Bounce

Basic contacts

Balls bounce on pads from dead to very bouncy.

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The steel preset (the default), recordedon an NVIDIA RTX PRO 6000 Blackwell.

Run it

cd examples
# the default preset (steel), then the others
uv run shelf play bounce
# change solver settings live
uv run shelf tune bounce

Source: examples/shelf/01_basic_contacts/bounce/ · ported from steel-webgpu tools/scenes.py bounce() @ ad03e49

Presets

Preset	Solver	Status	Checked
steeldefault	steel	works	2026-11-10, newton f6fc82a8b, NVIDIA RTX PRO 6000 Blackwell Server Edition

Solvers with no row are untested on this example. The full settings are inpresets.toml.

Code

example.py

examples/shelf/01_basic_contacts/bounce/example.py

# SPDX-FileCopyrightText: Copyright (c) 2026 Skild AI and the Steel contributors
# SPDX-License-Identifier: Apache-2.0
# Direct source: steel-webgpu; see presets.toml and THIRD_PARTY.md.
# Modified for Steel Shelf: preset-driven execution and shared runner integration.

###########################################################################
# Example Bounce
#
# Restitution: five balls, each on a pad of the same restitution (0 ... 0.9), dropped from 0.8 m.
#
# Ported from steel-webgpu's tools/scenes.py (bounce); the scene builds
# as it does there and the shelf's preset drives the solver.
#
# Command: python -m shelf run bounce
#
###########################################################################

import math

import numpy as np
import warp as wp

import newton
import newton.examples
from common.steel_scenes import BLUE, GOLD, GREY, RED, TEAL, SceneExample, cfg, tf


class Example(SceneExample):
    file = __file__
    camera = {'eye': (1.25, -1.6, 1.25), 'target': (0.1, 0.05, 0.15)}

    def build(self):
        """Restitution: five balls, each on a pad of the same restitution (0 ... 0.9), dropped from 0.8 m."""
        b = newton.ModelBuilder()
        b.add_ground_plane()
        twists = {}
        for i, e in enumerate((0.0, 0.25, 0.5, 0.75, 0.9)):
            x = (i - 2) * 0.25
            b.add_shape_box(-1, xform=tf((x, 0, 0.02)), hx=0.1, hy=0.1, hz=0.02, cfg=cfg(restitution=e), color=GREY)
            body = b.add_body(xform=tf((x, 0, 0.8)))
            b.add_shape_sphere(body, radius=0.04, cfg=cfg(restitution=e, density=1000), color=[BLUE, TEAL, GOLD, RED, BLUE][i])
        return b, twists, "Restitution: balls on pads of restitution 0, 0.25, 0.5, 0.75 and 0.9, left to right."

    def test_post_step(self):
        self._z = getattr(self, "_z", []) + [self.state_0.body_q.numpy()[:, 2].copy()]

    def check(self):
        """Each ball's first rebound is e^2 of its 0.72 m drop within 15 mm (16 x 50: 47, 182, 408, 587 mm)."""
        z = np.array(self._z)
        for ball, e in enumerate((0.0, 0.25, 0.5, 0.75, 0.9)):
            up = np.flatnonzero(np.diff(z[:, ball]) > 0)
            rebound = float(z[up[0] :, ball].max() - 0.08) if len(up) else 0.0
            assert abs(rebound - e * e * 0.72) < 0.015, (
                f"ball {ball} (e={e}) rebounds {1e3 * rebound:.0f} mm, e^2 h is {720 * e * e:.0f}"
            )


if __name__ == "__main__":
    parser = Example.create_parser()
    viewer, args = newton.examples.init(parser)
    newton.examples.run(Example(viewer, args), args)

example.py on GitHub

presets.toml

examples/shelf/01_basic_contacts/bounce/presets.toml

example = "bounce"
source = "steel-webgpu tools/scenes.py bounce() @ ad03e49"
default = "steel"

[check]
# Arguments `python -m shelf check` passes in test mode: 70 frames, the first rebounds; the example's check() asserts the outcome.
args = ["--num-frames", "70"]

[gallery]
short = "Balls bounce on pads from dead to very bouncy."
title = "Bounce"

[xpbd]
status = "untested"

[vbd]
status = "untested"

[mujoco]
status = "untested"

[featherstone]
status = "untested"

[semi_implicit]
status = "untested"

[feather_pgs]
status = "untested"

[steel]
status = "works"
# 4 substeps x 2 iterations: first rebounds of 54, 191, 418 and 598 mm from the 720 mm drop (16 x 50: 47, 182, 408,
# 587; e^2 h: 45, 180, 405, 583), the same as the browser's 4 x 12 at two thirds of the cost. The rebound error
# grows with the substep (2 substeps: 64, 203, 432, 613; 1 substep: 84 ... 643), and iterations do not change it:
# each ball has one contact, so 1 iteration gives the same motion; 2 keep a ball touching pad and floor at once solved.
# 8 x 1 comes within 2% of 16 x 50 at twice the cost. No joints: limits off.
sim = { fps = 60, substeps = 4 }
solver = { solve_mode = "matrix_free", iterations = 2, articulated_contact_response = "propagation_colored" }
checked = "2026-11-10, newton f6fc82a8b, NVIDIA RTX PRO 6000 Blackwell Server Edition"

[kamino]
status = "untested"

[implicit_mpm]
status = "untested"

[style3d]
status = "untested"

presets.toml on GitHub

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