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Cloth

Add a sheet of fabric with scene.add_cloth, drop it over an object, and read where every vertex ends up.

1. Drop a sheet over a stand

import alloy

scene = alloy.Scene(dt=0.01)
scene.add_ground()
scene.add_box("stand", size=(0.2, 0.2, 0.3), pos=(0, 0, 0.15), static=True)         # top at z = 0.3
scene.add_cloth("sheet", size=(0.5, 0.5), pos=(0, 0, 0.5), resolution=10, mass=0.1)
sim = alloy.build(scene)
print(sim.routes()["sheet"]["route"])

cloth_vbd

The sheet is flat, 50 cm square, 20 cm above the stand, with 11 x 11 vertices. sim.routes() says it runs on the cloth solver.

2. Run it and measure the drape

sim.run(1.0)
points = sim.state(["sheet"], world=0)["sheet.points"]
print(points.shape, (0.3 - points[:, 2].min()).round(1))

(121, 3) 0.2

"sheet.points" holds one row per vertex. The number is how far the lowest vertex hangs below the stand top, in metres. After 1 s the sheet has fallen onto the stand and its edges hang 20 cm down the sides.

3. Save a picture

sim.render(width=640, height=480, path="cloth.png")
print(sim.check()["ok"])

True

Keep in mind

  • A sheet thinner than the 2 mm default needs a smaller dt.
  • There is no cloth from a mesh. To cover a shape, drape a flat sheet sized to it from a little above.
  • Cloth is slow on the CPU; keep the resolution low while you try things out.
  • Sheets that fall onto each other can pass through (details).
  • A thin cloth (2 mm) slips out of a robot pinch (details).

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