Implement mesh extrema in a 3x3x3 grid corresponding to each feature of a unit cube. These are used as seeds for a better initial point in mesh hill climbing with up to a 2x speedup in mjc_Convex.

PiperOrigin-RevId: 959645299
Change-Id: I032ae534704e0cc440ddb7691fd21a29a359f521

doc/changelog.rst

@@ -34,6 +34,7 @@ Actuation
 Engine
 ^^^^^^
 
+- Optimized large-mesh convex collision detection with up to 2x speedup in certain cases.
 - Replaced the per-step sparse Cholesky factorization of the flex block of the implicit effective metric M + K with
   its prefactored per-vertex 3x3 diagonal blocks. The blocks precondition the CG constraint solver and drive an
   iterative solve for ``qacc_smooth``, which now converges on :ref:`tolerance<option-tolerance>` rather than a fixed

doc/includes/references.h

@@ -967,6 +967,7 @@ typedef struct mjModel_ {
   int*      mesh_texcoordadr;     // texcoord data address; -1: no texcoord   (nmesh x 1)
   int*      mesh_texcoordnum;     // number of texcoord                       (nmesh x 1)
   int*      mesh_graphadr;        // graph data address; -1: no graph         (nmesh x 1)
+  int*      mesh_extrema;         // extremum vertices in 3x3x3 directions    (nmesh x 27)
   float*    mesh_vert;            // vertex positions for all meshes          (nmeshvert x 3)
   float*    mesh_normal;          // normals for all meshes                   (nmeshnormal x 3)
   float*    mesh_texcoord;        // vertex texcoords for all meshes          (nmeshtexcoord x 2)