Using the VBD Solver

Using the VBD Solver#

Vertex Block Descent (VBD) is a Newton solver for cloth and soft-body simulation. In Isaac Lab, VBD is enabled by selecting a NewtonCfg whose solver_cfg is provided by isaaclab_contrib.deformable.

VBD support is experimental. The solver managers, configuration fields, and recommended tuning values may change while Newton deformable support is under active development. A task that works with PhysX or with Newton’s MuJoCo-Warp solver may still need deformable assets, materials, contacts, and coupling tuned before it works well with VBD.

VBD is usually exposed through a task-specific physics preset rather than a general newton_vbd preset. Deformable-only scenes can use VBDSolverCfg directly. Robot or rigid-body scenes usually use CoupledMJWarpVBDSolverCfg or CoupledFeatherstoneVBDSolverCfg so one solver advances rigid bodies and VBD advances deformable particles.

Start from a Supported Deformable Task#

Before adding VBD to a new task, first run one of the experimental Franka deformable tasks:

./isaaclab.sh -p scripts/environments/zero_agent.py --task Isaac-Lift-Soft-Franka-v0 --num_envs 1 --visualizer kit

For the surface-deformable cloth variant, use:

./isaaclab.sh -p scripts/environments/zero_agent.py --task Isaac-Lift-Cloth-Franka-v0 --num_envs 1 --visualizer kit

Both tasks configure MJWarp for the rigid Franka and VBD for the deformable object through CoupledMJWarpVBDSolverCfg. Use these tasks as starting points for asset setup, solver coupling, and contact tuning.

Add a VBD Physics Preset#

Tasks that support multiple physics options usually store SimulationCfg.physics as a PresetCfg. For deformable Newton tasks, the preset can use a small NewtonCfg subclass to carry NewtonModelCfg alongside the normal Newton fields:

from isaaclab.utils.configclass import configclass
from isaaclab_newton.physics import NewtonCfg

from isaaclab_contrib.deformable import NewtonModelCfg


@configclass
class DeformableNewtonCfg(NewtonCfg):
    model_cfg: NewtonModelCfg | None = None

The Franka soft-body task defines a newton_mjwarp_vbd preset that couples MJWarp and VBD:

class PhysicsCfg(PresetCfg):
    # Newton physics: MJWarp rigid + VBD soft, one-way coupled
    # (matches newton/examples/softbody/example_softbody_franka.py)
    newton_mjwarp_vbd: DeformableNewtonCfg = DeformableNewtonCfg(
        solver_cfg=CoupledMJWarpVBDSolverCfg(
            rigid_solver_cfg=MJWarpSolverCfg(
                njmax=40,
                nconmax=20,
                ls_iterations=20,
                cone="pyramidal",
                impratio=1,
                ls_parallel=False,
                integrator="implicitfast",
                ccd_iterations=100,
            ),
            soft_solver_cfg=VBDSolverCfg(
                iterations=10,
                integrate_with_external_rigid_solver=True,
                particle_enable_self_contact=False,
                particle_collision_detection_interval=-1,
            ),
            coupling_mode="two_way",
        ),
        model_cfg=NewtonModelCfg(
            soft_contact_ke=1e4,
            soft_contact_kd=1e-5,
            soft_contact_mu=5.0,
            shape_material_ke=4e4,
            shape_material_kd=1e-5,
            shape_material_mu=5.0,
        ),
        num_substeps=10,
        use_cuda_graph=True,
    )

    physx: PhysxCfg = PhysxCfg()

    default = newton_mjwarp_vbd

The important pieces are:

Add a Newton physics preset whose value is DeformableNewtonCfg.
Use CoupledMJWarpVBDSolverCfg when rigid bodies and deformables must interact in the same scene.
Use soft_solver_cfg=VBDSolverCfg(integrate_with_external_rigid_solver=True) inside a coupled solver so VBD advances only the deformable particles.
Add NewtonModelCfg when body-particle or self-contact values need task-level tuning.
Keep the preset at the same config path used by the task’s SimulationCfg, for example env.sim.physics.

You can select the deformable Newton preset globally:

./isaaclab.sh -p scripts/reinforcement_learning/rsl_rl/train.py --task=Isaac-Lift-Soft-Franka-v0 presets=newton_mjwarp_vbd

or select the physics field directly:

./isaaclab.sh -p scripts/reinforcement_learning/rsl_rl/train.py --task=Isaac-Lift-Soft-Franka-v0 env.sim.physics=newton_mjwarp_vbd

Use the direct path override when only one task field should use the VBD preset. Use presets=newton_mjwarp_vbd when you want every matching preset field in the task config to resolve to that preset. Isaac Lab training scripts accept these Hydra overrides after the regular command line flags; no separator is needed for the examples above.

