Conveyor belts

11 · Contact and friction

Three static mesh belts carry boxes through friction alone. A Mesh created with enable_surface_velocity=True has per-vertex velocities that the contacts read, and Steel uses the part tangent to each contact. The belts move at 0, 0.1 and 0.25 m/s.

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0.00 s

What to look at

Each box reaches its belt's speed in a fraction of a second.
The belt itself never moves.

Key parameter

Mesh(enable_surface_velocity=True)

After finalize, write mesh.mesh.velocities (m/s per vertex). The array is read in place, also inside a captured CUDA graph.

Try changing

Use velocities that turn around the z axis for a turntable.
Lower the boxes' μ and watch them lag behind the belt.

Concept

Surface velocity

What it is

A mesh with surface velocity tells each contact how fast its surface moves. Friction then drives the relative tangential speed to zero instead of the absolute one, so belts move objects while staying in place.

In this scene

Each box reaches its belt's speed (0.10 and 0.25 m/s) within about 0.1 s.

Considerations

Cost: One mesh query per contact on that mesh; meshes without it pay nothing.
Accuracy: Only the tangent component is used; normal motion needs moving geometry.
Interactions: The velocities array is read in place, also inside captured CUDA graphs.
Use it when: Belts, turntables and treads, without simulating the moving parts.

The script

cd tutorials && python 11_conveyor_surface_velocity.py prints each variant's final metric; --record out.json also writes the replay. Every tutorial imports replay.py from the same folder.

11_conveyor_surface_velocity.py
replay.py

# SPDX-FileCopyrightText: Copyright (c) 2026 Skild AI and the Steel contributors
# SPDX-License-Identifier: Apache-2.0

"""Conveyor belts: a static mesh with per-vertex surface velocity carries boxes through friction."""

import newton
import numpy as np
import warp as wp
from newton.solvers import SolverSteel

from replay import Recorder

DT = 1.0 / 240.0
SUBSTEPS = 8


def belt_mesh(hx, hy, hz):
    """A closed box mesh with counter-clockwise outward winding."""
    v = np.array(
        [[x, y, z] for z in (-hz, hz) for y in (-hy, hy) for x in (-hx, hx)], dtype=np.float32
    )
    faces = [
        (0, 2, 1),
        (1, 2, 3),
        (4, 5, 6),
        (5, 7, 6),  # bottom, top
        (0, 1, 4),
        (1, 5, 4),
        (2, 6, 3),
        (3, 6, 7),  # -y, +y
        (0, 4, 2),
        (2, 4, 6),
        (1, 3, 5),
        (3, 7, 5),  # -x, +x
    ]
    return newton.Mesh(v, np.array(faces, dtype=np.int32).reshape(-1), enable_surface_velocity=True)


def build(speeds):
    builder = newton.ModelBuilder()
    builder.add_ground_plane()
    belts = []
    for i, speed in enumerate(speeds):
        y = 0.3 * i - 0.3 * (len(speeds) - 1) / 2
        mesh = belt_mesh(0.6, 0.1, 0.02)
        builder.add_shape_mesh(
            -1,
            mesh=mesh,
            xform=wp.transform((0.0, y, 0.2), wp.quat_identity()),
            cfg=newton.ModelBuilder.ShapeConfig(mu=0.8),
        )
        belts.append((mesh, speed))
        for k in range(3):
            box = builder.add_body(
                xform=wp.transform((-0.45 + 0.15 * k, y, 0.26), wp.quat_identity())
            )
            builder.add_shape_box(
                box,
                hx=0.04,
                hy=0.04,
                hz=0.04,
                cfg=newton.ModelBuilder.ShapeConfig(density=400.0, mu=0.8),
            )
    model = builder.finalize()
    for mesh, speed in belts:
        # every vertex moves along +x; Steel uses the part tangent to each contact
        mesh.mesh.velocities.assign(np.tile([speed, 0.0, 0.0], (8, 1)).astype(np.float32))
    return model


def simulate(rec, label, speeds):
    model = build(speeds)
    solver = SolverSteel(model)
    pipeline = newton.CollisionPipeline(model)
    contacts = pipeline.contacts()
    state_0, state_1, control = model.state(), model.state(), model.control()

