Make shellinertia ignore exactmeshinertia, which is only used for legacy volume computations. Fixes #759.

PiperOrigin-RevId: 524884880
Change-Id: Ia5b64ee33802866f792cdad4acfec6d9baf2a9e9

src/user/user_mesh.cc

@@ -1047,7 +1047,7 @@ void mjCMesh::Process() {
       double vol = type==mjSHELL_MESH ? a : mjuu_dot3(vec, nrm) * a / 3;
 
       // if legacy computation requested, then always positive
-      if (!model->exactmeshinertia) {
+      if (!model->exactmeshinertia && type==mjVOLUME_MESH) {
         vol = fabs(vol);
       }
 
@@ -1093,7 +1093,7 @@ void mjCMesh::Process() {
       double vol = type==mjSHELL_MESH ? a : mjuu_dot3(cen, nrm) * a / 3;
 
       // if legacy computation requested, then always positive
-      if (!model->exactmeshinertia) {
+      if (!model->exactmeshinertia && type==mjVOLUME_MESH) {
         vol = fabs(vol);
       }
 

src/user/user_objects.cc

@@ -1201,7 +1201,7 @@ double mjCGeom::GetVolume(void) {
     }
 
     mjCMesh* pmesh = model->meshes[meshid];
-    if (model->exactmeshinertia) {
+    if (model->exactmeshinertia || typeinertia==mjSHELL_MESH) {
       return pmesh->GetVolumeRef(typeinertia);
     } else {
       return pmesh->boxsz_volume[0]*pmesh->boxsz_volume[1]*pmesh->boxsz_volume[2]*8;

test/user/testdata/inertia_shell.xml

@@ -1,5 +1,5 @@
 <mujoco>
-  <compiler exactmeshinertia="true"/>
+  <compiler/>
   <asset>
     <mesh file="cube.obj" name="hollow_cube"/>
   </asset>