merge lang/horse-walker: the horse on legs (VehicleLegs), convex and scaled shapes in ludic.physics

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/ludic.physics/README.md

@@ -29,7 +29,7 @@ import "ludic.physics"
 | | |
 | --- | --- |
 | `phys_open(st, max_bodies, threads) -> bool`, `phys_close(st)`, `phys_is_open(st)` | a world, and everything in it let go |
-| `phys_box`, `phys_sphere`, `phys_capsule`, `phys_cylinder`, `phys_dome(r, h)`, `phys_offset(shape, x, y, z, yaw)` | shapes, each an int (`-1` refused); a dome is a boulder standing on its origin |
+| `phys_box`, `phys_sphere`, `phys_capsule`, `phys_cylinder`, `phys_dome(r, h)`, `phys_convex(points, n)`, `phys_scaled(shape, k)`, `phys_offset(shape, x, y, z, yaw)` | shapes, each an int (`-1` refused); a dome is a boulder standing on its origin; a convex hull of n points (x, y, z) is a rock's own shape, made once and scaled for every rock of that shape; a body's yaw turns it as a renderer turns an instance (x' = cos x + sin z, z' = -sin x + cos z) |
 | `phys_heightfield(st, h, n, ox, oz, cell)`, `phys_mesh(st, v, nv, tri, nt)` | the ground (`PHYS_HOLE` is none) and a dock or cabin |
 | `phys_ground_add`, `phys_static_add`, `phys_kinematic_add`, `phys_body_add(..., mass)` | bodies: the ground, still things, what the game moves (the player), what falls and floats |
 | `phys_settle(st)` | after adding many still things, before the first query |

packages/ludic.physics/native.ludic

@@ -14,6 +14,8 @@ extern function jph_shape_capsule(half_h: float, r: float) -> pointer = "jph_sha
 extern function jph_shape_cylinder(half_h: float, r: float) -> pointer = "jph_shape_cylinder"
 extern function jph_shape_dome(r: float, h: float) -> pointer = "jph_shape_dome"
 extern function jph_shape_offset(s: pointer, x: float, y: float, z: float, yaw: float) -> pointer = "jph_shape_offset"
+extern function jph_shape_convex(v: pointer, n: int, radius: float) -> pointer = "jph_shape_convex"
+extern function jph_shape_scaled(s: pointer, k: float) -> pointer = "jph_shape_scaled"
 extern function jph_shape_heightfield(h: pointer, n: int, ox: float, oz: float, cell: float) -> pointer = "jph_shape_heightfield"
 extern function jph_shape_mesh(v: pointer, nv: int, tri: pointer, nt: int) -> pointer = "jph_shape_mesh"
 extern function jph_shape_free(s: pointer) -> void = "jph_shape_free"

packages/ludic.physics/native/shim/jph_shapes.inl

@@ -17,6 +17,19 @@ JPH_SHIM void *jph_shape_dome(float r, float h) {
   return keep(RotatedTranslatedShapeSettings(Vec3(0.0f, h - big, 0.0f), Quat::sIdentity(), s).Create());
 }
 
+// a rock: the convex hull of n points (x, y, z) about its origin, its edges rounded by radius
+JPH_SHIM void *jph_shape_convex(const float *v, int n, float radius) {
+  Array<Vec3> pts;
+  pts.reserve(n);
+  for (int i = 0; i < n; ++i) pts.push_back(Vec3(v[3 * i], v[3 * i + 1], v[3 * i + 2]));
+  return keep(ConvexHullShapeSettings(pts, radius).Create());
+}
+
+// another shape, k times its size on every axis (one hull, every boulder of that shape)
+JPH_SHIM void *jph_shape_scaled(void *s, float k) {
+  return keep(ScaledShapeSettings(static_cast<Shape *>(s), Vec3::sReplicate(k)).Create());
+}
+
 // a shape moved and turned about y, inside its body
 JPH_SHIM void *jph_shape_offset(void *s, float x, float y, float z, float yaw) {
   return keep(RotatedTranslatedShapeSettings(Vec3(x, y, z), yaw_q(yaw), static_cast<Shape *>(s)).Create());

