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Remove "global" option from [compiler/coordinate](https://mujoco.readthedocs.io/en/latest/XMLreference.html#compiler-coordinate) attribute.

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Keeping this rarely-used setting complicates the compiler logic and is blocking future improvements. In order to convert older models which used this option, load and save them in MuJoCo 2.3.3 or older.

PiperOrigin-RevId: 521739137
Change-Id: If0cb40d5db4879a4db9318cc2be258a7923e6e85
This commit is contained in:
Yuval Tassa
2023-04-04 05:58:53 -07:00
committed by Copybara-Service
parent b327058641
commit 7cc42ecf2a
14 changed files with 705 additions and 806 deletions
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doc/modeling.rst
+10 -45
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@@ -182,11 +182,11 @@ no class is specified in the present element but one or more of its ancestor bod
childclass from the nearest ancestor body is used. If the present element specifies a class, that class is used
regardless of any childclass attributes in its ancestor bodies.
Some attributes, such as body position in models defined in global coordinates, can be in a special undefined state.
This instructs the compiler to infer the corresponding value from other information, in this case the positions of the
geoms attached to the body. The undefined state cannot be entered in the XML file. Therefore once an attribute is
defined in a given class, it cannot be undefined in that class or in any of its child classes. So if the goal is to
leave a certain attribute undefined in a given model element, it must be undefined in the active defaults class.
Some attributes, such as body inertia, can be in a special undefined state. This instructs the compiler to infer the
corresponding value from other information, in this case the inertias of the geoms attached to the body. The undefined
state cannot be entered in the XML file. Therefore once an attribute is defined in a given class, it cannot be undefined
in that class or in any of its child classes. So if the goal is to leave a certain attribute undefined in a given model
element, it must be undefined in the active defaults class.
A final twist here is actuators. They are different because some of the actuator-related elements are actually
shortcuts, and shortcuts interact with the defaults setting mechanism in a non-obvious way. This is explained in the
@@ -197,47 +197,12 @@ shortcuts, and shortcuts interact with the defaults setting mechanism in a non-o
Coordinate frames
~~~~~~~~~~~~~~~~~
After compilation the positions and orientations of all elements defined in the kinematic tree are expressed in local
coordinates, relative to the parent body for bodies, and relative to the body that owns the element for geoms, joints,
sites, cameras and lights. Consequently, when a compiled model is saved as MJCF, the coordinates are always local.
However when the user writes an MJCF file, the coordinates can be either local or global, as specified by the
:at:`coordinate` attribute of :ref:`compiler <compiler>`. This setting applies to all position and
orientation data in the MJCF file. A related attribute is :at:`angle`. It specifies whether all angles in the MJCF
file are expressed in degrees or radians (after compilation angles are always expressed in radians). Awareness of
these global settings is essential when constructing new models.
The positions and orientations of all elements defined in the kinematic tree are expressed in local coordinates,
relative to the parent body for bodies, and relative to the body that owns the element for geoms, joints, sites, cameras
and lights.
Global coordinates can sometimes be more intuitive and have the added benefit that body positions and orientations can
be omitted. In that case the body frame is set to the body inertial frame, which can itself be omitted and inferred
from the geom masses and inertias. When an MJCF model is defined in local coordinates on the other hand, the user must
specify the positions and orientations of the body frames explicitly. This is because if they were omitted, and the
elements inside the body were specified in local coordinates relative to the body frame, there would be no way to
infer the body frame.
Here is an example of an MJCF fragment in global coordinates:
.. code-block:: xml
<body>
<geom type="box" pos="1 0 0" size="0.5 0.5 0.5"/>
</body>
When this model is compiled and saved as MJCF (in local coordinates) the same fragment becomes:
.. code-block:: xml
<body pos="1 0 0">
<inertial pos="0 0 0" mass="1000" diaginertia="166.667 166.667 166.667"/>
<geom type="box" pos="0 0 0" size="0.5 0.5 0.5"/>
</body>
The body position was set to the geom position (1 0 0), while the geom and inertial positions were set to (0 0 0)
relative to the body.
In principle the user always has a choice between local and global coordinates, but in practice this choice is viable
only when using geometric primitives rather than meshes. For meshes, the 3D vertex positions are expressed in either
local or global coordinates depending on how the mesh was designed - effectively forcing the user to adopt the same
convention for the entire model. The alternative would be to pre-process the mesh data outside MuJoCo so as to change
coordinates, but that effort is rarely justified.
A related attribute is :ref:`compiler/angle<compiler-angle>`. It specifies whether angles in the MJCF file are expressed
in degrees or radians (after compilation, angles are always expressed in radians).
.. _COrientation: