Add automatic free-joint alignment.
PiperOrigin-RevId: 671010083 Change-Id: Ib7d60f75c7f545b1886703715663db8de77feeef
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Yuval Tassa
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Copybara-Service
Copybara-Service
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8954a088ce
@@ -658,6 +658,15 @@ Type of limit specification.
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.. mujoco-include:: mjtLimited
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.. _mjtAlignFree:
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mjtAlignFree
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~~~~~~~~~~~~
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Whether to align free joints with the inertial frame.
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.. mujoco-include:: mjtAlignFree
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.. _mjtInertiaFromGeom:
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mjtInertiaFromGeom
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@@ -822,6 +822,14 @@ has any effect. The settings here are global and apply to the entire model.
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If this attribute is set to false, computes mesh inertia with the legacy algorithm, which is exact only for convex
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meshes. If set to true, it is exact for any closed mesh geometry.
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.. _compiler-alignfree:
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:at:`alignfree`: :at-val:`[false, true], "false"`
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This attribute toggles the default behaviour of an optimization that applies to bodies with a
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:ref:`free joint<body-freejoint>` and no child bodies.
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When true, the body frame and free joint will automatically be aligned with inertial frame, which leads to both
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faster and more stable simulation. See :ref:`freejoint/align<body-freejoint-align>` for details.
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.. _compiler-inertiagrouprange:
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:at:`inertiagrouprange`: :at-val:`int(2), "0 5"`
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@@ -2251,6 +2259,20 @@ inherited*. If the XML model is saved, it will appear as a regular joint of type
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Integer group to which the joint belongs. This attribute can be used for custom tags. It is also used by the
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visualizer to enable and disable the rendering of entire groups of joints.
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.. _body-freejoint-align:
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:at:`align`: :at-val:`[false, true, auto], "auto"`
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When set to :at-val:`true`, the body frame and free joint will automatically be aligned with inertial frame. When set
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to :at-val:`false`, no alignment will occur. When set to :at-val:`auto`, the compiler's
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:ref:`alignfree<compiler-alignfree>` global attribute will be respected.
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Inertial frame alignment is an optimization only applies to bodies with a free joint and no child bodies ("simple
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free bodies"). The alignment diagonalizes the 6x6 inertia matrix and minimizes bias forces, leading to faster and
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more stable simulation. While this behaviour is a strict improvement, it modifies the semantics of the free joint,
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making ``qpos`` and ``qvel`` values saved in older versions (for example, in :ref:`keyframes<keyframe>`) invalid.
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Note that the :at:`align` attribute is never saved to XML. Instead, the pose of simple free bodies and their children
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will be modified such that the body frame and inertial frame are aligned.
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.. _body-geom:
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+3
-1
@@ -56,6 +56,8 @@
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| | | +-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+ |
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| | | | :ref:`inertiafromgeom<compiler-inertiafromgeom>` | :ref:`inertiagrouprange<compiler-inertiagrouprange>` | :ref:`exactmeshinertia<compiler-exactmeshinertia>` | :ref:`assetdir<compiler-assetdir>` | |
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| | | +-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+ |
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| | | | :ref:`alignfree<compiler-alignfree>` | | | | |
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| | | +-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+ |
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+------------------------------------+----+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
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| |_| compiler |br| |_| |L| | | .. table:: |
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| :ref:`lengthrange | ? | :class: mjcf-attributes |
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@@ -290,7 +292,7 @@
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| :ref:`freejoint | \* | :class: mjcf-attributes |
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| <body-freejoint>` | | |
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| | | +-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+ |
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| | | | :ref:`name<body-freejoint-name>` | :ref:`group<body-freejoint-group>` | | | |
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| | | | :ref:`name<body-freejoint-name>` | :ref:`group<body-freejoint-group>` | :ref:`align<body-freejoint-align>` | | |
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| | | +-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+-----------------------------------------------------------------+ |
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+------------------------------------+----+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
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| |_| body |br| |_| |L| | | .. table:: |
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+16
-3
@@ -18,13 +18,26 @@ General
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- The functions ``mjs_findMesh`` and ``mjs_findKeyframe`` were replaced by ``mjs_findElement``, which allows to look
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for any object type.
