From 49e892b04f32eaf659a1d87bcd6ff24f152a1268 Mon Sep 17 00:00:00 2001
From: Russ Tedrake <russt@mit.edu>
Date: Fri, 14 May 2021 22:12:12 -0400
Subject: [PATCH] quadrotor example: add getter for geometry frame

---
 bindings/pydrake/examples/quadrotor_py.cc          | 2 ++
 bindings/pydrake/examples/test/quadrotor_test.py   | 3 ++-
 examples/quadrotor/quadrotor_geometry.h            | 4 ++++
 examples/quadrotor/test/quadrotor_geometry_test.cc | 2 ++
 4 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/bindings/pydrake/examples/quadrotor_py.cc b/bindings/pydrake/examples/quadrotor_py.cc
index 63229cc12c50..8ed671d00654 100644
--- a/bindings/pydrake/examples/quadrotor_py.cc
+++ b/bindings/pydrake/examples/quadrotor_py.cc
@@ -38,6 +38,8 @@ PYBIND11_MODULE(quadrotor, m) {
 
   py::class_<QuadrotorGeometry, LeafSystem<double>>(
       m, "QuadrotorGeometry", doc.QuadrotorGeometry.doc)
+      .def("get_frame_id()", &QuadrotorGeometry::get_frame_id,
+          doc.QuadrotorGeometry.get_frame_id.doc)
       .def_static("AddToBuilder", &QuadrotorGeometry::AddToBuilder,
           py::arg("builder"), py::arg("quadrotor_state_port"),
           py::arg("scene_graph"), py::return_value_policy::reference,
diff --git a/bindings/pydrake/examples/test/quadrotor_test.py b/bindings/pydrake/examples/test/quadrotor_test.py
index b82e90d46b4a..908247e7b4ec 100644
--- a/bindings/pydrake/examples/test/quadrotor_test.py
+++ b/bindings/pydrake/examples/test/quadrotor_test.py
@@ -25,4 +25,5 @@ def test_geometry(self):
         quadrotor = builder.AddSystem(QuadrotorPlant())
         scene_graph = builder.AddSystem(SceneGraph())
         state_port = quadrotor.get_output_port(0)
-        QuadrotorGeometry.AddToBuilder(builder, state_port, scene_graph)
+        geom = QuadrotorGeometry.AddToBuilder(builder, state_port, scene_graph)
+        self.assertTrue(geom.get_frame_id().is_valid())
diff --git a/examples/quadrotor/quadrotor_geometry.h b/examples/quadrotor/quadrotor_geometry.h
index 87183af40926..3613faa3df22 100644
--- a/examples/quadrotor/quadrotor_geometry.h
+++ b/examples/quadrotor/quadrotor_geometry.h
@@ -25,6 +25,10 @@ class QuadrotorGeometry final : public systems::LeafSystem<double> {
   DRAKE_NO_COPY_NO_MOVE_NO_ASSIGN(QuadrotorGeometry);
   ~QuadrotorGeometry() final;
 
+  /// Returns the frame of the geometry registered with a SceneGraph.  This can
+  /// be useful, e.g., if one would like to add a camera to the quadrotor.
+  geometry::FrameId get_frame_id() const { return frame_id_; }
+
   /// Creates, adds, and connects a QuadrotorGeometry system into the given
   /// `builder`.  Both the `quadrotor_state.get_system()` and `scene_graph`
   /// systems must have been added to the given `builder` already.
diff --git a/examples/quadrotor/test/quadrotor_geometry_test.cc b/examples/quadrotor/test/quadrotor_geometry_test.cc
index 840a3f93d8ef..2fbeb340346b 100644
--- a/examples/quadrotor/test/quadrotor_geometry_test.cc
+++ b/examples/quadrotor/test/quadrotor_geometry_test.cc
@@ -20,6 +20,8 @@ GTEST_TEST(QuadrotorGeometryTest, AcceptanceTest) {
       &builder, plant->get_output_port(0), scene_graph);
   auto diagram = builder.Build();
   ASSERT_NE(geom, nullptr);
+
+  EXPECT_TRUE(geom->get_frame_id().is_valid());
 }
 
 }  // namespace