Improve quality of maxhullvert coarse convex hulls using Qhull's Q9 option.

PiperOrigin-RevId: 928417339
Change-Id: I59ecd454f063f2e32e1c3ce08eb4a18506ddf8c8

doc/changelog.rst

@@ -7,15 +7,17 @@ Upcoming version (not yet released)
 
 General
 ^^^^^^^
+- Added :ref:`mju_threadpool`, a new function for creating a thread pool on an ``mjData`` instance. When a thread pool
+  is initialized, parts of the simulation pipeline, such as collision detection and constraint solving across islands,
+  are parallelized. The thread pool is automatically destroyed when the ``mjData`` is freed.
 - Improved primal solver convergence under float32. Improvements initially proposed by :github:user:`n3b` in
   :issue:`2313` and :github:user:`denzeler-nvidia` in :doc:`MJWarp <mjwarp/index>` pull request
   `1374 <https://github.com/google-deepmind/mujoco_warp/pull/1374>`__.
 - Added :ref:`mjs_makeFlex`, a new C API function equivalent to the :ref:`flexcomp<body-flexcomp>` element for
   programmatically creating flex objects with auto-generated bodies, joints, and equality constraints. Exposed as
   ``body.make_flex()`` in Python.
-- Added :ref:`mju_threadpool`, a new function for creating a thread pool on an ``mjData`` instance. When a thread pool
-  is initialized, parts of the simulation pipeline, such as collision detection and constraint solving across islands,
-  are parallelized. The thread pool is automatically destroyed when the ``mjData`` is freed.
+- Significantly improved the quality of coarse convex hulls produced by the :ref:`maxhullvert<asset-mesh-maxhullvert>`
+  attribute by invoking Qhull's `Q9 <http://www.qhull.org/html/qh-optq.htm#Q9>`__ option.
 
   .. admonition:: Breaking API changes
      :class: attention

src/user/user_mesh.cc

@@ -1730,8 +1730,9 @@ void mjCMesh::MakeGraph(const double* dvert) {
 
   std::string qhopt = "qhull Qt";
   if (maxhullvert_ > -1) {
+    // qhull "Q9" picks the furthest of all furthest points across facets.
     // qhull "TA" actually means "number of vertices added after the initial simplex"
-    qhopt += " TA" + std::to_string(maxhullvert_ - 4);
+    qhopt += " Q9 TA" + std::to_string(maxhullvert_ - 4);
   }
 
   // graph not needed for small meshes