NVIDIA Newton vs. FEniCSx (FEM) — a soft-body accuracy benchmark

A quantitative, apples-to-apples comparison of one deformable soft body simulated two ways:

• NVIDIA Newton (on Warp, CUDA) — three solvers: XPBD (fast positional projection), VBD (implicit), SemiImplicit (explicit, differentiable);
• FEniCSx / dolfinx — implicit FEM, used as the reference solve.

Same mesh, same material parameters (Lamé μ, λ), same gravity — only the solver differs (the FEM side uses a compressible Neo-Hookean law, Newton an StVK/co-rotational one at the same μ, λ — equal at small strain). The goal is to make it measurable how far the fast game/robotics solver deviates from an accurate FEM solve, and exactly why.

Key result (hanging bar): at a fast budget all three Newton solvers settle noticeably softer than the FEM/analytic answer — the explicit solver is closest, and the implicit VBD does not automatically track FEM. Exact tip ratios with provenance: docs/STATUS.md; the why (XPBD's force-balance residual vs. VBD's unconverged iterations): 10_hanging_bar.

The references are layered: analytic → FEM → Newton

Where a closed-form solution exists, it anchors both sides — so a skeptic can follow the whole trust chain, not just take FEM on faith:

• the 1-D self-weight bar (hanging bar) — tip elongation ρgL²/2E (an approximate anchor; it omits Poisson contraction and 3-D effects);
• the confined uniaxial Neo-Hookean stress law (material test) — matched to machine precision by a test in tests/;
• the Coulomb μ·W plateau and N = W (friction);
• the Hertz sphere-on-half-space force F = (4/3)·E*·√R·δ^(3/2) (indentation, an approximate anchor for a finite soft slab; the full contact method — penalty / AL law, Hertz derivation, Newmark drop — is in docs/CONTACT.md).

What the FEM reference is (for non-FEM readers): a finite-element solve in FEniCSx/dolfinx that discretises the body into small elements and solves the discretised force balance to convergence — a genuine static equilibrium (net nodal force ≈ 0), unlike a fast positional solver that only projects positions. Its material is a compressible Neo-Hookean law (hyperelastic, i.e. rubber-like) at the Lamé parameters μ (shear modulus — resistance to change of shape) and λ (volumetric stiffness). The FEM is itself checked against these analytic solutions in the simple cases; the fast Newton solvers are then scored against it on the same mesh and material. So the comparison is analytic → FEM → Newton, with each link testable.

The scenarios (named for what they do)

scenario	what it does	the point
hanging bar	a soft bar stretches under self-weight	a closed-form deformation (1-D tip elongation) to score every solver against node-for-node — with the FEM solve as the reference
indentation	a rigid sphere is pressed into a soft slab	FEM gives a calibrated contact-force curve; the fast XPBD gives deformation, not a force (VBD/explicit selectable via --solver; method in docs/CONTACT.md)
drop	a sphere is dropped onto a block (dynamic impact)	transient impact; FEM Newmark + contact vs. Newton solvers (implicit VBD is the natural match; see docs/CONTACT.md)
friction	a block is dragged on a rigid floor	FEM friction force + dissipated work vs. analytic μ·W; XPBD slip only
material test	confined uniaxial squeeze/stretch	stress vs. stretch into large strain (constitutive fidelity)
convergence	swept budgets / meshes	discretisation error (FEM) vs. solver-budget error (XPBD)

Each scenario has newton_run/run_<x>.py, fenics_run/run_<x>.py and compare/<x>.py (the overlay). The analysis lives in the 10/20/30/40_* notebooks, each written as a 10-minute read for a skeptical expert (verdict first, then the mechanism), with a rendered 3D scene of what is being simulated. The contact scenarios (indentation / drop / friction) run all three Newton solvers via --solver on the same soft_contact scene, so the implicit VBD — not just XPBD — is the apples-to-apples partner for the implicit FEM; docs/CONTACT.md covers the version caveats and what "no calibrated contact force" does and does not mean.

The three Newton solvers

• XPBD — positional projection; fast; leaves a finite equilibrium residual → reads soft (the softest of the three Newton solvers at this budget, tip ratio ≈ 3.7× FEM). The canonical run (writes the shared mesh).
• VBD — Vertex Block Descent; implicit (minimises the backward-Euler objective). In principle converges to the implicit solution as iterations grow; on the slender hanging bar at the budget used it still settles ~3× too soft (slow block-Gauss-Seidel propagation of the clamp).
• SemiImplicit — explicit, force-based; the differentiable one → used by the diffsim θ* fit and the material test.

Platform

Newton needs a CUDA GPU — that is the only requirement, so the repo runs on any CUDA-capable machine. The easiest zero-setup option is Google Colab (free GPU; an A100 / high-RAM runtime is comfortable but not required). FEniCSx is CPU (installed via fem-on-colab on the same instance, or conda-forge locally). The repo is public, so Colab opens it directly (File → Open notebook → GitHub) — see 00_setup.ipynb.

If the two large stacks clash in one runtime (numpy/petsc/mpi), run the Newton and FEM sides in separate notebooks — they only exchange data/*.npz.

Quickstart

pip install -e .# registers common/newton_run/fenics_run/compare
pytest tests/ # validate the diagnostics (no GPU needed)
python -m newton_run.run_hanging_bar # XPBD (CUDA); add --solver vbd | semi_implicit
python -m fenics_run.run_hanging_bar --element tet # FEM on Newton's mesh (node-for-node)
python -m fenics_run.run_hanging_bar --element hex # FEM, independent hex mesh
python -m compare.hanging_bar # overlay + figures/ + a text report

Then the other scenarios: run_indentation, run_drop, run_friction, run_stress_strain, convergence (each on both newton_run / fenics_run, then compare.<scenario>), and newton_run.diffsim. The 00_setup.ipynb notebook installs everything and runs the whole pipeline on any CUDA GPU (Colab is the easy path); each scenario appends an OK/ERR line + key results to logs/summary.txt — a running health report.

Layout

src/common/params.py single source of truth (geometry, material, gravity, paths, per-scenario params)
src/common/mesh_io.py shared mesh Newton <-> FEM (tet orientation)
src/common/runlog.py notebook pipeline-stage runner (live stream + logs/summary.txt)
src/newton_run/ run_hanging_bar · run_indentation · run_drop · run_friction · run_stress_strain · convergence · diffsim · _solver (shared solver factory)
src/fenics_run/ run_hanging_bar · run_indentation · run_drop · run_friction · run_stress_strain · convergence
src/compare/ hanging_bar · indentation · drop · friction · stress_strain · convergence · energies · scene
tests/ test_energies.py (finite-difference force check + machine-precision stress check)
10_hanging_bar · 15_material · 20_contact · 25_dynamic · 30_convergence · 40_friction (analysis notebooks)
00_setup.ipynb install + run the whole pipeline (any CUDA GPU; Colab = easy path)

compare/energies.py (pure numpy) computes the diagnostics identically for both solvers — strain energy, Jacobian (det F), internal nodal forces f = −∂U/∂x, and the equilibrium residualf_internal + f_gravity (the sharpest "solve vs. project" measure). Its correctness claims are backed by tests/test_energies.py. compare/scene.py renders the deformed body in 3D (matplotlib, headless-safe), solver-agnostic.

Documentation

• docs/METHOD.md — problem setup, material matching, element variants (tet/hex locking), the three solvers, the energy/residual diagnostics.
• docs/CONTACT.md — the indentation contact law (penalty / Augmented Lagrangian, no C++) and the dynamic drop.
• docs/EXPERIMENTS.md — friction, the material test, the convergence study, and the differentiable θ* fit.
• docs/STATUS.md — what is verified vs. still being confirmed on Colab, with the result numbers and their provenance.
• CLAUDE.md — concise project guide / conventions.

Development

pip install -e ".[dev]"# ruff, pytest, build
ruff check .# lint (config lives in pyproject.toml)
pytest tests/ # the pure-numpy validation tests (no GPU/FEM)
pre-commit install # optional: ruff + hygiene hooks on each commit

CI (.github/workflows/ci.yml) runs ruff + pytest tests/ + a package build on every push. It deliberately does not run the GPU (Newton) or FEM (dolfinx) stages — those have no runner on GitHub Actions and are exercised on a CUDA GPU (Colab or local) via 00_setup.ipynb. The src/ layout means the packages must be installed (pip install -e .) before python -m … resolves them.