Check Task and Asset Compatibility#

VBD uses the Newton model built from the task assets. When adding VBD to a new task, validate the following before tuning solver parameters:

The task must already be compatible with the Newton backend. If a rigid-only newton_mjwarp preset fails during model construction, fix the asset or task configuration first.
The scene must include Newton-compatible deformable assets and materials. Use NewtonDeformableBodyMaterialCfg for volume deformables and NewtonSurfaceDeformableBodyMaterialCfg for cloth or surface deformables.
Coupled robot tasks should start with coupling_mode="two_way" when the robot should feel contact forces from the deformable object.
Contact-heavy scenes usually need task-specific num_substeps, VBDSolverCfg, and NewtonModelCfg values. Start from the Franka soft-body or cloth preset that most closely resembles the scene.
Use a small visual smoke test before training. Confirm that the deformable spawns, renders, deforms, and contacts rigid bodies as expected.

VBD Solver Parameters#

The following fields are specific to VBDSolverCfg. They are grouped by the part of the solver they affect.

Core Solve#

Parameter	Description
`iterations`	Default: `10`. Number of VBD iterations per substep. Increasing this value improves deformation and contact convergence, especially for stiff materials or rigid gripper contacts, but increases runtime.
`integrate_with_external_rigid_solver`	Default: `False`. Set to `True` when VBD is used inside a coupled solver so the rigid sub-solver owns rigid-body integration. Leave `False` for deformable-only VBD scenes.

Self-Contact#

Parameter	Description
`particle_enable_self_contact`	Default: `False`. Enables deformable self-contact. Use this for cloth folds or soft bodies that collide with themselves. It increases contact work and usually needs additional tuning.
`particle_self_contact_radius`	Default: `0.005` [m]. Effective self-contact thickness. VBD applies vertex-triangle and edge-edge self-contact response when the current primitive distance is smaller than this radius.
`particle_self_contact_margin`	Default: `0.005` [m]. Self-contact candidate search distance. VBD uses this envelope when building self-contact lists, then applies contact response using `particle_self_contact_radius`. Keep this greater than or equal to the radius to avoid missed contacts.
`particle_collision_detection_interval`	Default: `-1`. Controls how often self-contact detection runs. A negative value detects before initialization only. `0` detects before and immediately after initialization. A positive value `k` detects before every `k` VBD iterations.
`particle_vertex_contact_buffer_size`	Default: `32`. Preallocation size for each vertex’s vertex-triangle self-contact buffer. Increase it if dense folds or high-resolution cloth exceed the default capacity.
`particle_edge_contact_buffer_size`	Default: `64`. Preallocation size for each edge’s edge-edge self-contact buffer. Increase it if dense folds or high-resolution cloth exceed the default capacity.
`particle_topological_contact_filter_threshold`	Default: `2`. Filters contacts between mesh primitives that are close in topology. Increase this to suppress contact between neighboring elements of the same surface. Values greater than `3` can significantly increase compute time.
`particle_rest_shape_contact_exclusion_radius`	Default: `0.0` [m]. Filters self-contact candidates whose rest-configuration distance is shorter than this distance. Increase it when rest-neighbor contacts produce unwanted resistance.

Coupled Solver Parameters#

Use the coupled solver configs when one solver should advance rigid bodies and VBD should advance deformables:

Parameter	Description
`rigid_solver_cfg`	Rigid-body sub-solver configuration. `CoupledMJWarpVBDSolverCfg` uses `MJWarpSolverCfg`; `CoupledFeatherstoneVBDSolverCfg` uses `FeatherstoneSolverCfg`.
`soft_solver_cfg`	VBD sub-solver configuration. In coupled scenes, set `integrate_with_external_rigid_solver=True` so VBD advances only deformable particles.
`coupling_mode="one_way"`	Rigid solver advances first, and VBD reacts to the updated rigid poses. The rigid solver does not feel particle contact forces.
`coupling_mode="two_way"`	Contact reactions from deformables are injected into the rigid solver before the rigid step, then VBD advances deformables against the shared contacts. Use this for manipulation tasks where the robot should be pushed back by deformable contact.
`coupling_mode="kinematic"`	Available on `CoupledFeatherstoneVBDSolverCfg`. Rigid bodies are kinematically updated by Featherstone, and VBD reacts to them. The rigid solver does not feel particle contacts.

The rigid solver parameters still matter. For example, MJWarp’s nconmax and njmax must be large enough for the rigid contacts in the scene, and ccd_iterations can affect fast rigid contacts near deformables. See MJWarp Solver for the MJWarp-side parameters.

Contact and Material Parameters#

Contact Model#

NewtonModelCfg applies contact parameters to the finalized Newton model:

Parameter	Description
`soft_contact_ke`	Default: `1.0e3` [N/m]. Stiffness for body-particle and particle self-contact. Increase it to reduce clipping through rigid shapes or through other deformable particles. If it is too high, the object can stop visibly deforming or require more VBD iterations and substeps.
`soft_contact_kd`	Default: `1.0e-2` [N*s/m]. Contact damping. Increase it to reduce chatter or bouncing. Too much damping can make contact response sticky or overdamped.
`soft_contact_mu`	Default: `0.5`. Friction coefficient for body-particle and particle self-contact. Increase it when a gripper cannot carry the deformable object without slipping.
`shape_material_ke`	Default: `None` [N/m]. Optional override for all rigid collision-shape contact stiffness values in the Newton model. Use this when the rigid-side material parsed from the asset is not appropriate for deformable contact.
`shape_material_kd`	Default: `None` [N*s/m]. Optional override for all rigid collision-shape contact damping values in the Newton model.
`shape_material_mu`	Default: `None`. Optional override for all rigid collision-shape friction values in the Newton model. Body-particle friction depends on both the soft contact and rigid shape friction coefficients.

Volume Deformable Materials#

Use NewtonDeformableBodyMaterialCfg for volume deformables:

Parameter	Description
`density`	Default: `1.0` [kg/m^3]. Material density. Higher density increases particle mass and inertia, so the object accelerates and deforms less for the same contact forces.
`particle_radius`	Default: `0.008` [m]. Particle contact radius used by Newton. Increase it when contacts are missed or detected too late. If it is too large relative to the mesh resolution, contacts can start too early.
`k_mu`	Default: `1.0e5` [Pa]. First Lame material parameter. Higher values make the deformable object stiffer and usually require more VBD iterations, more substeps, or a smaller timestep.
`k_lambda`	Default: `1.0e5` [Pa]. Second Lame material parameter. Higher values make the deformable object stiffer and usually require more VBD iterations, more substeps, or a smaller timestep.
`k_damp`	Default: `0.0` [Pa*s]. Damping for tetrahedral elements. Increase it to reduce oscillations after deformation, but avoid overdamping if the object should rebound.

Surface Deformable Materials#

Use NewtonSurfaceDeformableBodyMaterialCfg for cloth or surface deformables:

Parameter	Description
`density`	Default: `1.0` [kg/m^3]. Material density. Higher density increases particle mass and inertia.
`particle_radius`	Default: `0.008` [m]. Particle contact radius used by Newton.
`tri_ke`	Default: `1.0e4` [Pa]. Triangle area-preserving stiffness. Increase it to reduce cloth stretch.
`tri_ka`	Default: `1.0e4` [Pa]. Triangle area stiffness. Increase it to reduce cloth area change.
`tri_kd`	Default: `1.5e-6` [Pa*s]. Triangle area damping. Increase it to reduce cloth vibration after stretching.
`edge_ke`	Default: `5.0` [N*m]. Bending stiffness. Increase it for stiffer cloth folds; decrease it for softer draping.
`edge_kd`	Default: `1.0e-2` [Nms]. Bending damping. Increase it to damp fold oscillations.

Tuning Workflow#

Use the following sequence when bringing up a new VBD task:

Run one of the supported Franka deformable tasks and confirm your installation, visualizer, and deformable rendering path work.
Add a task-specific VBD or coupled VBD preset copied from the closest supported task.
Run a small visual smoke test with --num_envs 1 before training.
Tune deformable material stiffness and damping until the object deforms in the expected range without rigid contact.
Increase num_substeps or decrease dt if the object is unstable before increasing stiffness further.
Increase iterations when contacts or stiff materials do not converge within a substep.
Tune soft_contact_ke to reduce rigid/deformable clipping, then tune soft_contact_mu for grip and soft_contact_kd for chatter.
Enable self-contact only after body-particle contact is stable, then tune particle_self_contact_radius for active self-contact thickness, particle_self_contact_margin for missed contacts, and particle_collision_detection_interval for detection frequency.
Increase num_envs and profile only after the single-environment scene is stable.

Symptoms and First Parameters to Check#

Symptom	First parameters to check
Rigid bodies visibly clip through the deformable.	Increase `soft_contact_ke`, VBD `iterations`, `num_substeps`, or the deformable material `particle_radius`.
The robot cannot lift the deformable.	Use `coupling_mode="two_way"`, then increase `soft_contact_mu` and rigid-side `shape_material_mu`. Also check gripper actuator stiffness and effort limits.
The deformable barely deforms.	Reduce material stiffness, `soft_contact_ke`, or shape contact stiffness.
Contact chatters or bounces.	Increase `soft_contact_kd` or material damping, and consider using more substeps.
Cloth passes through itself.	Enable `particle_enable_self_contact`, increase `particle_self_contact_radius` if the active self-contact thickness is too small, increase `particle_self_contact_margin` if contacts are missed, and use a positive `particle_collision_detection_interval`.
Self-contact is too expensive.	Increase `particle_collision_detection_interval`, reduce mesh resolution, or disable self-contact until the rest of the scene is tuned.

For implementation details of the VBD and coupled solver managers, see Newton Manager Abstraction.