Make "acc" the default computation stage for sensors of type user.

PiperOrigin-RevId: 492689134
Change-Id: I21c07c74cb4b94d39c091ae98b169fd1235769d4

doc/XMLreference.rst

@@ -3943,7 +3943,7 @@ bodies whose center of mass is of interest.
    The type of output generated by this sensor. "axis" means a unit-length 3D vector. "quat" means a unit quaternion.
    These need to be declared because when MuJoCo adds noise, it must respect the vector normalization. "real" means a
    generic array (or scalar) of real values to which noise can be added independently.
-:at:`needstage`: :at-val:`[pos, vel, acc], required`
+:at:`needstage`: :at-val:`[pos, vel, acc], "acc"`
    The MuJoCo computation stage that must be completed before the user callback mjcb_sensor() is able to evaluate the
    output of this sensor.
 :at:`dim`: :at-val:`int, required`

doc/changelog.rst

@@ -69,8 +69,9 @@ General
   register one or more MuJoCo plugins on load.
 - Add an optional ``visualize`` callback to plugins, which is called during ``mjv_updateScene``. This callback allows
   custom plugin visualizations. Enable stree visualization for the Cable plugin as an example.
-- Sensors of type :ref:`user<sensor-user>` no longer require :at:`objtype` and :at:`objname`. If unspecified, the
-  objtype will be :ref:`mjOBJ_UNKNOWN<mjtObj>`. ``user`` sensors :at:`datatype` default is now :at-val:`"real"`.
+- Sensors of type :ref:`user<sensor-user>` no longer require :at:`objtype`, :at:`objname` and :at:`needstage`. If
+  unspecified, the objtype is now :ref:`mjOBJ_UNKNOWN<mjtObj>`. ``user`` sensors :at:`datatype` default is now 
+  :at-val:`"real"`, :at:`needstage` default is now :at-val:`"acc"`.
 - Add support for capsules in URDF import.
 - On macOS, issue an informative error message when run under `Rosetta 2 <https://support.apple.com/en-gb/HT211861>`_
   translation on an Apple Silicon machine. Pre-built MuJoCo binaries make use of

src/xml/xml_native_reader.cc

@@ -3159,8 +3159,9 @@ void mjXReader::Sensor(XMLElement* section) {
       ReadAttrInt(elem, "dim", &psen->dim, true);
 
       // keywords
-      MapValue(elem, "needstage", &n, stage_map, stage_sz, true);
-      psen->needstage = (mjtStage)n;
+      if (MapValue(elem, "needstage", &n, stage_map, stage_sz)) {
+        psen->needstage = (mjtStage)n;
+      }
       if (MapValue(elem, "datatype", &n, datatype_map, datatype_sz)) {
        psen->datatype = (mjtDataType)n;
       }

test/xml/xml_native_reader_test.cc

@@ -967,6 +967,25 @@ TEST_F(SensorParseTest, UserObjNameNoType) {
   EXPECT_THAT(error.data(), HasSubstr("objname 'kevin' given but"));
 }
 
+TEST_F(SensorParseTest, UserNeedstageAcc) {
+  static constexpr char xml[] = R"(
+  <mujoco>
+    <sensor>
+      <user dim="1" needstage="vel"/>
+      <user dim="1"/>
+      <user dim="1" needstage="pos"/>
+    </sensor>
+  </mujoco>
+  )";
+  std::array<char, 1024> error;
+  mjModel* model = LoadModelFromString(xml, error.data(), error.size());
+  ASSERT_THAT(model, NotNull()) << error.data();
+  EXPECT_EQ(model->sensor_needstage[0], mjSTAGE_VEL);
+  EXPECT_EQ(model->sensor_needstage[1], mjSTAGE_ACC);
+  EXPECT_EQ(model->sensor_needstage[2], mjSTAGE_POS);
+  mj_deleteModel(model);
+}
+
 // ------------- test general parsing ------------------------------------------
 
 TEST_F(XMLReaderTest, ZnearZeroNotAllowed) {