Module rollout 

Differentiable rollouts: exact parameter and contact-surface adjoints.

Re-exported by the umbrella crate as phyz::diff.

One backward pass of the discrete trajectory adjoint yields, for a semi-implicit Euler rollout with a final-state objective:

1. dJ/dπ per body — the 10 spatial-inertia scalars [m, cx, cy, cz, Ixx, Iyy, Izz, Ixy, Ixz, Iyz], exact (dual-number lanes through a scalar-generic ABA; no finite differences). This is the parameter sensitivity StepJacobians never covered.
2. ∂J/∂x per collision-mesh vertex under the differentiable per-vertex ground-penalty contact model — the cotangent a CAD-side surface pullback (vcad’s surface_gradient) consumes.

See step and adjoint for the forward-model and contract details.

Which adjoint to use

The contact model here is the module’s own differentiable per-vertex ground-penalty law — not the convex solve the forward simulator (phyz::Simulator::step_with_contacts) integrates. It exists for the vcad surface-gradient seam (∂J/∂vertex), which needs a smooth force per vertex. For gradients of the physics the engine actually runs — state, control and inertia channels through the convex contact solve — use crate::contact_adjoint instead; its gradients match a finite difference of the real forward path, and this module’s do not (measured: ~94% error on a box-drop mass gradient, phyz’s diff_convex_contact.rs gate).

Re-exports

pub use adjoint::AdjointGradients;

pub use adjoint::AdjointRollout;

pub use adjoint::ContactSetup;

pub use adjoint::FinalStateObjective;

pub use adjoint::adjoint_rollout_gradient;

pub use adjoint::rollout_objective;

pub use step::CollisionMesh;

pub use step::DofLayout;

pub use step::GroundContact;

pub use step::N_INERTIA_PARAMS;

pub use step::inertia_params;

pub use step::joint_nq;

Modules

adjoint

Discrete trajectory adjoint for semi-implicit Euler rollouts.

step

Scalar-polymorphic dynamics step: ABA + forward kinematics + per-vertex ground contact + semi-implicit Euler, generic over T: tang::Scalar.