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Implement midpoint integrator for free bodies.

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PiperOrigin-RevId: 899043541
Change-Id: I0bb38f6ad94e189b45ab16777a04ad6fefc6adf7
This commit is contained in:
Yuval Tassa
2026-04-13 09:37:51 -07:00
committed by Copybara-Service
parent d9b5d8babb
commit 0c337799bd
12 changed files with 913 additions and 29 deletions
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doc/XMLreference.rst
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@@ -657,13 +657,22 @@ from its default.
.. _option-flag-invdiscrete:
:at:`invdiscrete`: :at-val:`[disable, enable], "disable"`
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.
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.
.. _option-flag-multiccd: