building obstacles; CM_TerrainPointIsWalkable and CM_TerrainPointIsSwimmable expose that distinction.

Walkable destructables normally use their authored entity Z over the rectangular pathing-texture footprint. The DTrx/DTrf elevators add their mutable occluder_height to that Z immediately, matching ChangeElevatorHeight; unit collision size extends the elevator support footprint so units can stand near the deck edge. Other bridges retain their authored Z. Renderer mesh conformance may raise a supported unit to the visible surface, but it cannot lower the server-authored altitude. See Warcraft III elevators for the elevator state and animation contract.

Projectile Height

Basic and spell missile entities already launch from their server-side source Z. Once unit altitude is part of s.origin.z, a flying attacker's launch point naturally inherits its fly height.

Missile steering uses:

target Z = target s.origin.z + target impactZ

G_UnitImpactZ resolves the ROC/TFT storage split:

• TFT: UnitWeapons.impactZ
• ROC: UnitData.impactZ

The existing G_UnitAttack1LaunchX/Y/Z helpers provide the equivalent source-side ROC/TFT split.

Horizontal Flying Pathing

Flight altitude and horizontal flight pathing remain separate systems. AI_FLYING now selects the static PATHING_FLAG_UNFLYABLE/0x04 rule for WC3 movement, while ordinary ground movers retain PATHING_FLAG_UNWALKABLE/0x02. The distinction is applied to direct and swept static validation, destination resolution, bounded A*, shared flow fields, closest-reachable fallback, trained-unit exit placement, and the SetUnitPosition/unstuck search. Static pathing textures also preserve the authored green-channel UNFLYABLE cells.

Dynamic collision remains an independent layer rule: flying units collide with/avoid other flying units, ground units collide with/avoid ground units, and the two layers ignore each other. FlyHeight is still only the vertical offset from the selected support surface; OpenRealm does not compare model geometry or model Z extents when deciding whether a flyer can cross a building, tree, bridge, or other object. An object blocks a flyer only when its authored/static pathing contributes UNFLYABLE cells.

Deliberately Not Implemented Here

These related Warsmash features need broader renderer/network/pathing work and should not be faked inside M_CheckGround:

• FLOAT swimability and full AMPH horizontal routing;
• terrain slope pitch/roll from elevRad, maxPitch, and maxRoll;
• selZ-driven authored selection-circle height; water-supported and walkable-destructable selection, hover, and indicator rings already use their support surface;
• shadow bridge/water semantics;
• gradual SetUnitFlyHeight rate interpolation and transform takeoff/landing timers.

Verification

Focused tests are in games/warcraft-3/game/tests/t_movement.c, t_pathfinding.c, and t_api.c:

• wc3_movement.fly_height_is_added_to_support_surface
• wc3_movement.flyer_uses_water_surface_before_fly_height
• wc3_pathfinding.movement_class_pathing_distinguishes_walk_and_fly_bits
• wc3_pathfinding.static_path_texture_green_channel_marks_unflyable
• wc3_pathfinding.flyer_move_validation_uses_unflyable_static_pathing
• wc3_pathfinding.heatmap_cache_separates_ground_and_flying_pathing
• wc3_api.flyer_unstuck_search_uses_unflyable_instead_of_unwalkable
• wc3_movement.float_unit_uses_water_surface_and_ignores_bridge
• wc3_api.fly_height_native_keeps_authored_default_separate

A future runtime check should compare a known movetp="fly" unit and a movetp="horse" unit on flat terrain, water, and a walkable bridge. The horse model origin should remain on the support surface; the flying model origin should remain moveHeight above that support surface.

See Also

• WC3 Data Model
• WC3 Pathfinding
• WC3 Attack Damage

Ownership

skills/s_move.c owns M_CheckGround, movement-type support-surface rules, steering, collision-aware steps, and waypoint/flow-field handling. Other abilities call these shared locomotion operations. g_monster.c retains unit initialization and the generic animation/behavior scheduler; g_ai.c retains acquisition and behavior transitions. Raven Form's ascent target, timing, and interpolation belong to skills/s_raven.c, which updates the ordinary unitinfo.FlyHeight value and calls Move's ground/link operations. See Raven Form.