packages/ludic.physics/native/shim/jph_shim.cpp

@@ -16,6 +16,8 @@
 #include <Jolt/Physics/Collision/Shape/HeightFieldShape.h>
 #include <Jolt/Physics/Collision/Shape/MeshShape.h>
 #include <Jolt/Physics/Collision/Shape/RotatedTranslatedShape.h>
+#include <Jolt/Physics/Collision/Shape/ConvexHullShape.h>
+#include <Jolt/Physics/Collision/Shape/ScaledShape.h>
 #include <Jolt/Physics/Collision/RayCast.h>
 #include <Jolt/Physics/Collision/CastResult.h>
 #include <Jolt/Physics/Collision/ShapeCast.h>

packages/ludic.physics/shapes.ludic

@@ -59,6 +59,18 @@ export function phys_cylinder(physics_st: mut PhysicsState, half_h: float, r: fl
 # a boulder: a dome r across and h tall, standing on its origin
 export function phys_dome(physics_st: mut PhysicsState, r: float, h: float) -> int { return ph_keep(physics_st, jph_shape_dome(r, h)) }
 
+# a rock: the convex hull of n points (x, y, z) about its origin, its edges rounded by 2 cm
+export function phys_convex(physics_st: mut PhysicsState, v: []float, n: int) -> int {
+  if n < 4 or len(v) < n * 3 { return -1 }
+  return ph_keep(physics_st, jph_shape_convex(v, n, 0.02))
+}
+# another shape k times its size, so one hull serves every boulder of its shape
+export function phys_scaled(physics_st: mut PhysicsState, shape: int, k: float) -> int {
+  let s = ph_shape(physics_st, shape)
+  if s == null { return -1 }
+  return ph_keep(physics_st, jph_shape_scaled(s, k))
+}
+
 # another shape moved and turned about y inside its body (a post whose foot is its origin)
 export function phys_offset(physics_st: mut PhysicsState, shape: int, x: float, y: float, z: float, yaw: float) -> int {
   let s = ph_shape(physics_st, shape)

packages/ludic.physics/tests/hull_shape_test.ludic

@@ -0,0 +1,44 @@
+# hull_shape_test.ludic - a rock's shape: a convex hull of points, scaled for one boulder and
+# turned by its yaw the way a renderer turns an instance (x' = cos x + sin z, z' = -sin x + cos z)
+import "ludic.physics"
+import "ludic.base"
+program HullShapeTest {
+  numbers float
+
+  # a cube two metres across, its centre two metres along x from its origin
+  function cube(physics_st: mut PhysicsState) -> int {
+    let v = floats(8 * 3)
+    for i in 0 .. 8 {
+      v[i * 3] = 2.0 + float(i % 2) * 2.0 - 1.0
+      v[i * 3 + 1] = float((i / 2) % 2) * 2.0 - 1.0
+      v[i * 3 + 2] = float(i / 4) * 2.0 - 1.0
+    }
+    return phys_convex(physics_st, v, 8)
+  }
+  function down(physics_st: mut PhysicsState, x: float, z: float) -> PhysHit { return phys_ray(physics_st, x, 10.0, z, 0.0, -20.0, 0.0, PHYS_M_ALL) }
+
+  test "a hull stands where its points are, and too few points are no shape" (physics_st: mut PhysicsState) {
+    expect(phys_open(physics_st, 16, 1))
+    phys_static_add(physics_st, cube(physics_st), 0.0, 0.0, 0.0, 0.0)
+    phys_settle(physics_st)
+    let h = down(physics_st, 2.0, 0.0)
+    expect(h.body >= 0)
+    expect(Math.abs(h.y - 1.0) < 0.01)
+    expect(down(physics_st, 0.0, 0.0).body < 0)
+    expect_eq(phys_convex(physics_st, floats(9), 3), -1)
+    phys_close(physics_st)
+  }
+
+  test "scaled twice as big, and turned a quarter the renderer's way" (physics_st: mut PhysicsState) {
+    expect(phys_open(physics_st, 16, 1))
+    phys_static_add(physics_st, phys_scaled(physics_st, cube(physics_st), 2.0), 0.0, 0.0, 0.0, 1.5707963)
+    phys_settle(physics_st)
+    # (4, 0, 0) turned by +90 degrees about y is (0, 0, -4): x' = cos x + sin z, z' = -sin x + cos z
+    let h = down(physics_st, 0.0, -4.0)
+    expect(h.body >= 0)
+    expect(Math.abs(h.y - 2.0) < 0.01)
+    expect(down(physics_st, 4.0, 0.0).body < 0)
+    expect(down(physics_st, 0.0, 4.0).body < 0)
+    phys_close(physics_st)
+  }
+}

