Report gravity in accelerometer and framelinacc on static and mocap bodies

`mj_objectAcceleration` returned zero for dof-less (static and mocap) bodies, dropping the gravitational proper acceleration that `mj_rnePostConstraint` already stores in `cacc` for every body. Sensors on such bodies now read like a dynamic body at rest, matching MJX and MuJoCo Warp, which already did this.

- `mj_objectAcceleration`: always transform `cacc`, only skip the Coriolis term for dof-less bodies (zero velocity).
- Test accelerometer, framelinacc and frameangacc on world, static and mocap bodies, with the sleep filter active.
- Changelog entry.

PiperOrigin-RevId: 1012012486
Change-Id: I1b8b1574c5c53f45c7a7802439bfad24478470c9

Author: yuvaltassa

doc/changelog.rst

@@ -320,6 +320,9 @@ Bug fixes
 - Legacy ``.msh`` meshes with mirrored :ref:`scale<asset-mesh-scale>` no longer double-flip face winding (which
   inverted faces), and ``.msh`` files without vertex normals no longer populate empty face normals or reject custom
   normals on recompilation.
+- :ref:`accelerometer<sensor-accelerometer>` and :ref:`framelinacc<sensor-framelinacc>` sensors and
+  :ref:`mj_objectAcceleration` on static and mocap bodies now report the gravitational proper acceleration, like a
+  dynamic body at rest. Previously they returned zero, a regression introduced in 3.3.7.
 
 Documentation
 ^^^^^^^^^^^^^

src/engine/engine_core_util.c

@@ -1272,15 +1272,14 @@ void mj_objectAcceleration(const mjModel* m, const mjData* d,
     mjERROR("invalid object type %d", objtype);
   }
 
-  // dof-less body (static or mocap): quick return
+  // transform com-based acceleration to local frame
+  mju_transformSpatial(res, d->cacc+6*bodyid, 0, pos, d->subtree_com+3*m->body_rootid[bodyid], rot);
+
+  // dof-less body (static or mocap): zero velocity, quick return
   if (m->body_dofnum[m->body_weldid[bodyid]] == 0) {
-    mju_zero(res, 6);
     return;
   }
 
-  // transform com-based acceleration to local frame
-  mju_transformSpatial(res, d->cacc+6*bodyid, 0, pos, d->subtree_com+3*m->body_rootid[bodyid], rot);
-
   // transform com-based velocity to local frame
   mjtNum vel[6];
   mju_transformSpatial(vel, d->cvel+6*bodyid, 0, pos, d->subtree_com+3*m->body_rootid[bodyid], rot);

test/engine/engine_sensor_test.cc

@@ -2046,6 +2046,62 @@ TEST_F(SensorTest, AccelerometerAfterStep1) {
   EXPECT_NEAR(data->sensordata[2], 9.81, 1e-2);
 }
 
+// accelerometer and framelinacc on static and mocap bodies read gravity
+TEST_F(SensorTest, AccelerometerStaticAndMocap) {
+  constexpr char xml[] = R"(
+  <mujoco>
+    <option>
+      <flag sleep="enable"/>
+    </option>
+    <worldbody>
+      <site name="world"/>
+      <body name="static" pos="1 2 3">
+        <geom size="0.1"/>
+        <site name="static" euler="90 0 0"/>
+        <camera name="static" euler="90 0 0"/>
+      </body>
+      <body name="mocap" mocap="true" pos="-1 0 2">
+        <geom size="0.1"/>
+        <site name="mocap"/>
+      </body>
+      <body pos="0 0 5" sleep="init">
+        <freejoint/>
+        <geom size="0.1"/>
+      </body>
+    </worldbody>
+    <sensor>
+      <accelerometer site="world"/>
+      <accelerometer site="static"/>
+      <accelerometer site="mocap"/>
+      <framelinacc objtype="body" objname="static"/>
+      <frameangacc objtype="body" objname="mocap"/>
+    </sensor>
+  </mujoco>
+  )";
+  MjModelPtr model = LoadModelFromString(xml);
+  ASSERT_THAT(model, NotNull());
+  MjDataPtr data = MakeData(model);
+  mj_forward(model.get(), data.get());
+  EXPECT_LT(data->nbody_awake, model->nbody);
+
+  std::vector<mjtNum> expected = {
+      0, 0,    9.81,  // accelerometer: world
+      0, 9.81, 0,     // accelerometer: static site rotated +90 deg about x
+      0, 0,    9.81,  // accelerometer: mocap
+      0, 0,    9.81,  // framelinacc: static
+      0, 0,    0,     // frameangacc: mocap
+  };
+  EXPECT_THAT(AsVector(data->sensordata, 15),
+              Pointwise(MjNear(1e-12, 1e-5), expected));
+
+  mjtNum acc[6];
+  int cam_id = mj_name2id(model.get(), mjOBJ_CAMERA, "static");
+  mj_objectAcceleration(model.get(), data.get(), mjOBJ_CAMERA, cam_id, acc, 1);
+  EXPECT_THAT(
+      AsVector(acc, 6),
+      Pointwise(MjNear(1e-12, 1e-5), std::vector<mjtNum>{0, 0, 0, 0, 9.81, 0}));
+}
+
 // Test tactile sensor reads non-zero values when contacts occur
 TEST_F(SensorTest, TactileSkipTangents) {
   constexpr char xml[] = R"(