Do not modify mesh position when fitting a geom to the mesh AABB.

Also, unify the meaning of fitting a geom to an AABB: it now means to find the smallest geom such that its AABB contains the mesh AABB.

Fixes #2881.

PiperOrigin-RevId: 817097544
Change-Id: I883c686feffa803bd16ba461db24a474dfec7712

doc/XMLreference.rst

@@ -735,12 +735,12 @@ has any effect. The settings here are global and apply to the entire model.
 
 :at:`fitaabb`: :at-val:`[false, true], "false"`
    The compiler is able to replace a mesh with a geometric primitive fitted to that mesh; see :ref:`geom <body-geom>`
-   below. If this attribute is "true", the fitting procedure uses the axis-aligned bounding box (aabb) of the mesh.
-   Otherwise it uses the equivalent-inertia box of the mesh. The type of geometric primitive used for fitting is
-   specified separately for each geom. The models used to generate the image on the right can be found
-   `here <https://github.com/google-deepmind/mujoco/blob/main/test/user/testdata/fitmesh_inertiabox.xml>`__ (fit inertia
-   box) and `here <https://github.com/google-deepmind/mujoco/blob/main/test/user/testdata/fitmesh_aabb.xml>`__ (fit
-   aabb).
+   below. If this attribute is "true", the fitting procedure uses the axis-aligned bounding box (AABB) of the mesh,
+   choosing the smallest primitive whose AABB contains the mesh AABB. Otherwise it uses the equivalent-inertia box of
+   the mesh. The type of geometric primitive used for fitting is specified separately for each geom. The models used to
+   generate the image on the right can be found `here
+   <https://github.com/google-deepmind/mujoco/blob/main/test/user/testdata/fitmesh_inertiabox.xml>`__ (fit inertia box)
+   and `here <https://github.com/google-deepmind/mujoco/blob/main/test/user/testdata/fitmesh_aabb.xml>`__ (fit aabb).
 
 .. _compiler-eulerseq:
 

doc/changelog.rst

@@ -38,6 +38,12 @@ MJX
 - Pull in MuJoCo Warp updatest to ``io.py``, and use ``naconmax`` instead of ``naconmax`` to set the maximum number of
   contacts over all environments.
 
+Bug fixes
+^^^^^^^^^
+- Fix :github:issue:`2881`, :at:`fitaabb` was adding an offset to the mesh and applying an incorrect frame
+  transformation. Also, unify the meaning of fitting a geom to a mesh AABB: it now means to find the smallest geom such
+  that its AABB contains the mesh AABB.
+
 Version 3.3.6 (September 15, 2025)
 ----------------------------------
 

src/user/user_mesh.cc

@@ -979,10 +979,7 @@ void mjCMesh::DelTexcoord() {
 
 
 // set geom size to match mesh
-void mjCMesh::FitGeom(mjCGeom* geom, double* meshpos) {
-  // copy mesh pos into meshpos
-  mjuu_copyvec(meshpos, GetPosPtr(), 3);
-
+void mjCMesh::FitGeom(mjCGeom* geom, double center[3]) {
   // use inertial box
   if (!model->compiler.fitaabb) {
     // get inertia box type (shell or volume)
@@ -1016,64 +1013,35 @@ void mjCMesh::FitGeom(mjCGeom* geom, double* meshpos) {
 
   // use aamm
   else {
-    // find aabb box center
-    double cen[3] = {(aamm_[0]+aamm_[3])/2, (aamm_[1]+aamm_[4])/2, (aamm_[2]+aamm_[5])/2};
+    // find aabb box center and size
+    center[0] = (aamm_[0]+aamm_[3])/2;
+    center[1] = (aamm_[1]+aamm_[4])/2;
+    center[2] = (aamm_[2]+aamm_[5])/2;
+    double size[3] = {aamm_[3] - center[0], aamm_[4] - center[1], aamm_[5] - center[2]};
 
-    // add box center into meshpos
-    meshpos[0] += cen[0];
-    meshpos[1] += cen[1];
-    meshpos[2] += cen[2];
-
-    // compute depending on type
+    // compute smallest geom whose aabb contains the mesh aabb
     switch (geom->type) {
       case mjGEOM_SPHERE:
-        // find maximum distance
-        geom->size[0] = 0;
-        for (int i=0; i < nvert(); i++) {
-          double v[3] = {vert_[3*i], vert_[3*i+1], vert_[3*i+2]};
-          double dst = mjuu_dist3(v, cen);
-          geom->size[0] = max(geom->size[0], dst);
-        }
+        geom->size[0] = max(max(size[0], size[1]), size[2]);
         break;
 
       case mjGEOM_CAPSULE:
       case mjGEOM_CYLINDER:
         // find maximum distance in XY, separately in Z
-        geom->size[0] = 0;
-        geom->size[1] = 0;
-        for (int i=0; i < nvert(); i++) {
-          double v[3] = {vert_[3*i], vert_[3*i+1], vert_[3*i+2]};
-          double dst = sqrt((v[0]-cen[0])*(v[0]-cen[0]) +
-                            (v[1]-cen[1])*(v[1]-cen[1]));
-          geom->size[0] = max(geom->size[0], dst);
-
-          // proceed with z: valid for cylinder
-          double dst2 = abs(v[2]-cen[2]);
-          geom->size[1] = max(geom->size[1], dst2);
-        }
+        geom->size[0] = max(size[0], size[1]);
+        geom->size[1] = size[2];
 
         // special handling of capsule: consider curved cap
         if (geom->type == mjGEOM_CAPSULE) {
-          geom->size[1] = 0;
-          for (int i=0; i < nvert(); i++) {
-            // get distance in XY and Z
-            double v[3] = {vert_[3*i], vert_[3*i+1], vert_[3*i+2]};
-            double dst = sqrt((v[0]-cen[0])*(v[0]-cen[0]) +
-                              (v[1]-cen[1])*(v[1]-cen[1]));
-            double dst2 = abs(v[2]-cen[2]);
-
-            // get spherical elevation at horizontal distance dst
-            double h = geom->size[0] * sin(acos(dst/geom->size[0]));
-            geom->size[1] = max(geom->size[1], dst2-h);
-          }
+          geom->size[1] -= geom->size[0];
         }
         break;
 
       case mjGEOM_ELLIPSOID:
       case mjGEOM_BOX:
-        geom->size[0] = aamm_[3] - cen[0];
-        geom->size[1] = aamm_[4] - cen[1];
-        geom->size[2] = aamm_[5] - cen[2];
+        geom->size[0] = size[0];
+        geom->size[1] = size[1];
+        geom->size[2] = size[2];
         break;
 
       default:

src/user/user_objects.cc

@@ -3732,22 +3732,26 @@ void mjCGeom::Compile(void) {
 
     // save reference in case this is not an mjGEOM_MESH
     mjCMesh* pmesh = mesh;
+    double center[3] = {0, 0, 0};
 
     // fit geom if type is not mjGEOM_MESH
     if (type != mjGEOM_MESH && type != mjGEOM_SDF) {
-      double meshpos[3];
-      mesh->FitGeom(this, meshpos);
+      mesh->FitGeom(this, center);
 
       // remove reference to mesh
       meshname_.clear();
       mesh = nullptr;
-      mjuu_copyvec(pmesh->GetPosPtr(), meshpos, 3);
     } else if (typeinertia == mjINERTIA_SHELL) {
       throw mjCError(this, "for mesh geoms, inertia should be specified in the mesh asset");
     }
 
-    // apply geom pos/quat as offset
-    mjuu_frameaccum(pos, quat, pmesh->GetPosPtr(), pmesh->GetQuatPtr());
+    // rotate center to geom frame and add it to mesh frame
+    double meshpos[3];
+    mjuu_rotVecQuat(meshpos, center, pmesh->GetQuatPtr());
+    mjuu_addtovec(meshpos, pmesh->GetPosPtr(), 3);
+
+    // accumulate mesh frame into geom frame
+    mjuu_frameaccum(pos, quat, meshpos, pmesh->GetQuatPtr());
   }
 
   // check size parameters

src/user/user_objects.h

@@ -1186,7 +1186,7 @@ class mjCMesh: public mjCMesh_, private mjsMesh {
   double* GetQuatPtr();                             // get orientation
   double* GetInertiaBoxPtr();                       // get inertia box
   double GetVolumeRef() const;                      // get volume
-  void FitGeom(mjCGeom* geom, double* meshpos);     // approximate mesh with simple geom
+  void FitGeom(mjCGeom* geom, double center[3]);    // approximate mesh with simple geom
   bool HasTexcoord() const;                         // texcoord not null
   void DelTexcoord();                               // delete texcoord
   bool IsVisual(void) const { return visual_; }     // is geom visual

test/user/testdata/fitmesh_aabb.xml

@@ -22,19 +22,19 @@
       <geom type="mesh" mesh="stanford_bunny"/>
     </body>
     <body euler="90 0 0">
-      <geom name="bunny_sphere" type="sphere" mesh="stanford_bunny" rgba="1 0 1 1"/>
+      <geom name="bunny_sphere" type="sphere" mesh="stanford_bunny" rgba="1 0 1 .5"/>
     </body>
     <body euler="90 0 0" pos=".25 0 0">
-      <geom name="bunny_capsule" type="capsule" mesh="stanford_bunny" rgba="1 0 1 1"/>
+      <geom name="bunny_capsule" type="capsule" mesh="stanford_bunny" rgba="1 0 1 .5"/>
     </body>
     <body euler="90 0 0" pos=".5 0 0">
-      <geom name="bunny_ellipsoid" type="ellipsoid" mesh="stanford_bunny" rgba="1 0 1 1"/>
+      <geom name="bunny_ellipsoid" type="ellipsoid" mesh="stanford_bunny" rgba="1 0 1 .5"/>
     </body>
     <body euler="90 0 0" pos=".75 0 0">
-      <geom name="bunny_cylinder" type="cylinder" mesh="stanford_bunny" rgba="1 0 1 1"/>
+      <geom name="bunny_cylinder" type="cylinder" mesh="stanford_bunny" rgba="1 0 1 .5"/>
     </body>
     <body euler="90 0 0" pos="1 0 0">
-      <geom name="bunny_box" type="box" mesh="stanford_bunny" rgba="1 0 1 1"/>
+      <geom name="bunny_box" type="box" mesh="stanford_bunny" rgba="1 0 1 .5"/>
     </body>
     <light name="left" pos="0 0 1"/>
     <light name="right" pos="1 0 1"/>