Phase 4.7: in progress

assets/blender/scripts/gen_vehicle.py

@@ -32,7 +32,15 @@
 # chassis frame).  See assets/blender/vehicles/CAR_MODELING.md.
 #
 # Usage:
-#   blender <vehicle>.blend -b -Y -P gen_vehicle.py -- <out>.glb <out>.vehicle.json
+#   blender <vehicle>.blend -b -Y -P gen_vehicle.py -- <out>.glb \
+#       <out>.vehicle.json [downforce]
+#
+# downforce (optional, default 25): aerodynamic downforce coefficient
+# (N per (m/s)^2, straight down at the COM while moving) — the
+# speed-dependent grip lever: at speed it multiplies the tire normal
+# load (and thus the lateral impulse budget) without adding inertia,
+# which is exactly where plain tires start understeering.  Higher =
+# tighter turns at speed.
 
 import os
 import sys
@@ -58,6 +66,7 @@ except:
 
 glb_file = argv[0]
 json_file = argv[1]
+downforce = float(argv[2]) if len(argv) > 2 else 25.0
 outdir = os.path.dirname(glb_file)
 base = os.path.splitext(os.path.basename(bpy.data.filepath))[0]
 
@@ -294,7 +303,9 @@ SUSP_MAX = 0.35
 # Softer than the 1.8/0.7 starting point: gives the body some slack
 # over bumps instead of pogo-stiff wheels (which also cost grip).
 SUSP_FREQ = 1.3
-SUSP_DAMP = 0.6
+# Critical-ish damping: less and the body rocks for seconds after a
+# bump (the "almost endlessly bouncy" report).
+SUSP_DAMP = 1.0
 SUSP_REST = SUSP_MAX - 9.81 / (2.0 * math.pi * SUSP_FREQ) ** 2
 for w in sorted(wheel_info, key=lambda w: w["pos"].y):
     front = w["pos"].y < axle_mid_y
@@ -320,6 +331,17 @@ for s in seat_info:
         "driver": s["driver"],
     })
 
+# Center of mass: a box collider puts the COM at the box center — for
+# a sedan that is ~1 m above the tire contact patches (twice the real
+# value) and the car trips onto its side in hard corners (the taxi,
+# with its taller box, tips even easier).  Shift the COM to just above
+# wheel-hub height instead (Jolt OffsetCenterOfMassShape: only the
+# physics COM moves — geometry, wheel attachments and the render node
+# stay put).
+collider_center_ogre_y = (smx.z + smn.z) / 2.0
+hub_ogre_y = sum(w["pos"].z for w in wheel_info) / len(wheel_info)
+com_offset_y = round(hub_ogre_y + 0.1 - collider_center_ogre_y, 4)
+
 driver_offset = ogre_vec3(driver_seats[0]["pos"])
 
 children = []
@@ -410,6 +432,13 @@ vehicle = {
     # soft 1.3 Hz springs a full-lock turn otherwise rolls the car past
     # 45 deg onto two wheels.
     "antiRollBarStiffness": 2000.0,
+    "centerOfMassOffset": {"x": 0.0, "y": com_offset_y, "z": 0.0},
+    # Aerodynamic downforce (N per (m/s)^2, straight down at the COM
+    # while moving): presses the car onto its suspension at speed —
+    # less bounce, and a bigger lateral impulse budget exactly where
+    # plain tires start understeering (turn radius stops growing with
+    # speed).  Per-model value from the CMake invocation.
+    "downforceCoefficient": downforce,
 }
 if steering:
     vehicle["steeringWheelMeshName"] = base + "-steering-wheel.mesh"