House of cards (mujoco.org)

Many contacts

A four-storey house of 26 thin playing cards stands on friction alone.

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Loop

The steel preset (the default), recordedon an NVIDIA RTX PRO 6000 Blackwell, sped up 2× (the readout shows sim time).

Run it

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

Source: examples/shelf/02_many_contacts/mjorg_house_of_cards/ · ported from google-deepmind/mujoco model/cards/house_of_cards.xml (MuJoCo 4.0 tour on mujoco.org)

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/02_many_contacts/mjorg_house_of_cards/example.py

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

###########################################################################
# Example mujoco.org house of cards
#
# A four-storey house of 26 playing cards, 0.7 mm thick, standing on friction alone.
#
# Replicates the "House of Cards" scene of the MuJoCo 4.0 tour on mujoco.org from its
# MJCF (scene.xml, flattened from google-deepmind/mujoco model/cards/house_of_cards.xml).
# See README.md for the gaps.
#
# Command: python -m shelf run mjorg_house_of_cards
#
###########################################################################

from pathlib import Path

import numpy as np

import newton
import newton.examples
from common.steel_scenes import SceneExample

# How far a card may move before the house counts as failing [m].
CARD_SHIFT_MAX = 0.005


class Example(SceneExample):
    file = __file__
    # MuJoCo's free camera for the scene: azimuth 150, elevation -15 around the statistic centre
    camera = {"eye": (0.44, -0.25, 0.24), "target": (0.0, 0.0, 0.1)}

    def build(self):
        builder = newton.ModelBuilder()
        self.preset.apply_model(builder)
        builder.add_mjcf(str(Path(__file__).with_name("scene.xml")))
        self.start = np.array([list(builder.body_q[b])[:3] for b in range(builder.body_count)])
        return builder, {}

    def check(self):
        """Every card stays where it was built."""
        self.solver.check_constraint_capacity()  # dropped contact rows invalidate the run
        q = self.state_0.body_q.numpy()
        shift = np.linalg.norm(q[:, :3] - self.start, axis=1)
        assert shift.max() < CARD_SHIFT_MAX, (
            f"{int((shift >= CARD_SHIFT_MAX).sum())} cards moved, up to {shift.max():.4f} m"
        )


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/02_many_contacts/mjorg_house_of_cards/presets.toml

example = "mjorg_house_of_cards"
source = "google-deepmind/mujoco model/cards/house_of_cards.xml (MuJoCo 4.0 tour on mujoco.org)"
license = "Apache-2.0"
default = "steel"

[check]
# Arguments `python -m shelf check` passes in test mode.
args = ["--num-frames", "1200"]

[gallery]
short = "A four-storey house of 26 thin playing cards stands on friction alone."
title = "House of cards (mujoco.org)"
frame = 60

[xpbd]
status = "untested"

[vbd]
status = "untested"

[mujoco]
status = "untested"

[featherstone]
status = "untested"

[semi_implicit]
status = "untested"

[feather_pgs]
status = "untested"

[steel]
status = "works"
# Nesterov momentum with warm start keeps the apex pair standing; every colored propagation setting tried
# (4-32 substeps, 16-64 iterations) loses it within 2 s.
sim = { fps = 60, substeps = 8 }
model = { rigid_gap = 0.002 }
solver = { solve_mode = "serial", iterations = 32, articulated_contact_response = "immediate", nesterov_momentum = true, contact_warmstart = true, free_body_constraint_max = 4096 }
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

README

Steel replica of “House of Cards” from the MuJoCo 4.0 tour on mujoco.org. 26 playing cards, 0.7 mm thick, make a four-storey house that stands on friction alone.

• Source: google-deepmind/mujoco model/cards/house_of_cards.xml and its assets/, Apache-2.0 (text in LICENSE). scene.xml is that file compiled with MuJoCo 3.17 and re-emitted flat; the card textures are downscaled to 256 x 256.
• Check: after 20 s no card has moved more than 5 mm, and no contact rows were dropped.

Known gaps:

• Steel needs Nesterov momentum with contact warm start (serial solve, immediate response) at 2 ms steps. With the colored solve and propagation response the apex pair slides apart within 2 s at every setting tried.
• MuJoCo’s contacts are soft and over-damped (solref 0.005 3), with an elliptic cone, impratio 10 and two noslip iterations; Steel’s contacts are hard. Air density and viscosity are not modelled.
• The tour lets the user drag a card; the shelf scene runs untouched.
• Rendering uses the shelf viewer, not MuJoCo’s Filament renderer.

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