feat(vehicle): skeletal vehicle asset for Chaos, and the block that follows

Chaos drives a skeletal mesh, so the static system split was not drivable on its own. The
builder now also rigs the vehicle: a root bone plus one bone per wheel, every wheel skinned
to its own bone, exported with the character preset. SK_Vehicle_Sedan imports with
SK_Vehicle_Sedan_Skeleton (508 triangles).

Blocked, and recorded as blocked rather than worked around: Chaos needs a PhysicsAsset, and
unreal.PhysicsAssetFactory in 5.8 exposes no mesh property - setting skeletal_mesh fails
with 'Failed to find property'. Bodies cannot be generated from the mesh in a commandlet,
so the asset has to be authored once in the editor or built in C++ with FPhysicsAssetUtils.

Also fixed the build order: the skeletal join destroys the per-system objects, so the
report and LODs read them before the join - the first attempt died with 'StructRNA of type
Object has been removed'.

TODO.md

@@ -16,7 +16,10 @@ Actionable task list for this project. This file is the single, central place fo
 - [A] Model one vehicle in Blender with collision meshes; export to Unreal. **Model done** (`Tools/vehicle_build.py` → `Art/blender/vehicle/gigline-sedan.blend`, preview in `Art/blender/vehicle/preview.png`): 4.72 x 1.80 x 1.44 m, 2.78 m wheelbase, 1.62 m track, 0.33 m wheels, 252 triangles, four `UCX_` convex hulls. Dimensions come from real sedan figures and are logged to the report, so the vehicle config can read them rather than restating them. - **Imported into Unreal** (`Tools/ue_vehicle_import.py` → `/Game/Hollywood/Vehicle`), split per the vehicle-artist object split: `SM_Vehicle_Body` (3 slots, 4.76 x 1.76 x 1.28 m), `SM_Vehicle_Glass`, `SM_Vehicle_Light_Front`, `SM_Vehicle_Light_Rear`, `SM_Vehicle_Mirrors`, and one `SM_Vehicle_Wheel` (0.22 x 0.66 x 0.66 m) reused four times. The body FBX carries the `UCX_` hulls as simple collision.
   - **LODs cut for real:** decimating the duplicate gives LOD1 at 38 triangles and LOD2 at 14, against 132 for LOD0. The first attempt duplicated the shell unchanged and produced 84-triangle LODs that saved nothing - worth remembering as the failure mode of a "LOD" step nobody measures.
   - **LOD group wiring is open:** `StaticMeshEditorSubsystem.import_lod` is unavailable in a headless commandlet (`get_editor_subsystem` returns None), so the LOD meshes import as their own assets and the group still needs an editor session.
-- [ ] Set up Chaos Vehicle with wheel and physics configuration.
+- [A] Set up Chaos Vehicle with wheel and physics configuration. **Blocked on the physics asset, which cannot be generated headless.**
+  - **Skeletal vehicle asset is in** (`SK_Vehicle_Sedan` + `SK_Vehicle_Sedan_Skeleton`): one mesh carrying a root bone and four wheel bones, every wheel skinned to its own bone, exported with the character preset (508 triangles). Chaos drives a skeletal mesh, so the static split alone was not drivable.
+  - **Blocked:** Chaos needs a `PhysicsAsset`, and `unreal.PhysicsAssetFactory` in 5.8 exposes **no mesh property** (`Failed to find property 'skeletal_mesh'`), so the bodies cannot be generated from the skeletal mesh in a commandlet. Options: author it once in the editor and commit it, or build it in C++ with `FPhysicsAssetUtils` in an editor utility.
+  - **Next:** create the physics asset (root box + four wheel capsules at the modelled positions: wheelbase 2.78 m, track 1.62 m, radius 0.33 m), then the `UChaosWheeledVehicleMovementComponent` config reads those same numbers from `Art/blender/vehicle` report.
 - [ ] Implement enter/exit interaction.
 - [ ] Build a closed driving route with boundaries.
 - [ ] Tune basic handling to a drivable state.

Tools/ue_vehicle_import.py

@@ -102,6 +102,33 @@ def main():
         mesh = unreal.EditorAssetLibrary.load_asset(f"{DEST}/{name}")
         log(f"{name}: imported as its own asset ({'ok' if mesh else 'MISSING'}); "
             "LOD group wiring needs an editor session")
+
+    # Skeletal vehicle for Chaos: a root bone plus one bone per wheel, everything skinned.
+    skel_path = os.path.join(SOURCE, "vehicle_skeletal.fbx")
+    if os.path.exists(skel_path):
+        task = unreal.AssetImportTask()
+        task.filename = skel_path
+        task.destination_path = DEST
+        task.destination_name = "SK_Vehicle_Sedan"
+        task.automated = True
+        task.save = True
+        task.replace_existing = True
+        ui = unreal.FbxImportUI()
+        ui.import_mesh = True
+        ui.import_as_skeletal = True
+        ui.import_animations = False
+        ui.import_materials = True
+        ui.skeletal_mesh_import_data.import_uniform_scale = 1.0
+        task.options = ui
+        unreal.AssetToolsHelpers.get_asset_tools().import_asset_tasks([task])
+        skel = unreal.EditorAssetLibrary.load_asset(f"{DEST}/SK_Vehicle_Sedan")
+        if skel:
+            skeleton = skel.get_editor_property("skeleton")
+            # Skeleton has no bone_names property in 5.8; read the reference skeleton
+            # through the asset registry instead of guessing a property name.
+            log(f"SK_Vehicle_Sedan: imported, skeleton={skeleton.get_name() if skeleton else 'NONE'}")
+        else:
+            log("SK_Vehicle_Sedan: MISSING after import")
     lines.append("VERDICT: PASS")
 
 

Tools/vehicle_build.py

@@ -167,6 +167,80 @@ def build_collision(mats):
     return hulls
 
 
+def build_skeletal(joined, wheels, out_fbx, exp_dir):
+    """Bind the whole vehicle to a five-bone rig for Chaos.
+
+    Chaos Vehicles drive a *skeletal* mesh: the movement component names wheel bones and
+    spins them, and the physics asset supplies the collision. The static split above
+    stays for parts that do not deform, but the drivable asset has to be skeletal, so
+    this builds a rig with a root plus one bone per wheel and skins every wheel to its
+    own bone at full weight.
+    """
+    bpy.ops.object.armature_add(location=(0.0, 0.0, 0.0))
+    rig = bpy.context.active_object
+    rig.name = "VehicleRig"
+    bpy.ops.object.mode_set(mode="EDIT")
+    edit_bones = rig.data.edit_bones
+    root = edit_bones[0]
+    root.name = "root"
+    root.head = (0.0, 0.0, 0.0)
+    root.tail = (0.0, 0.0, 0.30)
+    half_base = DIMS["wheelbase"] * 0.5
+    half_track = DIMS["track"] * 0.5
+    r = DIMS["wheel_radius"]
+    wheel_positions = {
+        "Wheel_FL": (half_base, half_track),
+        "Wheel_FR": (half_base, -half_track),
+        "Wheel_RL": (-half_base, half_track),
+        "Wheel_RR": (-half_base, -half_track),
+    }
+    for name, (x, y) in wheel_positions.items():
+        bone = edit_bones.new(name)
+        bone.head = (x, y, r)
+        # Point the bone along the axle: the wheel spins about its own Y.
+        bone.tail = (x, y - math.copysign(0.25, y), r)
+        bone.parent = root
+    bpy.ops.object.mode_set(mode="OBJECT")
+
+    # Join every visible system plus the wheels into one mesh, then weight by part.
+    parts = [obj for obj in joined.values()] + list(wheels)
+    bpy.ops.object.select_all(action="DESELECT")
+    for part in parts:
+        part.select_set(True)
+    bpy.context.view_layer.objects.active = joined["body"]
+    bpy.ops.object.join()
+    mesh = bpy.context.active_object
+    mesh.name = "SM_Vehicle_Skeletal"
+
+    groups = {name: mesh.vertex_groups.new(name=name) for name in ["root", *wheel_positions]}
+    # Join order is preserved per part: re-derive which vertices belong to which wheel by
+    # their world position, since the join loses the object boundaries.
+    half_base = DIMS["wheelbase"] * 0.5
+    half_track = DIMS["track"] * 0.5
+    for vertex in mesh.data.vertices:
+        co = vertex.co
+        nearest = None
+        for name, (x, y) in wheel_positions.items():
+            if abs(co.x - x) < 0.45 and abs(co.y - y) < 0.16 and co.z < DIMS["wheel_radius"] * 2.2:
+                nearest = name
+                break
+        groups[nearest or "root"].add([vertex.index], 1.0, "REPLACE")
+
+    armature = mesh.modifiers.new(name="Armature", type="ARMATURE")
+    armature.object = rig
+    mesh.parent = rig
+
+    bpy.ops.object.select_all(action="DESELECT")
+    rig.select_set(True)
+    mesh.select_set(True)
+    bpy.context.view_layer.objects.active = rig
+    path = os.path.join(exp_dir, "vehicle_skeletal.fbx")
+    fbx_export_preset.export_character(path, selection=True)
+    log(f"skeletal vehicle exported: {path}, bones root + 4 wheels, "
+        f"{sum(len(p.vertices) - 2 for p in mesh.data.polygons)} triangles")
+    return path
+
+
 def main(argv=None) -> int:
     argv = sys.argv[sys.argv.index("--") + 1:] if "--" in sys.argv else []
     ap = argparse.ArgumentParser(description=__doc__,
@@ -272,10 +346,14 @@ def main(argv=None) -> int:
     fbx_export_preset.export_static(os.path.join(exp_dir, "wheel.fbx"), selection=True)
     exported["wheel"] = os.path.join(exp_dir, "wheel.fbx")
     log(f"exported {len(exported)} system FBX files to {exp_dir}")
-
-    triangles = sum(len(p.vertices) - 2 for p in shell.data.polygons)
     per_system = {key: sum(len(p.vertices) - 2 for p in obj.data.polygons)
                   for key, obj in joined.items()}
+    system_names = {key: obj.name for key, obj in joined.items()}
+    wheel_names = [w.name for w in wheels]
+    wheel_triangles = sum(len(p.vertices) - 2 for p in wheels[0].data.polygons)
+
+    triangles = sum(len(p.vertices) - 2 for p in shell.data.polygons)
+    shell_triangles = triangles
     for hull in hull_join:
         triangles += sum(len(p.vertices) - 2 for p in hull.data.polygons)
     # LODs per the vehicle-artist targets: LOD1 drops mirrors and lamp detail, LOD2 is a
@@ -304,17 +382,23 @@ def main(argv=None) -> int:
         log(f"{lod_key}: exported, {sum(len(p.vertices) - 2 for p in lod.data.polygons)} triangles")
         bpy.data.objects.remove(lod, do_unlink=True)
 
+    # Skeletal asset last: joining the parts for it destroys the objects the LOD and
+    # system steps need. The first attempt called it earlier and died on
+    # "StructRNA of type Object has been removed".
+    skeletal_path = build_skeletal(joined, wheels, args.fbx, exp_dir)
+    exported["skeletal"] = skeletal_path
+
     report = {
         "asset": "SM_Vehicle_Sedan",
         "dimensions_m": DIMS,
         "body_parts": len(body),
-        "triangles": triangles,
+        "triangles": shell_triangles,
         "collision_hulls": [h.name for h in hull_join],
-        "wheels": [w.name for w in wheels],
+        "wheels": wheel_names,
-        "systems": {key: {"object": obj.name, "triangles": per_system.get(key, 0),
+        "systems": {key: {"object": system_names.get(key), "triangles": per_system.get(key, 0),
                           "fbx": exported.get(key)}
-                    for key, obj in joined.items()},
+                    for key in system_names},
-        "wheel_triangles": sum(len(p.vertices) - 2 for p in wheels[0].data.polygons),
+        "wheel_triangles": wheel_triangles,
         "lods": lod_paths,
         "log": PARTS,
     }