Make constraint island discovery on by default.
Also fix latent bug in mjData serialization. PiperOrigin-RevId: 799505349 Change-Id: I299e7cc8133fa2ec0b339a7ff3d02543e06778b1
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Yuval Tassa
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Copybara-Service
Copybara-Service
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@@ -587,6 +587,14 @@ from its default.
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This flag enables the native convex collision detection pipeline instead of using the
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`libccd library <https://github.com/danfis/libccd>`__, see :ref:`convex collisions<coCCD>` for more details.
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.. _option-flag-island:
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:at:`island`: :at-val:`[disable, enable], "enable"`
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This flag enables discovery and construction of constraint islands: disjoint sets of constraints and
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degrees-of-freedom that do not interact and can be solved independently. Islanding is not yet supported by the PGS
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solver. See :ref:`soIsland` for more details. The :ref:`mjVIS_ISLAND <mjtVisFlag>` enables
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`island visualization <https://youtu.be/Vc1tq0fFvQA>`__.
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.. _option-flag-eulerdamp:
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:at:`eulerdamp`: :at-val:`[disable, enable], "enable"`
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@@ -647,14 +655,6 @@ from its default.
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to instabilities that typically manifest as sliding or wobbling. The implementation of this feature depends on the
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selected convex collision pipeline, see :ref:`convex collisions<coCCD>` for more details.
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.. _option-flag-island:
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:at:`island`: :at-val:`[disable, enable], "disable"`
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This flag enables discovery of constraint islands: disjoint sets of constraints and
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degrees-of-freedom that do not interact. The flag currently has no effect on the physics pipeline, but enabling it
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allows for `island visualization <https://youtu.be/Vc1tq0fFvQA>`__.
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In a future release, the constraint solver will exploit the disjoint nature of constraint islands.
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.. _compiler:
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**compiler** (*)
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+2
-2
@@ -37,9 +37,9 @@
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| | | +-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+ |
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| | | | :ref:`sensor<option-flag-sensor>` | :ref:`midphase<option-flag-midphase>` | :ref:`eulerdamp<option-flag-eulerdamp>` | :ref:`autoreset<option-flag-autoreset>` | |
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| | | +-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+ |
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| | | | :ref:`override<option-flag-override>` | :ref:`energy<option-flag-energy>` | :ref:`fwdinv<option-flag-fwdinv>` | :ref:`invdiscrete<option-flag-invdiscrete>` | |
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| | | | :ref:`nativeccd<option-flag-nativeccd>` | :ref:`island<option-flag-island>` | :ref:`override<option-flag-override>` | :ref:`energy<option-flag-energy>` | |
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| | | +-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+ |
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| | | | :ref:`multiccd<option-flag-multiccd>` | :ref:`island<option-flag-island>` | :ref:`nativeccd<option-flag-nativeccd>` | | |
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| | | | :ref:`fwdinv<option-flag-fwdinv>` | :ref:`invdiscrete<option-flag-invdiscrete>` | :ref:`multiccd<option-flag-multiccd>` | | |
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| | | +-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+ |
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+------------------------------------+----+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
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| mujoco |br| |L| | | .. table:: |
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@@ -7,9 +7,16 @@ Upcoming version (not yet released)
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General
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^^^^^^^
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- Constraint island discovery and construction, previously an experimental feature, is now :ref:`documented<soIsland>`
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and promoted to default; disable it with :ref:`option/flag/island <option-flag-island>`. We expect islanding to be
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a strict improvement over the monolithic constraint solver, please let us know if you experience any issues.
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.. admonition:: Breaking API changes
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:class: attention
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- The promotion of islanding to default involved removing the enable flag ``mjENBL_ISLAND`` and
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converting it to a disable flag :ref:`mjDSBL_ISLAND <mjtDisableBit>`.
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- The update of ``mjData.qacc_warmstart`` was moved from the end of the solver call (:ref:`mj_fwdConstraint`) to the
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end of :ref:`mj_step`, and is now updated with all other state variables. This change makes :ref:`mj_forward`
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fully idempotent.
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@@ -43,6 +50,11 @@ MJX
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- ``ten_length`` was moved from ``mjx.Data._impl.ten_length`` to a public field ``mjx.Data.ten_length``.
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Bug fixes
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^^^^^^^^^
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- Fixed a latent bug where MjData objects were not serialized correctly by the Python bindings when islanding was
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enabled.
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Version 3.3.5 (August 8, 2025)
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------------------------------
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@@ -1362,15 +1362,45 @@ representations of the constraint Jacobian and related matrices.
