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.397395619494580:eb91f111a0a5c785d9d2026cd67e3333_69e6210c180bcf8a442e5ffe.69e623c7180bcf8a442e6003.69e623c6966af563bdeec98d:Trae CN.T(2026/4/20 21:01:59) #113

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.397395619494580:eb91f111a0a5c785d9d2026cd67e3333_69e6210c180bcf8a442e5ffe.69e623c7180bcf8a442e6003.69e623c6966af563bdeec98d:Trae CN.T(2026/4/20 21:01:59) #113

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6 changes: 6 additions & 0 deletions demo/demo_3d.py
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Original file line number Diff line number Diff line change
Expand Up @@ -25,6 +25,12 @@

mpm.set_gravity((0, -50, 0))

mpm.add_wind_field(
lower_bound=[0, 0, 0],
upper_bound=[3, 10, 10],
force=[200, 0, 0]
)

for frame in range(1500):
mpm.step(4e-3)
colors = np.array([0x068587, 0xED553B, 0xEEEEF0, 0xFFFF00],
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20 changes: 20 additions & 0 deletions engine/mpm_solver.py
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Original file line number Diff line number Diff line change
Expand Up @@ -318,6 +318,26 @@ def set_gravity(self, g):
assert len(g) == self.dim
self.gravity[None] = g

def add_wind_field(self, lower_bound, upper_bound, force):
lower_bound = list(lower_bound)
upper_bound = list(upper_bound)
force = list(force)
assert len(lower_bound) == self.dim
assert len(upper_bound) == self.dim
assert len(force) == self.dim

@ti.kernel
def apply_wind(t: ti.f32, dt: ti.f32, grid_v: ti.template()):
for I in ti.grouped(grid_v):
grid_pos = I * self.dx
inside = True
for d in ti.static(range(self.dim)):
inside = inside and (lower_bound[d] <= grid_pos[d] < upper_bound[d])
if inside:
grid_v[I] += dt * ti.Vector(force)

self.grid_postprocess.append(apply_wind)
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@cursor cursor Bot Apr 20, 2026

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Wind postprocessor overrides bounding box boundary conditions

Medium Severity

The wind postprocessor is appended to grid_postprocess after the bounding box (which is always the first element, added in __init__). During each substep, the bounding box runs first and zeroes out boundary-violating velocities, but then apply_wind adds velocity back, potentially in the outward direction. Unlike existing colliders which only reduce or project velocity, wind adds velocity, so it can override the bounding box constraint and allow particles to escape the simulation domain.

Additional Locations (1)
Fix in Cursor Fix in Web

Reviewed by Cursor Bugbot for commit e333388. Configure here.


@ti.func
def sand_projection(self, sigma, p):
sigma_out = ti.Matrix.zero(ti.f32, self.dim, self.dim)
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