improvement: map URDF <mimic> joints to BB.Sensor.Mimic

`BB.Sensor.Mimic` already implements URDF mimic semantics — same
`position * multiplier + offset` formula, even documents the URDF
equivalent in its docstring. So instead of skipping `<mimic>` with a
warning, the importer now attaches a `BB.Sensor.Mimic` sensor to the
mimicking joint pointing back at the source joint, with multiplier and
offset only emitted when they differ from URDF's defaults of 1.0 and
0.0.

Confirmed on the Franka Panda fixture (`panda_finger_joint2` mimics
`panda_finger_joint1`): the generated module now wires up the mimic
sensor automatically instead of leaving the user to add it by hand.

Author: jimsynz

lib/bb/urdf/importer.ex

@@ -249,11 +249,31 @@ if Code.ensure_loaded?(Sourceror) do
           render_axis(joint) ++
           render_limit(joint) ++
           render_dynamics(joint.dynamics) ++
+          render_mimic(joint) ++
           [render_link(child_link, links_by_name, joints_by_parent)]
 
       call(:joint, [atom_name(joint.name)], body)
     end
 
+    defp render_mimic(%{mimic: nil}), do: []
+
+    defp render_mimic(%{name: joint_name, mimic: mimic}) do
+      sensor_name = String.to_atom("#{joint_name}_mimic")
+
+      opts =
+        [source: atom_name(mimic.joint)]
+        |> append_if(mimic.multiplier != 1.0, multiplier: mimic.multiplier)
+        |> append_if(mimic.offset != 0.0, offset: mimic.offset)
+
+      mimic_module = {:__aliases__, [], [:BB, :Sensor, :Mimic]}
+      child_spec = {mimic_module, opts}
+
+      [call(:sensor, [sensor_name, child_spec])]
+    end
+
+    defp append_if(list, false, _kv), do: list
+    defp append_if(list, true, kv), do: list ++ kv
+
     defp render_axis(%{type: :fixed}), do: []
     defp render_axis(%{axis: nil}), do: []
 
@@ -462,6 +482,8 @@ if Code.ensure_loaded?(Sourceror) do
         red: 1,
         roll: 1,
         scale: 1,
+        sensor: 2,
+        sensor: 3,
         settings: 1,
         sphere: 0,
         sphere: 1,

lib/bb/urdf/parser.ex

@@ -325,7 +325,7 @@ defmodule BB.Urdf.Parser do
     axis = parse_axis(first_by_name(children, :axis))
     limit = parse_limit(first_by_name(children, :limit))
     dynamics = parse_dynamics(first_by_name(children, :dynamics))
-    {mimic, mimic_warnings} = parse_mimic(first_by_name(children, :mimic), to_string(name))
+    mimic = parse_mimic(first_by_name(children, :mimic))
 
     safety_warnings =
       case first_by_name(children, :safety_controller) do
@@ -343,7 +343,7 @@ defmodule BB.Urdf.Parser do
        limit: limit,
        dynamics: dynamics,
        mimic: mimic
-     }, mimic_warnings ++ safety_warnings}
+     }, safety_warnings}
   end
 
   defp parse_origin(nil), do: nil
@@ -380,19 +380,14 @@ defmodule BB.Urdf.Parser do
     }
   end
 
-  defp parse_mimic(nil, _joint_name), do: {nil, []}
+  defp parse_mimic(nil), do: nil
 
-  defp parse_mimic(xml_element() = element, joint_name) do
-    mimic = %{
+  defp parse_mimic(xml_element() = element) do
+    %{
       joint: element |> attr(:joint) |> to_string(),
       multiplier: element |> attr(:multiplier, "1") |> parse_float(),
       offset: element |> attr(:offset, "0") |> parse_float()
     }
-
-    warning =
-      "joint #{inspect(joint_name)}: <mimic> is not supported by bb (mirrors #{inspect(mimic.joint)})"
-
-    {mimic, [warning]}
   end
 
   defp parse_geometry(nil), do: nil

lib/mix/tasks/bb.from_urdf.ex

@@ -23,12 +23,15 @@ if Code.ensure_loaded?(Igniter) do
       * `--module`, `-m` - The module name for the generated robot.
         Defaults to `{AppPrefix}.Robot`.
 
-    ## Limitations
+    ## URDF feature support
 
-    Some URDF features have no direct BB equivalent and are skipped with a
+    `<mimic>` joints are emitted as a `BB.Sensor.Mimic` attached to the
+    mimicking joint — BB's sensor implements the same
+    `position * multiplier + offset` semantics as URDF.
+
+    These URDF features have no direct BB equivalent and are skipped with a
     warning rather than failing the import:
 
-      * `<mimic>` joints
       * `<safety_controller>` blocks
       * `<transmission>` blocks
       * `<gazebo>` extensions

test/bb/urdf/importer_test.exs

@@ -90,11 +90,51 @@ defmodule BB.Urdf.ImporterTest do
     test "passes through parser warnings" do
       {_source, warnings} = import_fixture("mimic_and_transmission.urdf")
 
-      assert Enum.any?(warnings, &(&1 =~ "<mimic>"))
       assert Enum.any?(warnings, &(&1 =~ "<safety_controller>"))
       assert Enum.any?(warnings, &(&1 =~ "<transmission>"))
     end
 
+    test "emits a BB.Sensor.Mimic for URDF <mimic> joints" do
+      {source, _warnings} = import_fixture("mimic_and_transmission.urdf", MyApp.MimicGen)
+
+      assert source =~ "sensor :right_finger_joint_mimic"
+      assert source =~ "BB.Sensor.Mimic"
+      assert source =~ "source: :left_finger_joint"
+      assert source =~ "multiplier: -1.0"
+      assert [{MyApp.MimicGen, _}] = Code.compile_string(source)
+    end
+
+    test "omits default multiplier and offset on emitted mimic sensors" do
+      xml = """
+      <?xml version="1.0"?>
+      <robot name="t">
+        <link name="a"/>
+        <link name="b"/>
+        <link name="c"/>
+        <joint name="j1" type="prismatic">
+          <parent link="a"/>
+          <child link="b"/>
+          <axis xyz="1 0 0"/>
+          <limit lower="0" upper="1" effort="1" velocity="1"/>
+        </joint>
+        <joint name="j2" type="prismatic">
+          <parent link="b"/>
+          <child link="c"/>
+          <axis xyz="1 0 0"/>
+          <limit lower="0" upper="1" effort="1" velocity="1"/>
+          <mimic joint="j1"/>
+        </joint>
+      </robot>
+      """
+
+      {:ok, parsed} = Parser.parse_string(xml)
+      {:ok, source, _} = Importer.to_source(parsed, MyApp.DefaultMimic)
+
+      assert source =~ ~r/source: :j1[^,}]*\}/
+      refute source =~ "multiplier:"
+      refute source =~ "offset:"
+    end
+
     test "dedupes named materials so multiple visuals can share a URDF material" do
       {source, _warnings} = import_fixture("shared_materials.urdf", MyApp.SharedGen)
 

test/bb/urdf/parser_test.exs

@@ -45,12 +45,18 @@ defmodule BB.Urdf.ParserTest do
     test "collects warnings for unsupported features" do
       {:ok, robot} = Parser.parse_file(Path.join(@fixture_dir, "mimic_and_transmission.urdf"))
 
-      assert Enum.any?(robot.warnings, &(&1 =~ "<mimic>"))
       assert Enum.any?(robot.warnings, &(&1 =~ "<safety_controller>"))
       assert Enum.any?(robot.warnings, &(&1 =~ "<transmission>"))
       assert Enum.any?(robot.warnings, &(&1 =~ "<gazebo>"))
     end
 
+    test "parses <mimic> into a per-joint mimic struct" do
+      {:ok, robot} = Parser.parse_file(Path.join(@fixture_dir, "mimic_and_transmission.urdf"))
+
+      right = Enum.find(robot.joints, &(&1.name == "right_finger_joint"))
+      assert right.mimic == %{joint: "left_finger_joint", multiplier: -1.0, offset: 0.0}
+    end
+
     test "resolves top-level material references inside visuals" do
       {:ok, robot} = Parser.parse_file(Path.join(@fixture_dir, "two_link_arm.urdf"))
 

test/mix/tasks/bb.from_urdf_test.exs

@@ -38,7 +38,7 @@ defmodule Mix.Tasks.Bb.FromUrdfTest do
       test_project()
       |> Igniter.compose_task("bb.from_urdf", [urdf, "--module", "Test.Robot"])
 
-    assert Enum.any?(igniter.warnings, &(&1 =~ "<mimic>"))
+    assert Enum.any?(igniter.warnings, &(&1 =~ "<safety_controller>"))
     assert Enum.any?(igniter.warnings, &(&1 =~ "<transmission>"))
   end