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Add changelog entry for effective metric flex elasticity.

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Add a changelog entry documenting that flex elasticity (stretch, bending,
interpolation stiffness) is now integrated implicitly inside the CG
constraint solver via an effective metric, replacing the previous post-hoc
CG correction.

PiperOrigin-RevId: 948963146
Change-Id: Id9c2a2920aabd0776f630b2f3a1ab30a214d9bf2
This commit is contained in:
Alessio Quaglino
2026-07-16 07:34:20 -07:00
committed by Copybara-Service
parent 0ce5e98916
commit cadf3bdedd
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doc/changelog.rst
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@@ -57,6 +57,13 @@ General
``nu = sum(ctrlnum)`` and ``nout = sum(outnum)`` dimension ``mjData.ctrl`` and ``mjData.actuator_force``,
respectively, ``nactuator`` is the number of actuators. For existing actuators ``ctrnum = outnum = 1``, so
``nactuator == nu == nout`` and existing code is unaffected.
- Flex elasticity (stretch, bending, interpolation stiffness) is now integrated implicitly inside the CG constraint
solver via an *effective metric*: the mass matrix is augmented with the stiffness Hessian,
so contact and elastic forces are solved against one consistent metric. This replaces the previous post-hoc CG
correction, which modified ``qacc`` after the constraint solve. The gate is ``solver="CG"`` with an implicit
integrator and flex stiffness present; Newton and PGS are unaffected. Bending-only models pay zero per-step
factorization cost (the factor is precomputed in :ref:`mj_setConst<mj_setConst>`). Inverse dynamics
(:ref:`mj_inverse<mj_inverse>`) is now discrete-consistent with forward dynamics for gated models.
.. admonition:: Breaking API changes
:class: attention