    run = rec.variant(label, model)
    for _ in range(rec.frames(3.0)):
        qd = state_0.body_qd.numpy()
        run.frame(state_0, **{f"belt_{s}": qd[3 * i + 1, 0] for i, s in enumerate(speeds)})
        for _ in range(SUBSTEPS):
            pipeline.collide(state_0, contacts)
            solver.step(state_0, state_1, control, contacts, DT)
            state_0, state_1 = state_1, state_0
    run.check(solver)


if __name__ == "__main__":
    rec = Recorder()
    simulate(rec, "belts at 0, 0.1 and 0.25 m/s", [0.0, 0.1, 0.25])
    rec.save()

# SPDX-FileCopyrightText: Copyright (c) 2026 Skild AI and the Steel contributors
# SPDX-License-Identifier: Apache-2.0

"""Record Steel tutorial runs as replay JSON for the tutorials page, or print a summary.

Every tutorial imports this module. ``python NN_name.py`` runs the variants and prints
the final metric of each; ``--record out.json`` also writes body poses, shapes and metric
series that the page replays in 3D.
"""

import argparse
import base64
import json
import os
import subprocess
import sys
import time

import newton
import numpy as np
import warp as wp

from source_hash import source_sha256

_RENDERED_TYPES = {
    newton.GeoType.PLANE,
    newton.GeoType.SPHERE,
    newton.GeoType.CAPSULE,
    newton.GeoType.ELLIPSOID,
    newton.GeoType.CYLINDER,
    newton.GeoType.BOX,
    newton.GeoType.MESH,
    newton.GeoType.CONE,
    newton.GeoType.CONVEX_MESH,
}


class Recorder:
    """Collect the variants of one tutorial; ``fps`` is the replay frame rate."""

    def __init__(self, fps: int = 30):
        parser = argparse.ArgumentParser()
        parser.add_argument(
            "--record", metavar="OUT.json", help="write a replay file for the tutorials page"
        )
        parser.add_argument("--device", default=None, help="Warp device, for example cuda:0")
        self.args = parser.parse_args()
        if self.args.device:
            wp.set_device(self.args.device)
        self.fps = fps
        self.script = os.path.abspath(sys.argv[0])
        self.variants: list[Variant] = []

    def frames(self, seconds: float) -> int:
        """Number of replay frames in ``seconds``, including the first."""
        return int(round(seconds * self.fps)) + 1

    def variant(
        self,
        label: str,
        model: newton.Model,
        *,
        ghost: bool = False,
        slot: int | None = None,
        scene: str | None = None,
    ) -> "Variant":
        """Start one run; ``ghost`` overlays run ``slot``, ``scene`` groups runs for charts."""
        variant = Variant(label, model, self.fps, ghost, slot, scene)
        self.variants.append(variant)
        return variant

    def save(self) -> None:
        """Print a summary and, with ``--record``, write the replay file."""
        for variant in self.variants:
            last = variant.metrics[-1] if variant.metrics else {}
            flag = "  (rows dropped)" if variant.overflow else ""
            print(f"{variant.label:<48} {json.dumps(last)}  {variant.wall:.2f} s{flag}")
        if not self.args.record:
            return
        source_sha = source_sha256(self.script)
        doc = {
            "script": os.path.basename(self.script),
            "script_sha256": source_sha,
            "newton_commit": _newton_commit(),
            "newton_version": newton.__version__,
            "warp_version": wp.config.version,
            "gpu": wp.get_device().name,
            "fps": self.fps,
            "variants": [v.to_json() for v in self.variants],
        }
        with open(self.args.record, "w") as f:
            json.dump(doc, f, separators=(",", ":"))
        print(f"wrote {self.args.record}")


class Variant:
    """One recorded run: a pose per frame for every body, plus named metric values."""