packages/ludic.vehicles/README.md

@@ -28,6 +28,11 @@ through the game).
   the oars pulled against each other - and reads back where the hull is and how fast it goes. The
   water floats it, the wind (`VehicleWorld.wind`) drifts every boat, ridden or not, and the lake bed
   stops it at the shore; the swell at the sail limit takes the outward share of a stroke.
+- **A horse walks on legs in the game's physics** (`VehicleLegs`; a Jolt walker in Maroon Lake). The
+  package decides its pace - a walk, a canter on stamina, hay for every metre - and never steps it
+  into water; the legs meet the ground, its steps and slopes, and whatever stands in its way. What
+  they cover along its heading is its speed, so a fence brings it to a stand and a trunk it glances
+  pushes it aside. A horse called to its bay or led round where it is tied is stood there (`stand`).
 - **A horse's steering follows signed speed**: backing up with the stick left swings the nose right.
 - A stale rider locks a vehicle for ever, so getting off is `vehicle_dismount` or `vehicle_drop`.
 
@@ -35,12 +40,13 @@ through the game).
 
 `VehicleHull { put(nid, x, y, z, yaw), row(nid, forward, turn), drift(nid, fx, fz), read(nid) -> bool,
 x, y, z, yaw, speed, gone(nid) }` is required: the boat's body, named by the vehicle's nid.
+`VehicleLegs { walk(nid, x, y, z, vx, vz, dt) -> bool, x, y, z, stand(nid, x, y, z), gone(nid) }` is the
+horse's: unbound, the ground is walked as it is and only a rise steeper than it can take stops it.
 
 ```ludic
 export port VehicleWorld {
   ground: fn(float, float) -> float                       # required
   water: fn(float, float) -> float                        # the surface (unbound: never wet)
-  blocked: fn(float, float, float) -> bool                # (x, z, r): a collider in a horse's way
   swell: fn(float, float, float, float) -> float          # (x, z, fx, fz): a stroke's share lost
   wind: fn() -> float                                     # 0..1
   wind_yaw: fn() -> float                                 # the yaw the wind pushes toward

packages/ludic.vehicles/call.ludic

@@ -20,12 +20,14 @@ function ve_add(vehicles_st: VehiclesState, kind: int, x: float, z: float, yaw:
   return v
 }
 
-# a boat's hull put where the vehicle now is, at rest
-function ve_settle(v: Vehicle) -> void { if v.kind == VEHICLE_BOAT { VehicleHull.put(v.nid, v.x, v.y, v.z, v.yaw) } }
+# a boat's hull or a horse's legs put where the vehicle now is, at rest
+function ve_settle(v: Vehicle) -> void {
+  if v.kind == VEHICLE_BOAT { VehicleHull.put(v.nid, v.x, v.y, v.z, v.yaw) } else { VehicleLegs.stand(v.nid, v.x, v.y, v.z) }
+}
 
-# gone from the world: its model and, for a boat, its hull
+# gone from the world: its model and its hull or legs
 function ve_gone(v: Vehicle) -> void {
-  if v.kind == VEHICLE_BOAT { VehicleHull.gone(v.nid) }
+  if v.kind == VEHICLE_BOAT { VehicleHull.gone(v.nid) } else { VehicleLegs.gone(v.nid) }
   VehicleWorld.gone(v)
 }
 
@@ -73,40 +75,3 @@ export function vehicles_release(vehicles_st: mut VehiclesState, pid: int) -> vo
   }
   vehicles_st.ve_list = keep
 }
-
-# hired or rented for the trip
