fix: handle URDF material name reuse and integer-default mesh scale

Tested the importer against UR5, Franka Panda, ANYmal B, and Allegro
Hand URDFs from public ROS packages. Two issues fell out:

- URDF allows many visuals to reference one named `<material>`. BB's
  DSL requires globally-unique entity names, so the generated module
  failed to compile. Keep the URDF name on the first visual that uses
  each material; strip it on later occurrences (BB's `material` entity
  auto-generates an identifier when `name` is absent).
- `BB.Dsl.Mesh.scale` defaults to the integer `1`, and the URDF
  exporter calls `:erlang.float_to_binary/2` on it, which crashes.
  Always emit `scale` as a float so the resulting module round-trips
  through `mix bb.to_urdf` without hitting that bug.

After these fixes, all four corpus URDFs parse, generate, compile, and
round-trip through the exporter with the same link/joint counts as the
inputs.

Author: jimsynz

lib/bb/urdf/importer.ex

@@ -50,6 +50,7 @@ if Code.ensure_loaded?(Sourceror) do
     @spec to_quoted(Parser.robot(), module) ::
             {:ok, Macro.t(), [String.t()]} | {:error, term}
     def to_quoted(robot, module_name) when is_atom(module_name) do
+      robot = dedupe_material_names(robot)
       links_by_name = Map.new(robot.links, &{&1.name, &1})
       joints_by_parent = Enum.group_by(robot.joints, & &1.parent)
       child_links = MapSet.new(robot.joints, & &1.child)
@@ -75,6 +76,31 @@ if Code.ensure_loaded?(Sourceror) do
       end
     end
 
+    # URDF lets many visuals reference a single named material; BB's DSL
+    # requires globally-unique entity names. Keep the URDF name on the first
+    # visual that uses each material and strip it on later occurrences (BB
+    # auto-generates a unique identifier when `name` is absent).
+    defp dedupe_material_names(robot) do
+      {links, _seen} = Enum.map_reduce(robot.links, MapSet.new(), &dedupe_link_material/2)
+      %{robot | links: links}
+    end
+
+    defp dedupe_link_material(%{visual: %{material: %{name: name}}} = link, seen)
+         when is_binary(name) do
+      if MapSet.member?(seen, name) do
+        {strip_material_name(link), seen}
+      else
+        {link, MapSet.put(seen, name)}
+      end
+    end
+
+    defp dedupe_link_material(link, seen), do: {link, seen}
+
+    defp strip_material_name(link) do
+      visual = %{link.visual | material: %{link.visual.material | name: nil}}
+      %{link | visual: visual}
+    end
+
     defp validate_referenced_links(joints, links_by_name) do
       Enum.reduce_while(joints, :ok, fn joint, _acc ->
         cond do
@@ -186,11 +212,10 @@ if Code.ensure_loaded?(Sourceror) do
     end
 
     defp render_geometry({:mesh, %{filename: filename, scale: scale}}) do
-      children =
-        [call(:filename, [filename])] ++
-          if scale == 1.0, do: [], else: [call(:scale, [scale])]
-
-      call(:mesh, [], children)
+      call(:mesh, [], [
+        call(:filename, [filename]),
+        call(:scale, [scale * 1.0])
+      ])
     end
 
     defp render_material(material) do

test/bb/urdf/importer_test.exs

@@ -95,6 +95,26 @@ defmodule BB.Urdf.ImporterTest do
       assert Enum.any?(warnings, &(&1 =~ "<transmission>"))
     end
 
+    test "dedupes named materials so multiple visuals can share a URDF material" do
+      {source, _warnings} = import_fixture("shared_materials.urdf", MyApp.SharedGen)
+
+      # First visual keeps the URDF material name…
+      assert Regex.match?(
+               ~r/link :base_link do.*?material do\s*name :grey/s,
+               source
+             )
+
+      # …and the resulting module compiles (would fail if the name leaked
+      # into later visuals — the DSL rejects duplicate entity names).
+      assert [{MyApp.SharedGen, _}] = Code.compile_string(source)
+    end
+
+    test "emits mesh scale as a float (the default integer trips the URDF exporter)" do
+      {source, _warnings} = import_fixture("shared_materials.urdf")
+
+      assert source =~ ~r/mesh do\s*filename "package:\/\/meshes\/arm\.stl"\s*scale 1\.0/
+    end
+
     test "errors out on multiple root links" do
       xml = """
       <?xml version="1.0"?>

test/fixtures/urdf/shared_materials.urdf

@@ -0,0 +1,47 @@
+<?xml version="1.0"?>
+<robot name="shared_materials_bot">
+  <material name="grey">
+    <color rgba="0.5 0.5 0.5 1"/>
+  </material>
+
+  <link name="base_link">
+    <visual>
+      <geometry>
+        <box size="0.1 0.1 0.1"/>
+      </geometry>
+      <material name="grey"/>
+    </visual>
+  </link>
+
+  <link name="arm1">
+    <visual>
+      <geometry>
+        <mesh filename="package://meshes/arm.stl"/>
+      </geometry>
+      <material name="grey"/>
+    </visual>
+  </link>
+
+  <link name="arm2">
+    <visual>
+      <geometry>
+        <cylinder radius="0.02" length="0.1"/>
+      </geometry>
+      <material name="grey"/>
+    </visual>
+  </link>
+
+  <joint name="j1" type="revolute">
+    <parent link="base_link"/>
+    <child link="arm1"/>
+    <axis xyz="0 0 1"/>
+    <limit lower="-1" upper="1" effort="5" velocity="1"/>
+  </joint>
+
+  <joint name="j2" type="prismatic">
+    <parent link="arm1"/>
+    <child link="arm2"/>
+    <axis xyz="1 0 0"/>
+    <limit lower="0" upper="0.1" effort="10" velocity="0.5"/>
+  </joint>
+</robot>

test/fixtures/urdf/shared_materials.urdf.license

@@ -0,0 +1,3 @@
+SPDX-FileCopyrightText: 2026 James Harton
+
+SPDX-License-Identifier: Apache-2.0