Presets

Presets are the most convenient way to declare a simulation. They combine state and energy registration, and optionally geometry generation in a single call. Most of the examples bundled with STARK use presets.

Access them via simulation.presets->deformables and simulation.presets->rigidbodies (C++), or simulation.presets().deformables() / simulation.presets().rigidbodies() (Python).

Handlers

Every preset returns a VCH (Vertex-Connectivity-Handler) struct with three fields:

• vertices — the generated vertex positions

• triangles / segments / tets — the generated connectivity (type depends on preset)

• handler — sub-handlers for each registered energy model, plus a PointSetHandler for transforms and a ContactHandler for friction

Deformable Presets

All deformable presets register: lumped inertia + the appropriate deformation energy + contact.

Volume (Soft Body)

A tet mesh filling an axis-aligned box, or any custom tet mesh. Registered energies: EnergyLumpedInertia + EnergyTetStrain.

// Uniform tet grid
auto [V, T, H] = simulation.presets->deformables->add_volume_grid(
    "body",
    Eigen::Vector3d(0.1, 0.1, 0.2),  // physical size (m)
    {3, 3, 6},                         // subdivisions per axis
    stark::Volume::Params::Soft_Rubber()
);

// Custom tet mesh (e.g. from TetGen / Gmsh)
auto [V, T, H] = simulation.presets->deformables->add_volume("body", vertices, tets, params);

H is a Volume::Handler with fields: point_set, inertia, strain, contact.

Material parameters:

stark::Volume::Params params = stark::Volume::Params::Soft_Rubber();

params.inertia.density                  = 1000.0;  // kg/m³
params.inertia.damping                  = 0.1;

params.strain.elasticity_only           = false;
params.strain.youngs_modulus            = 1e4;
params.strain.poissons_ratio            = 0.3;
params.strain.strain_limit              = 1.0;
params.strain.strain_limit_stiffness    = 1e2;
params.strain.damping                   = 0.0;

params.contact.thickness                = 0.001;

Surface (Cloth / Shell)

A triangle mesh, flat grid or custom, with in-plane strain and optional bending. Registered energies: EnergyLumpedInertia + EnergyTriangleStrain + EnergyDiscreteShells.

// Flat grid
auto [V, T, H] = simulation.presets->deformables->add_surface_grid(
    "cloth",
    Eigen::Vector2d(0.5, 0.5),         // physical size in X and Y
    {30, 30},                           // vertex count per axis
    stark::Surface::Params::Cotton_Fabric()
);

// Custom triangle mesh
auto [V, T, H] = simulation.presets->deformables->add_surface("cloth", vertices, triangles, params);

H is a Surface::Handler with fields: point_set, inertia, strain, bending, contact.

Material parameters:

stark::Surface::Params params = stark::Surface::Params::Cotton_Fabric();

params.inertia.density                  = 0.2;     // kg/m²
params.inertia.damping                  = 0.1;

params.strain.elasticity_only           = false;
params.strain.thickness                 = 0.001;   // m
params.strain.youngs_modulus            = 5e3;
params.strain.poissons_ratio            = 0.3;
params.strain.strain_limit              = 0.1;
params.strain.strain_limit_stiffness    = 1e6;
params.strain.damping                   = 0.1;

params.bending.flat_rest_angle          = true;
params.bending.stiffness                = 1e-6;
params.bending.damping                  = 0.01;

params.contact.thickness                = 0.001;

Line (Rod / Cable)

A 1D chain of segments. Registered energies: EnergyLumpedInertia + EnergySegmentStrain.

// Straight rod with uniform segments
auto [V, S, H] = simulation.presets->deformables->add_line_as_segments(
    "rod",
    Eigen::Vector3d(0, 0, 0),   // start
    Eigen::Vector3d(1, 0, 0),   // end
    20,                          // number of segments
    stark::Line::Params::Elastic_Rubberband()
);

// Arbitrary geometry
auto [V, S, H] = simulation.presets->deformables->add_line("rod", vertices, segments, params);

H is a Line::Handler with fields: point_set, inertia, strain, contact.

Material parameters:

stark::Line::Params params = stark::Line::Params::Elastic_Rubberband();

params.inertia.density                  = 0.05;    // kg/m
params.inertia.damping                  = 0.1;

params.strain.elasticity_only           = false;
params.strain.section_radius            = 0.002;   // m
params.strain.youngs_modulus            = 1e4;
params.strain.strain_limit              = 0.1;
params.strain.strain_limit_stiffness    = 1e5;
params.strain.damping                   = 1e-4;

params.contact.thickness                = 0.001;

Prescribed Surface

A surface with zero dynamic DOFs — vertices follow exactly the transformation you script. Useful for kinematic obstacles and animated rigid-looking objects that need contact. Registered energies: EnergyPrescribedPositions + contact.

auto H = simulation.presets->deformables->add_prescribed_surface(
    "obstacle", vertices, triangles, stark::PrescribedSurface::Params()
);

Rigid Body Presets

Rigid body presets register inertia and attach a collision/render mesh. The inertia tensor is computed automatically from the chosen shape.

// Box (uniform cube or non-uniform)
auto [V, T, H] = simulation.presets->rigidbodies->add_box("box", mass, 0.1);         // cube side
auto [V, T, H] = simulation.presets->rigidbodies->add_box("box", mass, size_vec3);    // non-uniform

// Sphere
auto [V, T, H] = simulation.presets->rigidbodies->add_sphere("ball", mass, radius);

// Cylinder
auto [V, T, H] = simulation.presets->rigidbodies->add_cylinder("cyl", mass, radius, height);

// Torus
auto [V, T, H] = simulation.presets->rigidbodies->add_torus("torus", mass, outer_r, inner_r);

// Custom mesh
auto H = simulation.presets->rigidbodies->add(
    "body", mass, inertia_tensor_local, vertices, triangles
);

H.rigidbody is the RigidBodyHandler for motion control (see Rigid Bodies). H.contact is the ContactHandler for friction pairing (see Contact).