    def __init__(self, label, model, fps, ghost, slot, scene):
        self.label, self.model, self.fps, self.ghost, self.slot = label, model, fps, ghost, slot
        self.scene = scene
        self.poses: list[np.ndarray] = []
        self.metrics: list[dict] = []
        self.awake: list[list[int]] = []
        self.particles: list[np.ndarray] = []
        self.overflow = False
        self.start = time.perf_counter()
        self.wall = 0.0

    def frame(self, state: newton.State, awake=None, particles=None, **metrics) -> None:
        """Record ``state.body_q``, optional awake flags, particle positions and metric values."""
        self.poses.append(state.body_q.numpy().astype(np.float32).copy())
        if particles is not None:
            self.particles.append(particles.numpy().astype(np.float32).copy())
        self.metrics.append({k: _round(v) for k, v in metrics.items()})
        if awake is not None:
            self.awake.append([int(x) for x in awake.numpy()])
        self.wall = time.perf_counter() - self.start

    def check(self, solver) -> None:
        """Note whether the solver dropped rows; call once after the run."""
        try:
            solver.check_constraint_capacity()
        except RuntimeError:
            self.overflow = True

    def to_json(self) -> dict:
        poses = (
            np.stack(self.poses)
            if self.poses
            else np.zeros((0, self.model.body_count, 7), np.float32)
        )
        return {
            "label": self.label,
            "fps": self.fps,
            "ghost": self.ghost,
            "slot": self.slot,
            "scene": self.scene,
            "bodies": int(self.model.body_count),
            "frames": int(poses.shape[0]),
            "poses": base64.b64encode(poses.astype(np.float32).tobytes()).decode(),
            "shapes": _shape_table(self.model),
            "metric": self.metrics,
            "awake": self.awake or None,
            "particles": self._particles_json(),
            "wall_s": round(self.wall, 3),
            "overflow": self.overflow,
        }

    def _particles_json(self):
        if not self.particles:
            return None
        frames = np.stack(self.particles)
        tris = self.model.tri_indices
        radius = self.model.particle_radius.numpy()
        return {
            "count": int(frames.shape[1]),
            "frames": base64.b64encode(frames.astype(np.float32).tobytes()).decode(),
            "radius": round(float(radius.mean()), 5) if radius.size else 0.0,
            "tris": [int(v) for v in tris.numpy().reshape(-1)]
            if tris is not None and tris.shape[0]
            else None,
        }


def _shape_table(model):
    types = model.shape_type.numpy()
    scales = model.shape_scale.numpy()
    xforms = model.shape_transform.numpy()
    bodies = model.shape_body.numpy()
    flags = model.shape_flags.numpy()
    shapes = []
    for i, t in enumerate(types):
        if int(t) not in _RENDERED_TYPES or not flags[i] & int(newton.ShapeFlags.VISIBLE):
            continue
        shape = {
            "b": int(bodies[i]),
            "t": int(t),
            "s": [round(float(v), 5) for v in scales[i]],
            "x": [round(float(v), 6) for v in xforms[i]],
        }
        if int(t) in (newton.GeoType.MESH, newton.GeoType.CONVEX_MESH):
            mesh = model.shape_source[i]
            shape["v"] = [round(float(v), 5) for v in np.asarray(mesh.vertices).reshape(-1)]
            shape["i"] = [int(v) for v in np.asarray(mesh.indices).reshape(-1)]
        shapes.append(shape)
    return shapes


def _round(value):
    value = float(value)
    return round(value, 5) if np.isfinite(value) else None


def _newton_commit():
    if os.environ.get("NEWTON_COMMIT"):
        return os.environ["NEWTON_COMMIT"]
    try:
        root = os.path.dirname(os.path.dirname(newton.__file__))
        return subprocess.check_output(["git", "-C", root, "rev-parse", "HEAD"], text=True).strip()
    except (OSError, subprocess.CalledProcessError):
        return None

Recorded with SolverSteel on Newton 1.9.0.dev0, Warp 1.19.0, NVIDIA RTX PRO 6000 Blackwell Server Edition. Replays read the state back every frame, so they are not a speed benchmark. Script · page text

Steel docs and tutorials are licensed under Apache-2.0. Third-party notices. "Skild", "Skild AI" and the Skild logo are trademarks of Skild AI and are not licensed under Apache-2.0.