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Update doc about MJX meshes, fixes #1325.

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PiperOrigin-RevId: 597342387
Change-Id: Iccb47b4b57adf2392fa5c5a38b2a28e87ace1e3d
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Baruch Tabanpour
2024-01-10 13:35:45 -08:00
committed by Copybara-Service
parent 85ab6183bc
commit d4e8070a3f
1 changed files with 16 additions and 8 deletions
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doc/mjx.rst
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@@ -288,6 +288,22 @@ Single scene simulation
Simulating a single scene (1 instance of :ref:`mjData`), MJX can be **10x** slower than MuJoCo, which has been
carefully optimized for CPU. MJX works best when simulating thousands or tens of thousands of scenes in parallel.
Collisions between large meshes
MJX supports collisions between convex mesh geometries. However the convex collision algorithms
in MJX are implemented differently than in MuJoCo. MJX uses a branchless version of the
`Separating Axis Test <https://ubm-twvideo01.s3.amazonaws.com/o1/vault/gdc2013/slides/822403Gregorius_Dirk_TheSeparatingAxisTest.pdf>`__
(SAT) to determine if geometries are colliding with convex meshes, while MuJoCo uses the Minkowski Portal Refinement (MPR)
algorithm as implemented in `libccd <https://github.com/danfis/libccd>`__.
SAT works well for smaller meshes but suffers in both runtime and memory for larger meshes.
For
collisions between convex meshes and primitives (spheres, capsules, planes), use **3000 vertices or less** for your convex meshes.
For collisions between convex meshes and other convex meshes, use **30 vertices or less**.
With careful
tuning, MJX can simulate scenes with mesh collisions -- see the MJX
`shadow hand <https://github.com/google-deepmind/mujoco/tree/main/mjx/mujoco/mjx/benchmark/model/shadow_hand>`__
config for an example. Speeding up mesh collision detection is an active area of development for MJX.
Large, complex scenes with many contacts
Accelerators exhibit poor performance for
`branching code <https://aschrein.github.io/jekyll/update/2019/06/13/whatsup-with-my-branches-on-gpu.html#tldr>`__.
@@ -310,14 +326,6 @@ Large, complex scenes with many contacts
**950K** and **2.7M** steps per second, respectively. Note that as we increase the number of humanoids (which
increases the number of potential contacts in a scene), MJX throughput decreases more rapidly than MuJoCo.
Scenes with collisions between meshes with many vertices
MJX supports mesh geometries and can determine if two meshes are colliding using branchless versions of
`mesh collision algorithms <https://ubm-twvideo01.s3.amazonaws.com/o1/vault/gdc2013/slides/822403Gregorius_Dirk_TheSeparatingAxisTest.pdf>`__.
These algorithms work well for smaller meshes (with hundreds of vertices) but suffer with large meshes. With careful
tuning, MJX can simulate scenes with mesh collisions well -- see the MJX
`shadow hand <https://github.com/google-deepmind/mujoco/tree/main/mjx/mujoco/mjx/benchmark/model/shadow_hand>`__
config for an example.
.. _MjxPerformance:
Performance tuning