-export function vehicle_own(v: Vehicle) -> void { if v != null { v.owned = true } }
-
-# hay for a horse, up to a full belly
-export function vehicle_feed(vehicles_st: VehiclesState, v: Vehicle, amount: float) -> void {
-  if v == null { return }
-  v.food = Math.min(v.food + amount, 100.0)
-  ve_say(vehicles_st, VEHICLE_FED, v, amount)
-}
-
-# a horse nobody rides goes where its animal goes, round the home it is tied at (home_x, home_z)
-export function vehicle_follow(v: Vehicle, x: float, y: float, z: float, yaw: float) -> void {
-  if v == null or v.rider >= 0 { return }
-  v.x = x
-  v.y = y
-  v.z = z
-  v.yaw = yaw
-}
-
-# a saved trip's word on this player's own: hired, and how fed
-export function vehicle_restore(vehicles_st: mut VehiclesState, kind: int, owned: bool, food: float) -> void {
-  if kind == VEHICLE_HORSE {
-    vehicles_st.ve_kept_horse = owned
-    vehicles_st.ve_kept_food = food
-  } else { vehicles_st.ve_kept_boat = owned }
-  let v = vehicle_find(vehicles_st, kind)
-  if v != null { ve_apply_kept(vehicles_st, v) }
-}
-
-
-function ve_apply_kept(vehicles_st: VehiclesState, v: Vehicle) -> void {
-  if v.kind == VEHICLE_HORSE {
-    v.owned = vehicles_st.ve_kept_horse
-    v.food = vehicles_st.ve_kept_food
-  } else { v.owned = vehicles_st.ve_kept_boat }
-}

packages/ludic.vehicles/horse.ludic

@@ -1,5 +1,5 @@
-# horse.ludic - a horse under the stick: a walk, a canter on stamina, hay for every metre; it stops at
-# water, at a rise it cannot take and at a still thing in its way
+# horse.ludic - a horse under the stick: a walk, a canter on stamina, hay for every metre; it will
+# not step into water, and its legs (VehicleLegs) meet the ground and whatever stands in its way
 const VE_HORSE_WALK: float = 4.5
 const VE_HORSE_GALLOP: float = 11.0
 const VE_HORSE_ACC: float = 4.0
@@ -15,26 +15,31 @@ function ve_ride_horse(vehicles_st: VehiclesState, v: Vehicle, ix: float, iz: fl
     if v.speed < 0.0 { rate = -rate }
     v.yaw = v.yaw - ix * VE_HORSE_TURN * (dt * rate)
   }
+  let fx = -Math.sin(v.yaw)
+  let fz = -Math.cos(v.yaw)
   let d = v.speed * dt
-  ve_travel(vehicles_st, v, v.x - Math.sin(v.yaw) * d, v.z - Math.cos(v.yaw) * d, d)
-  ve_stamina(v, d, dt)
-}
-
-# a horse only on dry ground, up a walkable rise, clear of what is in its way
-function ve_travel(vehicles_st: VehiclesState, v: Vehicle, nx: float, nz: float, d: float) -> void {
-  let nh = VehicleWorld.ground(nx, nz)
-  var ok = true
-  if nh < VehicleWorld.water(nx, nz) + 0.4 { ok = false }
-  if nh - v.y > Math.abs(d) * 0.9 { ok = false }
-  if VehicleWorld.blocked(nx, nz, 0.8) { ok = false }
-  if not ok {
+  let nh = VehicleWorld.ground(v.x + fx * d, v.z + fz * d)
+  if nh < VehicleWorld.water(v.x + fx * d, v.z + fz * d) + 0.4 {
     v.speed = 0.0
     return
   }
-  v.x = nx
-  v.z = nz
-  v.y = nh
-  if Math.abs(d) > 0.0 { ve_say(vehicles_st, VEHICLE_MOVED, v, Math.abs(d)) }
+  ve_legs_walk(vehicles_st, v, fx * v.speed, fz * v.speed, dt)
+  ve_stamina(v, d, dt)
+}
+
+# the legs carry it from where it is; what they covered along its heading is its speed now, never
+# more than was asked, so a trunk or a rise it cannot take brings it to a stand
+function ve_legs_walk(vehicles_st: VehiclesState, v: Vehicle, vx: float, vz: float, dt: float) -> void {
+  if not VehicleLegs.walk(v.nid, v.x, v.y, v.z, vx, vz, dt) { return }
+  let dx = VehicleLegs.x() - v.x
+  let dz = VehicleLegs.z() - v.z
+  v.x = VehicleLegs.x()
+  v.y = VehicleLegs.y()
+  v.z = VehicleLegs.z()
+  let got = (dx * -Math.sin(v.yaw) + dz * -Math.cos(v.yaw)) / dt
+  if v.speed > 0.0 { v.speed = Math.clamp(got, 0.0, v.speed) } else { v.speed = Math.clamp(got, v.speed, 0.0) }
+  let moved = Math.sqrt(dx * dx + dz * dz)
+  if moved > 0.0 { ve_say(vehicles_st, VEHICLE_MOVED, v, moved) }
 }
 
 # a gallop spends stamina and a walk gives it back; every metre costs hay

packages/ludic.vehicles/index.ludic

@@ -9,7 +9,9 @@ import "ports.ludic"
 import "state.ludic"
 import "places.ludic"
 import "call.ludic"
+import "keep.ludic"
 import "mount.ludic"
 import "ride.ludic"
 import "horse.ludic"
+import "legs.ludic"
 import "system.ludic"

packages/ludic.vehicles/keep.ludic

@@ -0,0 +1,39 @@
+# keep.ludic - what a player does with their own between rides: hires it, feeds the horse, lets it
+# wander where it is tied, and what a saved trip says of them
+# hired or rented for the trip
+export function vehicle_own(v: Vehicle) -> void { if v != null { v.owned = true } }
+
+# hay for a horse, up to a full belly
+export function vehicle_feed(vehicles_st: VehiclesState, v: Vehicle, amount: float) -> void {
+  if v == null { return }
+  v.food = Math.min(v.food + amount, 100.0)
+  ve_say(vehicles_st, VEHICLE_FED, v, amount)
+}
+
+# a horse nobody rides goes where its animal goes, round the home it is tied at (home_x, home_z)
+export function vehicle_follow(v: Vehicle, x: float, y: float, z: float, yaw: float) -> void {
+  if v == null or v.rider >= 0 { return }
+  v.x = x
+  v.y = y
+  v.z = z
+  v.yaw = yaw
+  VehicleLegs.stand(v.nid, x, y, z)
+}
+
+# a saved trip's word on this player's own: hired, and how fed
+export function vehicle_restore(vehicles_st: mut VehiclesState, kind: int, owned: bool, food: float) -> void {
+  if kind == VEHICLE_HORSE {
+    vehicles_st.ve_kept_horse = owned
+    vehicles_st.ve_kept_food = food
+  } else { vehicles_st.ve_kept_boat = owned }
+  let v = vehicle_find(vehicles_st, kind)
+  if v != null { ve_apply_kept(vehicles_st, v) }
+}
+
+
+function ve_apply_kept(vehicles_st: VehiclesState, v: Vehicle) -> void {
+  if v.kind == VEHICLE_HORSE {
+    v.owned = vehicles_st.ve_kept_horse
+    v.food = vehicles_st.ve_kept_food
+  } else { v.owned = vehicles_st.ve_kept_boat }
+}

packages/ludic.vehicles/legs.ludic

@@ -0,0 +1,30 @@
+# legs.ludic - a horse's legs in the world's physics (Maroon Lake: a Jolt walker), named by the
+# vehicle's nid. Gravity, steps, slopes and whatever stands in its way are theirs; unbound, the
+# ground is walked as it is, a rise steeper than it can take stops it and nothing else is in the way.
+export port VehicleLegs {
+  walk: fn(int, float, float, float, float, float, float) -> bool = fn ve__walk   # (nid, x, y, z, vx, vz, dt): its pose into x, y, z
+  x: fn() -> float = fn ve__lx
+  y: fn() -> float = fn ve__ly
+  z: fn() -> float = fn ve__lz
+  stand: fn(int, float, float, float) -> void = fn ve__stand   # (nid, x, y, z): tied or led there
+  gone: fn(int) -> void = fn ve__legs_gone
+}
+
+function ve__walk(vehicles_st: mut VehiclesState, nid: int, x: float, y: float, z: float, vx: float, vz: float, dt: float) -> bool {
+  vehicles_st.ve_lx = x
+  vehicles_st.ve_ly = y
+  vehicles_st.ve_lz = z
+  let nx = x + vx * dt
+  let nz = z + vz * dt
+  let nh = VehicleWorld.ground(nx, nz)
+  if nh - y > Math.sqrt(vx * vx + vz * vz) * dt * 0.9 { return true }
+  vehicles_st.ve_lx = nx
+  vehicles_st.ve_ly = nh
+  vehicles_st.ve_lz = nz
+  return true
+}
+function ve__lx(vehicles_st: VehiclesState) -> float { return vehicles_st.ve_lx }
+function ve__ly(vehicles_st: VehiclesState) -> float { return vehicles_st.ve_ly }
+function ve__lz(vehicles_st: VehiclesState) -> float { return vehicles_st.ve_lz }
+function ve__stand(nid: int, x: float, y: float, z: float) -> void { }
+function ve__legs_gone(nid: int) -> void { }

packages/ludic.vehicles/ports.ludic

@@ -4,7 +4,6 @@
 export port VehicleWorld {
   ground: fn(float, float) -> float
   water: fn(float, float) -> float = fn ve__dry            # the surface over a point
-  blocked: fn(float, float, float) -> bool = fn ve__open   # (x, z, r): a collider in a horse's way
   swell: fn(float, float, float, float) -> float = fn ve__calm   # (x, z, fx, fz): 0..1 of a stroke lost
   wind: fn() -> float = fn ve__zero                        # 0..1
   wind_yaw: fn() -> float = fn ve__zero                    # the yaw the wind pushes toward
@@ -56,10 +55,12 @@ export state VehiclesState {
   ve_rail_yaw: float = 0.0
   ve_have_dock: bool = false
   ve_swell_said: float = 0.0
+  ve_lx: float = 0.0            # the unbound legs' pose
+  ve_ly: float = 0.0
+  ve_lz: float = 0.0
   ve_fact_q: Queue<VehicleFact> = ve_fact_q__new()
 }
 function ve__dry(x: float, z: float) -> float { return -10000.0 }
-function ve__open(x: float, z: float, r: float) -> bool { return false }
 function ve__calm(x: float, z: float, fx: float, fz: float) -> float { return 0.0 }
 function ve__zero() -> float { return 0.0 }
 function ve__next(vehicles_st: mut VehiclesState) -> int {

packages/ludic.vehicles/system.ludic

@@ -3,7 +3,10 @@
 # read back and pushed by the wind in vehicles_tick, which the game calls where the world is run
 export function vehicles_reset(vehicles_st: mut VehiclesState) -> void {
   if vehicles_st.ve_list != null {
-    for i in 0 .. len(vehicles_st.ve_list) { if vehicles_st.ve_list[i].kind == VEHICLE_BOAT { VehicleHull.gone(vehicles_st.ve_list[i].nid) } }
+    for i in 0 .. len(vehicles_st.ve_list) {
+      let v = vehicles_st.ve_list[i]
+      if v.kind == VEHICLE_BOAT { VehicleHull.gone(v.nid) } else { VehicleLegs.gone(v.nid) }
+    }
   }
   vehicles_st.ve_list = new []Vehicle
   vehicles_st.ve_cur = null

packages/ludic.vehicles/tests/vehicles_test.ludic

@@ -1,8 +1,8 @@
 # vehicles_test.ludic - a fake shore: land at x < 10, a lake past it deepening a metre a metre, a
 # pier head at the shore facing out and a trough on the meadow, a trunk in a horse's way. Calling to
 # berths and bays, the owner and the rider, mounting a hungry horse, riding both, the swell, the wind,
-# getting off near a shore and far from one, a player leaving, and the save. The boat's hull is a
-# real ludic.physics body on the same lake.
+# getting off near a shore and far from one, a player leaving, and the save. The boat's hull and the
+# horse's legs are real ludic.physics bodies on the same ground, the trunk a post in it.
 import "ludic.vehicles"
 import "ludic.base"
 import "ludic.physics"
@@ -17,7 +17,6 @@ program VehiclesTest {
     if x > 9.5 { return 0.0 }
     return -10000.0
   }
-  function fk_blocked(x: float, z: float, r: float) -> bool { return Math.abs(x) < 1.0 + r and Math.abs(z + 60.0) < 1.0 + r }
   state VehiclesTestState {
     swell: float = 0.0
     wind: float = 0.0
@@ -38,6 +37,11 @@ program VehiclesTest {
     hyaw: float = 0.0
     hspeed: float = 0.0
     hulls: int = 0
+    legs: int = -1
+    lx: float = 0.0
+    ly: float = 0.0
+    lz: float = 0.0
+    stood: int = 0
   }
   function fk_swell(vehicles_test_st: VehiclesTestState, x: float, z: float, fx: float, fz: float) -> float {
     if fx > 0.0 { return vehicles_test_st.swell }
@@ -79,11 +83,28 @@ program VehiclesTest {
   function fk_lake(physics_st: mut PhysicsState) -> void {
     if phys_is_open(physics_st) { return }
     phys_open(physics_st, 64, 1)
-    let n = 96
+    let n = 160
     let h = floats(n * n)
-    for j in 0 .. n { for i in 0 .. n { h[j * n + i] = fk_ground(float(i) - 20.0, float(j) - 48.0) } }
-    phys_ground_add(physics_st, phys_heightfield(physics_st, h, n, -20.0, -48.0, 1.0))
+    for j in 0 .. n { for i in 0 .. n { h[j * n + i] = fk_ground(float(i) - 20.0, float(j) - 100.0) } }
+    phys_ground_add(physics_st, phys_heightfield(physics_st, h, n, -20.0, -100.0, 1.0))
+    phys_static_add(physics_st, phys_box(physics_st, 15.0, 5.0, 1.0), 0.0, 0.0, -60.0, 0.0)
   }
+
+  # the legs: the horse's own walker on that ground, made on its first step
+  function fk_walk(physics_st: mut PhysicsState, vehicles_test_st: mut VehiclesTestState, nid: int, x: float, y: float, z: float, vx: float, vz: float, dt: float) -> bool {
+    fk_lake(physics_st)
+    if vehicles_test_st.legs < 0 { vehicles_test_st.legs = phys_walker_add(physics_st, 0.8, 1.7, x, y, z, 40.0, 450.0) }
+    phys_walker_move(physics_st, vehicles_test_st.legs, x, y, z, vx, vz, 0.0, 0.45, 40.0, dt)
+    let p = phys_walker_pose(physics_st, vehicles_test_st.legs)
+    vehicles_test_st.lx = p.x
+    vehicles_test_st.ly = p.y
+    vehicles_test_st.lz = p.z
+    return true
+  }
+  function fk_lx(vehicles_test_st: VehiclesTestState) -> float { return vehicles_test_st.lx }
+  function fk_ly(vehicles_test_st: VehiclesTestState) -> float { return vehicles_test_st.ly }
+  function fk_lz(vehicles_test_st: VehiclesTestState) -> float { return vehicles_test_st.lz }
+  function fk_stand(vehicles_test_st: mut VehiclesTestState, nid: int, x: float, y: float, z: float) -> void { vehicles_test_st.stood += 1 }
   function fk_put(physics_st: mut PhysicsState, vehicles_test_st: mut VehiclesTestState, nid: int, x: float, y: float, z: float, yaw: float) -> void {
     fk_lake(physics_st)
     if vehicles_test_st.hull >= 0 and vehicles_test_st.hull_nid == nid {
@@ -120,8 +141,9 @@ program VehiclesTest {
   function fk_hspeed(vehicles_test_st: VehiclesTestState) -> float { return vehicles_test_st.hspeed }
   bind VehicleHull { put: fn fk_put, row: fn fk_row, drift: fn fk_drift, read: fn fk_read, x: fn fk_hx, y: fn fk_hy, z: fn fk_hz, yaw: fn fk_hyaw, speed: fn fk_hspeed, gone: fn fk_hull_gone }
   bind PhysWater { height: fn fk_water }
+  bind VehicleLegs { walk: fn fk_walk, x: fn fk_lx, y: fn fk_ly, z: fn fk_lz, stand: fn fk_stand }
 
-  bind VehicleWorld { ground: fn fk_ground, water: fn fk_water, blocked: fn fk_blocked, swell: fn fk_swell, wind: fn fk_wind, made: fn fk_made, gone: fn fk_gone, placed: fn fk_placed }
+  bind VehicleWorld { ground: fn fk_ground, water: fn fk_water, swell: fn fk_swell, wind: fn fk_wind, made: fn fk_made, gone: fn fk_gone, placed: fn fk_placed }
   bind VehicleRider { board: fn fk_board, ride_at: fn fk_ride, step_off: fn fk_off, safe_spot: fn fk_safe, safe_x: fn fk_sx, safe_z: fn fk_sz }
 
   const EAST: float = -1.5707963
@@ -207,7 +229,7 @@ program VehiclesTest {
     expect(vehicle_in_place(vehicles_st, VEHICLE_HORSE, 0) == null)
   }
 
-  test "a horse walks, gallops on stamina, stops at the water and at a trunk" (physics_st: mut PhysicsState, vehicles_st: mut VehiclesState, vehicles_test_st: mut VehiclesTestState) {
+  test "a horse walks, gallops on stamina, is pushed aside by a trunk it glances, stops at a fence and the water; called, it stands at its bay" (physics_st: mut PhysicsState, vehicles_st: mut VehiclesState, vehicles_test_st: mut VehiclesTestState) {
     fresh(vehicles_st, vehicles_test_st)
     vehicle_call_for(vehicles_st, 0, VEHICLE_HORSE, 0)
     let h = vehicle_find(vehicles_st, VEHICLE_HORSE)
@@ -221,10 +243,14 @@ program VehiclesTest {
     expect(h.z < -15.0)
     expect_near(vehicles_test_st.rz, h.z, 0.001)
     expect_near(vehicles_test_st.ry, 1.35, 0.001)
-    # the trunk at z = -60 stops it
+    # a trunk half a metre off its line pushes it aside and lets it by; the fence at z = -60 stops it
+    let x0 = h.x
+    phys_static_add(physics_st, phys_cylinder(physics_st, 3.0, 0.3), x0 + 0.5, 0.0, -35.0, 0.0)
     ride(physics_st, vehicles_st, 0.0, 1.0, true, 6.0)
-    expect(h.z > -60.0 + 1.7)
-    expect_near(h.speed, 0.0, 0.001)
+    expect(h.z > -59.0 - 0.85)
+    expect(h.z < -50.0)
+    expect(Math.abs(h.x - x0) > 0.2)
+    expect_near(h.speed, 0.0, 0.05)
     # and the lake to the east does too
     vehicle_drop(vehicles_st)
     h.x = 7.0
@@ -233,6 +259,11 @@ program VehiclesTest {
     vehicle_mount(vehicles_st, h)
     ride(physics_st, vehicles_st, 0.0, 1.0, false, 4.0)
     expect(h.x < 9.6)
+    # a horse brought to its bay is stood there
+    vehicle_drop(vehicles_st)
+    let before = vehicles_test_st.stood
+    expect(vehicle_call_for(vehicles_st, 0, VEHICLE_HORSE, 1))
+    expect_eq(vehicles_test_st.stood, before + 1)
   }
 
   test "a boat is rowed out, loses its stroke to the swell, turns, is stopped by the shore's bed, drifts on the wind" (physics_st: mut PhysicsState, vehicles_st: mut VehiclesState, vehicles_test_st: mut VehiclesTestState) {