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- Added the :ref:`nativeccd<option-flag-nativeccd>` flag. When this flag is enabled, general convex collision
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detection is handled natively, as opposed to using `libccd <https://github.com/danfis/libccd>`__. This feature is in
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early stages of testing.
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- Added a new way of defining :ref:`connect<equality-connect>` equality constraints, using two sites rather than bodies.
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The new semantic is useful when the assumption that the constraint is satisfied in the base configuration does not
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hold. In this case the sites will "snap together" at the beginning of the simulation. Additionally, changing the site
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positions in ``mjModel.site_pos`` at runtime can be used to modify the constraint.
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- Added the :ref:`nativeccd<option-flag-nativeccd>` flag. When this flag is enabled, general convex collision
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detection is handled natively, as opposed to using `libccd <https://github.com/danfis/libccd>`__. This feature is in
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early stages of testing.
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- Introduced an optimization that applies to bodies with a :ref:`free joint<body-freejoint>` and no child bodies (i.e.
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simple free-floating bodies): aligning the free joint (body frame) with the inertial frame. The alignment diagonalizes
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the related 6x6 inertia sub-matrix, leading to faster simulation, and minimizes bias forces, leading to more
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stable simulation of free bodies.
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While this optimization is a strict improvement over unaligned free joints, it also changes the semantics of the
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joint's degrees-of-freedom w.r.t to previous versions. Therefore, ``qpos`` and ``qvel`` values saved in older versions
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(for example, in :ref:`keyframes<keyframe>`) will become invalid.
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This feature can be toggled individually using the :ref:`freejoint/align<body-freejoint-align>` attribute or globally
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using the compiler :ref:`alignfree<compiler-alignfree>` attribute. The latter attribute currently defaults to "false"
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due to the potential breakage described above, but could be changed to "true" in a future release. Aligned free joints
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are recommended for all new models.
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- Added :ref:`mjSpec` option for creating a texture from a buffer.
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- :ref:`shellinertia <body-geom-shellinertia>` is now supported by all geom types.
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- When :ref:`attaching<meAttachment>` sub-models, :ref:`keyframes<keyframe>` will now be correctly merged into the
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@@ -1650,6 +1650,11 @@ typedef enum mjtLimited_ { // type of limit specification
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mjLIMITED_TRUE, // limited
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mjLIMITED_AUTO, // limited inferred from presence of range
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} mjtLimited;
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typedef enum mjtAlignFree_ { // whether to align free joints with the inertial frame
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mjALIGNFREE_FALSE = 0, // don't align
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mjALIGNFREE_TRUE, // align
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mjALIGNFREE_AUTO, // respect the global compiler flag
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} mjtAlignFree;
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typedef enum mjtInertiaFromGeom_ { // whether to infer body inertias from child geoms
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mjINERTIAFROMGEOM_FALSE = 0, // do not use; inertial element required
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mjINERTIAFROMGEOM_TRUE, // always use; overwrite inertial element
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@@ -1688,6 +1693,7 @@ typedef struct mjSpec_ { // model specification
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int inertiafromgeom; // use geom inertias (mjtInertiaFromGeom)
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int inertiagrouprange[2]; // range of geom groups used to compute inertia
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mjtByte exactmeshinertia; // if false, use old formula
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int alignfree; // align free joints with inertial frame
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mjLROpt LRopt; // options for lengthrange computation
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// engine data
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@@ -1778,6 +1784,7 @@ typedef struct mjsJoint_ { // joint specification
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double pos[3]; // anchor position
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double axis[3]; // joint axis
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double ref; // value at reference configuration: qpos0
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int align; // align free joint with body com (mjtAlignFree)
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// stiffness
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double stiffness; // stiffness coefficient
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