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axes. The right panel illustrates our solution to this problem. We still update one contact at a time, but within a
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contact we update along non-orthogonal axes adapted to the constraint surface, as follows. First, we optimize the
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quadratic cost along the ray from the tip of the cone through the current solution. Then we slice the cone with a
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hyperplane passing through the current solution and orthogonal to the contact normal. This yields an ellipsoid -which
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can be up to 5-dimensional given our contact model. Now we optimize the quadratic cost within this ellipsoid. This is
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an instance of quadratically constrained quadratic programming (QCQP). Since there is only one scalar constraint
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hyperplane passing through the current solution and orthogonal to the contact normal. This yields an ellipsoid which
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can be up to 5-dimensional, given our contact model. Now we optimize the quadratic cost within this ellipsoid. This
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is an instance of quadratically constrained quadratic programming (QCQP). Since there is only one scalar constraint
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(however nonlinear it may be), the dual is a scalar optimization problem over the unknown Lagrange multiplier. We
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solve this problem with Newton's method applied until convergence -- which in practice takes less than 10 iterations,
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and involves small matrices. Overall this algorithm has similar behavior to PGS for pyramidal cones, but it can
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handle elliptic cones without approximating them. It does more work per contact, however the contact dimensionality
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is smaller, and these two factors roughly balance each other.
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.. _soIsland:
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Constraint islands
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~~~~~~~~~~~~~~~~~~
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.. image:: ../images/computation/island.svg
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:width: 58%
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:align: right
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Consider the abstract graph defined by degrees of freedom (dofs) and constraints. A vertex is all the dofs in a single
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kinematic subtree; an edge is a constraint (a contact or equality) between two bodies belonging to different subtrees. A
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*constraint island* is a disjoint sub-graph which can be solved for independently, because constraint forces cannot
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propagate between islands. Constraint island discovery and construction ("islanding") invloves finding the disjoint
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subgraphs and reordering both the dofs and constraints to make them memory-contiguous. This amounts to a
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block-diagonalization of the constraint Jacobian :math:`J`, as illustrated in the figure. On the left is the monolithic
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Jacobian of size :math:`\nc \times \nv`, where we use the :ref:`corresponding <Framework>` size names from MuJoCo's data
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structures ``mjData.nefc`` and ``mjModel.nv``. On the right is the block-diagonalized Jacobian with 3 islands that can
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be solved indepenently. Note that islanding also identifies unconstrained dofs, so ``mjData.nidof``, the total number of
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dofs in all islands, might be smaller than ``mjModel.nv``. While islanding is not free (see implementation in
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`engine_island.c <https://github.com/google-deepmind/mujoco/blob/main/src/engine/engine_island.c>`__), it is worth the
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effort:
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- Different islands require different numbers of iterations to converge, and a monolithic solve would run for the
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number required by the slowest island.
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- Unconstrained dofs are completely untouched by the solver, which otherwise needs to discover that they are unaffected.
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- Solving separate islands can be multi-threaded.
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Islanding is not yet supported by the PGS solver.
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.. _soParameters:
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Parameters
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@@ -1694,7 +1724,8 @@ The stages below compute quantities that depend on the generalized positions ``m
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7. Compute the sparse factorization of the joint-space inertia matrix: :ref:`mj_factorM`
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8. Construct the list of active contacts. This includes both broad-phase and near-phase collision detection:
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:ref:`mj_collision`
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9. Construct the constraint Jacobian and compute the constraint residuals: :ref:`mj_makeConstraint`
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9. Construct the constraint Jacobian, compute the constraint residuals, construct islands: :ref:`mj_makeConstraint`,
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``mj_island`` (not yet exposed in the API)
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10. Compute the actuator lengths and moment arms: :ref:`mj_transmission`
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11. Compute the matrices and vectors needed by the constraint solvers: :ref:`mj_projectConstraint`
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12. Compute sensor data that only depends on position, and the potential energy if enabled: :ref:`mj_sensorPos`,
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File diff suppressed because one or more lines are too long
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After Width: | Height: | Size: 6.7 KiB |
@@ -464,8 +464,9 @@ typedef enum mjtDisableBit_ { // disable default feature bitflags
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mjDSBL_EULERDAMP = 1<<14, // implicit integration of joint damping in Euler integrator
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mjDSBL_AUTORESET = 1<<15, // automatic reset when numerical issues are detected
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mjDSBL_NATIVECCD = 1<<16, // native convex collision detection
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mjDSBL_ISLAND = 1<<17, // constraint island discovery
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mjNDISABLE = 17 // number of disable flags
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mjNDISABLE = 18 // number of disable flags
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} mjtDisableBit;
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typedef enum mjtEnableBit_ { // enable optional feature bitflags
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mjENBL_OVERRIDE = 1<<0, // override contact parameters
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@@ -474,9 +475,8 @@ typedef enum mjtEnableBit_ { // enable optional feature bitflags
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mjENBL_INVDISCRETE = 1<<3, // discrete-time inverse dynamics
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// experimental features:
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mjENBL_MULTICCD = 1<<4, // multi-point convex collision detection
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mjENBL_ISLAND = 1<<5, // constraint island discovery
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mjNENABLE = 6 // number of enable flags
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mjNENABLE = 5 // number of enable flags
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} mjtEnableBit;
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typedef enum mjtJoint_ { // type of degree of freedom
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mjJNT_FREE = 0, // global position and orientation (quat) (7)
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