Add missing gravity attribute authoring to Mjcf SdfFileFormat plugin.

PiperOrigin-RevId: 779239797
Change-Id: I039b1089e5b436498d14df384c1aee830fd4b48a

src/experimental/usd/plugins/mjcf/mujoco_to_usd.cc

@@ -554,6 +554,16 @@ class ModelWriter {
         break;
     }
 
+    pxr::GfVec3f gravity(spec_->option.gravity[0], spec_->option.gravity[1],
+                      spec_->option.gravity[2]);
+    // Normalize will normalize gravity in place and return the magnitude before normalization.
+    float gravity_magnitude = gravity.Normalize();
+
+    WriteUniformAttribute(physics_scene_path, pxr::SdfValueTypeNames->Float,
+                          pxr::UsdPhysicsTokens->physicsGravityMagnitude, gravity_magnitude);
+    WriteUniformAttribute(physics_scene_path, pxr::SdfValueTypeNames->Vector3f,
+                          pxr::UsdPhysicsTokens->physicsGravityDirection, gravity);
+
     pxr::GfVec3d wind(spec_->option.wind[0], spec_->option.wind[1],
                       spec_->option.wind[2]);
     WriteUniformAttribute(physics_scene_path, pxr::SdfValueTypeNames->Double3,

test/experimental/usd/plugins/mjcf/mjcf_file_format_test.cc

@@ -976,6 +976,38 @@ TEST_F(MjcfSdfFileFormatPluginTest, TestPhysicsScenePrimSDFIterations) {
                        60);
 }
 
+TEST_F(MjcfSdfFileFormatPluginTest, TestPhysicsScenePrimGravity) {
+  auto stage = pxr::UsdStage::Open(LoadLayer(R"(
+    <mujoco model="test">
+      <option gravity="-123 0 0"> </option>
+    </mujoco>
+  )"));
+
+  ExpectAttributeEqual(stage,
+                       kPhysicsScenePrimPath.AppendProperty(
+                           pxr::UsdPhysicsTokens->physicsGravityMagnitude),
+                       123.0f);
+  ExpectAttributeEqual(stage,
+                       kPhysicsScenePrimPath.AppendProperty(
+                           pxr::UsdPhysicsTokens->physicsGravityDirection),
+                       pxr::GfVec3f(-1.0f, 0.0f, 0.0f));
+
+  stage = pxr::UsdStage::Open(LoadLayer(R"(
+    <mujoco model="test">
+      <option gravity="2 3 6"> </option>
+    </mujoco>
+  )"));
+
+  ExpectAttributeEqual(stage,
+                       kPhysicsScenePrimPath.AppendProperty(
+                           pxr::UsdPhysicsTokens->physicsGravityMagnitude),
+                       7.0f);
+  ExpectAttributeEqual(stage,
+                       kPhysicsScenePrimPath.AppendProperty(
+                           pxr::UsdPhysicsTokens->physicsGravityDirection),
+                       pxr::GfVec3f(0.2857143f, 0.42857143f, 0.85714287f));
+}
+
 TEST_F(MjcfSdfFileFormatPluginTest, TestPhysicsScenePrimDisableFlags) {
   auto stage = pxr::UsdStage::Open(LoadLayer(R"(
     <mujoco model="test">