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Remove midpoint integration, superseded by free-body gyroscopic derivatives.

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The gyroscopic (bias) derivatives applied to standalone free bodies by the
implicitfast integrator provide comparable stability for spinning bodies,
with none of midpoint's restrictions: they apply under contacts, fluid
forces and constraints, and preserve the linear force-velocity relation
required by discrete-time inverse dynamics. The invdiscrete flag reverts to
its original single meaning and no longer affects forward dynamics.

Restore implicitfast coverage in the DiscreteInverseMatch test, removed
when midpoint made discrete inverse dynamics untestable.

Add implicit gyroscopic (bias) derivatives for free bodies in implicitfast.

The implicitfast integrator drops the RNE (bias) derivative to stay on the
symmetric Cholesky path, so fast-spinning free bodies integrate gyroscopic
forces explicitly and can gain energy. Symmetrizing the gyroscopic Jacobian
is not an option: its stabilizing content is the antisymmetric part, and
adding only the symmetric part is destabilizing.

Instead, exploit the fact that for a standalone free body the 6x6 block of
M - h*D is decoupled from the rest of the system (qDeriv sparsity is
tree-local): after the global solve, rebuild the block with the exact bias
derivative in closed form (mjd_freeBias_vel) and re-solve it with dense
unsymmetric LU, overwriting the block's rows of qacc. For lone spinning
bodies this makes implicitfast match implicit to rounding, at ~150ns per
eligible body: cheaper than the midpoint machinery it will replace.
Eligibility is structural only; contacts, fluid and constraints need no
gating. The same block is mirrored in discrete inverse dynamics
(mj_discreteAcc), making invdiscrete exact for spinning free bodies.

PiperOrigin-RevId: 948472495
Change-Id: I813ef3d98c7b399881bc8603b9f9208cfb02eb58
This commit is contained in:
Yuval Tassa
2026-07-15 12:07:10 -07:00
committed by Copybara-Service
parent b2106db52f
commit f0fa3d8260
12 changed files with 631 additions and 840 deletions
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doc/XMLreference.rst
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@@ -661,22 +661,13 @@ from its default.
.. _option-flag-invdiscrete:
:at:`invdiscrete`: :at-val:`[disable, enable], "disable"`
This dual-purpose flag enables discrete-time inverse dynamics and disables :ref:`midpoint integration<geMidpoint>`.
Enable discrete-time inverse dynamics
This flag **enables** discrete-time inverse dynamics with :ref:`mj_inverse` for all
:ref:`integrators<option-integrator>` other than ``RK4``. Recall from the :ref:`numerical
integration<geIntegration>` section that the one-step integrators (``Euler``, ``implicit`` and ``implicitfast``),
modify the mass matrix :math:`M \rightarrow M-hD`. This implies that finite-differenced accelerations
:math:`(v_{t+h} - v_t)/h` will not correspond to the continuous-time acceleration ``mjData.qacc``. When this flag
is enabled, :ref:`mj_inverse` will interpret ``qacc`` as having been computed from the difference of two sequential
velocities, and undo the above modification.
Disable midpoint integration
Additionally and relatedly, this flag **disables** :ref:`midpoint integration<geMidpoint>` for free bodies, which
would otherwise break the linear relationship between finite-differenced velocities and forces assumed by discrete
inverse dynamics. Note that disabling midpoint integration might be useful for debugging or for other reasons,
regardless or whether inverse dynamics are used.
This flag enables discrete-time inverse dynamics with :ref:`mj_inverse` for all
:ref:`integrators<option-integrator>` other than ``RK4``. Recall from the
:ref:`numerical integration<geIntegration>` section that the one-step integrators (``Euler``, ``implicit`` and
``implicitfast``), modify the mass matrix :math:`M \rightarrow M-hD`. This implies that finite-differenced
accelerations :math:`(v_{t+h} - v_t)/h` will not correspond to the continuous-time acceleration ``mjData.qacc``.
When this flag is enabled, :ref:`mj_inverse` will interpret ``qacc`` as having been computed from the difference of
two sequential velocities, and undo the above modification.
.. _option-flag-multiccd: