chenlin/Mujoco_WASM
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

Improve documentation of mesh pre-processing.

Browse Source 
PiperOrigin-RevId: 606268723
Change-Id: I1d939cbb214036ab477e12ae3fd266a25ff89399
This commit is contained in:
Yuval Tassa
2024-02-12 09:03:20 -08:00
committed by Copybara-Service
parent b224367a9b
commit 72cf304e2b
1 changed files with 20 additions and 19 deletions
Download Patch File Download Diff File Expand all files Collapse all files
doc/XMLreference.rst
+20 -19
View File
@@ -1218,40 +1218,41 @@ construct simple shapes directly in the XML. For example, a pyramid can be creat
.. code-block:: xml
<asset>
<mesh name="tetrahedron" vertex="0 0 0 1 0 0 0 1 0 0 0 1"/>
<mesh name="tetrahedron" vertex="0 0 0 1 0 0 0 1 0 0 0 1"/>
</asset>
Positioning and orienting is complicated by the fact that vertex data are often designed relative to coordinate frames
whose origin is not inside the mesh. In contrast, MuJoCo expects the origin of a geom's local frame to coincide with the
geometric center of the shape. We resolve this discrepancy by pre-processing the mesh in the compiler, so that it is
centered around (0,0,0) and its principal axes of inertia are the coordinate axes. We also save the translation and
rotation offsets needed to achieve such alignment in :ref:`mjModel.mesh_pos<mjModel>` and
:ref:`mjModel.mesh_quat<mjModel>`. These offsets are then applied to the referencing geom's position and orientation; see
also :at:`mesh` attribute of :ref:`geom <body-geom>` below. Fortunately most meshes used in robot models are designed in
a coordinate frame centered at the joint. This makes the corresponding MJCF model intuitive: we set the body frame at the
joint, so that the joint position is (0,0,0) in the body frame, and simply reference the mesh. Below is an MJCF model
fragment of a forearm, containing all the information needed to put the mesh where one would expect it to be. The body
position is specified relative to the parent body, namely the upper arm (not shown). It is offset by 35 cm which is the
typical length of the human upper arm. If the mesh vertex data were not designed in the above convention, we would have
to use the geom position and orientation (or the new refpos, refquat mechanism) to compensate, but in practice this is
Positioning and orienting is complicated by the fact that vertex data in the source asset are often relative to
coordinate frames whose origin is not inside the mesh. In contrast, MuJoCo expects the origin of a geom's local frame to
coincide with the geometric center of the shape. We resolve this discrepancy by pre-processing the mesh in the compiler,
so that it is centered around (0,0,0) and its principal axes of inertia are the coordinate axes. We save the translation
and rotation offsets applied to the source asset in :ref:`mjModel.mesh_pos<mjModel>` and
:ref:`mjModel.mesh_quat<mjModel>`; these are required if one reads vertex data from the source and needs to re-apply the
transform. These offsets are then composed with the referencing geom's position and orientation; see also the :at:`mesh`
attribute of :ref:`geom <body-geom>` below. Fortunately most meshes used in robot models are designed in a coordinate
frame centered at the joint. This makes the corresponding MJCF model intuitive: we set the body frame at the joint, so
that the joint position is (0,0,0) in the body frame, and simply reference the mesh. Below is an MJCF model fragment of
a forearm, containing all the information needed to put the mesh where one would expect it to be. The body position is
specified relative to the parent body, namely the upper arm (not shown). It is offset by 35 cm which is the typical
length of the human upper arm. If the mesh vertex data were not designed in the above convention, we would have to use
the geom position and orientation (or the :at:`refpos`, :at:`refquat`` mechanism) to compensate, but in practice this is
rarely needed.
.. code-block:: xml
<asset>
<mesh file="forearm.stl"/>
<mesh file="forearm.stl"/>
</asset>
<body pos="0 0 0.35"/>
<joint type="hinge" axis="1 0 0"/>
<geom type="mesh" mesh="forearm"/>
<joint type="hinge" axis="1 0 0"/>
<geom type="mesh" mesh="forearm"/>
</body>
The inertial computation mentioned above is part of an algorithm used not only to center and align the mesh, but also to
infer the mass and inertia of the body to which it is attached. This is done by computing the centroid of the triangle
faces, connecting each face with the centroid to form a triangular pyramid, computing the mass and signed inertia of all
pyramids (considered solid or hollow if :at:`shellinertia` is true) and accumulating them. The sign ensures that
pyramids on the outside of the surfaces are subtracted, as it can occur with concave geometries. This algorithm can be
pyramids (considered solid, or hollow if :at:`shellinertia` is true) and accumulating them. The sign ensures that
pyramids on the outside of the surfaces are subtracted, as can occur with concave geometries. This algorithm can be
found in section 1.3.8 of Computational Geometry in C (Second Edition) by Joseph O'Rourke.
The full list of processing steps applied by the compiler to each mesh is as follows: