Cushion · noun
A soft pad or pillow, usually set on a sofa, that makes a rigid structure comfortable to work with.
Also: an intermediate layer that softens the interface between two hard things.

Cushion is the soft layer between Grasshopper and SOFA.

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Soft-robotics simulation in Grasshopper, powered by SOFA.

You describe a soft robot the way you already model in Grasshopper: a deformable body, the pneumatic cavities or tendons that drive it, and a schedule of pressures or pulls over time. Cushion meshes the geometry, writes a runnable SOFA scene, launches the simulation, and brings the results (trajectories, forces, contacts, deformed shapes) back onto the canvas.

Developed by Trevor Exley and Anderson Nardin.

The scene structure follows the Monolithic Unit soft-robotics pipeline (Exley, Nardin, Trunin, Cafiso, Beccai; arXiv:2511.13120): corotational tetrahedral FEM, a homogenized lattice material, MeshROI-based local stiffening, and SoftRobots pressure and cable constraints.

GH geometry ──► STL/STEP ──► gmsh tets ──► scene.json + input.csv
                                              │
      GH points/curves ◄── monitor data ◄── runSofa (GUI or batch)

Status

Working prototype (v0.1), tested on Windows 11 and macOS with Rhino 8 and SOFA v24.06. The interface may still change.

Licensed under the GNU Lesser General Public License v2.1 or later. See LICENSE.

📖 Manual (PDF): component reference, installation, and a worked example.

Requirements

• Rhino 8 with Grasshopper. Document units in millimeters.
• Cushion plugin — get it from food4Rhino or Rhino's Package Manager (_PackageManager, search "Cushion"), or drop dist/Cushion.Components.gha into Grasshopper's Components folder. Needed even to open the examples.
• SOFA v24.06.00 binaries with SofaPython3, SoftRobots, STLIB, and SofaValidation (all in the official release bundle): github.com/sofa-framework/sofa/releases/tag/v24.06.00
• Python 3.10 on the system (SofaPython3 v24.06 is built against it) with numpy. This is SOFA's interpreter, separate from Rhino's.
• gmsh in Rhino's own CPython. Install it once: "<rhino-python>" -m pip install gmsh (see docs/INSTALL.md §5).

Full platform-specific steps: docs/INSTALL.md.

Quickstart (inflate a cylinder)

1. Install everything per docs/INSTALL.md: the Cushion plugin, Pufferfish, SOFA, Python 3.10, and gmsh.
2. In Grasshopper, drop the Cushion components from the Cushion tab onto the canvas and wire them as below. (Or skip to the examples for ready-made definitions.)
3. Model a solid cylinder (r 15, h 100 mm) → SofaBody.geo. Put a smaller closed cylinder inside → SofaCavity.geo, channel 1.
4. Box around one end cap → SofaBody.fixations; a few points along the axis → SofaBody.monitorPts.
5. PressureProfile: channel 1, times [0, 1.0], values [0, 20] (kPa).
6. SofaScene: wire the body and the profile, set projDir (a local disk folder), repo (this folder), gravityZ 0, toggle write → True.
7. SofaRun: sofaRoot = your SOFA folder, gui True, run → True. Press Animate in the SOFA window.
8. SofaResults: wire scenePath and bodyName, toggle refresh → the trajectories come back as a DataTree, one branch per frame.

Examples

Two worked definitions live in examples/. Install the Cushion plugin first (see docs/INSTALL.md), then open the .gh. Each has a .md walkthrough with a zoomable screenshot of the full canvas.

Note: both examples use the Reduce Mesh component from the free Pufferfish plugin (food4Rhino) — install it before opening them.

Tendon continuum robot: a cylindrical backbone driven by three cables at 0° / 120° / 240°. A single run sweeps the pull direction a full turn while the Monitor logs the backbone every timestep, so the visited configurations trace out the workspace; the convex hull of the tip cloud gives its bounds. Nothing to touch, so it runs fast and makes a good first sim.

PneuNet two-finger gripper: a fast-PneuNet finger whose chambers and profile are shaped by Graph Mappers and meshed with TriRemesh; the cavities become internal voids automatically (SofaBody.hollow), so there is no boolean subtraction anywhere. One finger is designed flat, oriented twice into a gripper, and carried through a pick-and-place by a SofaProtocol while the chambers inflate: it descends, grips an object, and lifts it.

Components

Component	Role
SofaBody	Deformable FEM body: solid, material, fixations, monitor points, placement
SofaCavity	Pneumatic chamber (SurfacePressureConstraint), driven by channel
SofaTendon	Cable/tendon path (CableConstraint), displacement- or force-driven
SofaRegion	Locally stiffened sub-region (membrane/sensor), closed mesh or auto-shell
PressureProfile	Actuation keyframes for one channel (linear-interpolated, resampled)
SofaProtocol	Keyframed rigid motion of a whole body (pick-and-place); drags it by its fixations
SofaTarget	Rigid interaction object (graspable target, obstacle)
SofaScene	Writes the runnable project (meshes, scene.json, input.csv); optional camera and targets
SofaRun	Launches runSofa (GUI or headless batch)
SofaResults	Reads trajectories, forces, contacts, and actuation history back into GH
SofaShape	Rebuilds the deformed body surface at any exported frame
SetupPreview	Rebuilds the whole written scene as Rhino geometry, gmsh volume included

SofaTendons and SofaProfiles are tree-input variants that drive several channels from one component. Details, input tables, and modeling notes: docs/COMPONENTS.md.

Units & conventions

Millimeters, kPa, kg, and seconds throughout; gravity is −9810 mm/s² in z. SOFA has no built-in up-axis; the gravity vector defines it, and geometry passes from Rhino to SOFA 1:1 with no transform, so the Rhino document must be in millimeters.

Young's moduli are effective values: lattice bodies use the homogenized modulus (Monolithic Unit 1.0×: 18.34 kPa). Heavy lattice geometry belongs in visualGeo only; it is displayed (barycentric-mapped), never meshed or simulated.

Common issues

Most problems announce themselves in a component's info output or in <projDir>/runsofa.log. Read those first. The ones you're most likely to hit:

• Meshing fails ("PLC error", "a segment and a facet intersect"). Two input surfaces overlap, usually a cavity poking through a thin body wall. Keep cavities fully inside the body, keep walls ≥ 2 mm, and refine the mesh (smaller meshSize / TriRemesh length, ≤ half the wall thickness).
• Body free-falls or drifts instead of deforming. The fixation box isn't catching any mesh nodes, or gravity is on with nothing clamped. Make the box swallow part of the mesh, or set gravityZ 0.
• Grasp target slips through or flies away. Check the collision meshes are wired on both the body and target, slow the approach, and confirm a floor catches the object.
• SofaResults is empty. write was left off, the sim wasn't animated, no monitorPts were wired, or bodyName doesn't match.

Full, field-tested list: docs/TROUBLESHOOTING.md.

Repo layout

sofagh/          pure-Python core (no Rhino): meshing, project writer,
                 runner, results parsers, also usable from plain Python
sofagh/runtime/  generic SOFA scene + controller (copied into projects)
gh/ghcommon.py   RhinoCommon helpers (STL/STEP export, shell offset)
gh/components/   the Grasshopper component scripts
docs/            install, component reference, troubleshooting, manual
examples/        the two walkthroughs above
dist/            compiled Grasshopper plugin (.gha)

Any written project folder stands on its own: python -m sofagh.runner path/to/project/scene.py [--batch].