# Material Override > Tune PBR metallic / roughness on URDF visual meshes without re-exporting DAE — author an inline `` element inside any ``, and let the engine's URDF→USD pipeline patch the converter's defaults post-conversion. Useful for sim-to-real visual fidelity (DAE materials round-trip poorly; Isaac Sim's importer lands every embedded material at roughness=0.5, metallic=0.0 by default). Trigger: When the user asks to "tune material", "change roughness / metallic", "make the gripper shinier", "fix dull PBR", "override material", "PBR override", or any time the simulator looks washed out after a URDF→USD import. - Skill: `agibottech/material-override` (Agent Skill) - Install (CLI): `npx skillmds@latest add agibottech/material-override` - Raw SKILL.md: https://api.skillmd.com/api/skills/agibottech/material-override/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: DevOps & Infra - License: MPL-2.0 - Author: agibottech (https://skillmd.com/u/agibottech) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/agibottech/material-override --- ## When to Use - Robot or asset visual meshes import with washed-out / flat shading after the URDF→USD converter runs. - User wants to A/B PBR values without re-exporting DAE meshes from Blender / Maya. - Polishing a robot for sim-to-real visual transfer. - Author of a new URDF (see `add-robot` skill) wants the meshes to look right without round-tripping DAE materials. Do **not** use for: - Replacing albedo / textures → must be done in the DAE export. - Per-prim material at runtime → that's a runtime USD edit, not the `` element. - Adding a brand-new material — only patches metallic / roughness on the embedded one. ## Critical Patterns 1. **Author inside ``, not on the link.** The override auto-scopes to exactly the mesh it sits under. A link with N `` elements just gets N independent overrides. 2. **`urdfdom` / RSP / MoveIt / Isaac's converter all ignore it.** `assemble_robot.py` strips the element from the URDF it feeds the converter, then re-applies the values to the converted USD via `_parse_material_override_blocks` / `_apply_material_overrides`. 3. **Only `roughness` and `metallic` are exposed.** Other UsdPreviewSurface inputs (specular, opacity, IOR) are not on the contract. 4. **Range 0..1.** Outside that range the importer warns and clamps. 5. **No attributes needed.** Scoping is positional. Don't try to target a sibling visual by name — author one override per visual. ## Schema ```xml 0.45 0.85 ``` Either child is optional — author only what you need. Default for an absent child is "leave as the converter wrote it" (typically 0.5 / 0.0 for DAE imports). ## PBR cheat-sheet | Look | `roughness` | `metallic` | Notes | |---|---|---|---| | Brushed steel | 0.40 | 0.95 | Most arm shells | | Anodised aluminium | 0.55 | 0.30 | UR-style structural | | Polished chrome | 0.05 | 1.00 | Decorative trims, mirror parts | | Matte plastic | 0.70 | 0.00 | Cable guards, end-stops | | Rubber pads | 0.85 | 0.00 | Soft contact surfaces | | Anodised gold (gripper bushings) | 0.30 | 0.90 | Highlights | These are starting points; A/B against a photograph of the real robot if you have one. ## Workflow ### Step 1 — Find the visual you want to tune ```bash # Locate the URDF / xacro under genie_sim_robot_model grep -nR "" source/geniesim_ros/src/ros_ws/src/genie_sim_robot_model/robots/ | head ``` ### Step 2 — Add the override Open the xacro, drop `` into the `` block. Existing reference: aloha arm xacro carries an override on every visual: ```xml 0.450.85 ``` ### Step 3 — Rebuild the workspace ```bash geniesim ros build dev && source devel/setup.bash ``` ### Step 4 — Force a USD regenerate The robot USD is cache-gated by `manifest.json`. Bust the cache: ```bash rm -rf assets/scenes// # OR pass at launch: ros2 launch genie_sim_bringup app.launch.py … always_regenerate_robot_usd:=true ``` ### Step 5 — Launch and inspect ```bash ros2 launch genie_sim_bringup app.launch.py \ scene:= \ launcher_config:=launcher_ovrtx_isaac_physx \ headless:=false # local workstation with a screen; flip to true on a remote/headless host ``` In the OVRtx viewport, compare against the unmodified baseline. Iterate values, rebuild, re-launch. ### Step 6 — Verify the converted USD ```bash grep -n "roughness\|metallic" assets/scenes//robot/robot.usda | head ``` You should see the patched values on the relevant material prims. ## Notes - `assemble_robot.py` walks each `` in author order; if a visual has multiple meshes (rare in our robots), the override applies to every embedded material of that visual. - The override is stripped from the URDF *before* it reaches the converter, so MoveIt / RSP / urdfdom never see it. They'll happily consume the same xacro. - For brand-new materials (no DAE-embedded source), prefer authoring a real `` block — material_override is patch-only. - Material is rendered by OVRtx + Isaac Sim. The Newton GL viewer uses simple shading and won't reflect roughness / metallic — use OVRtx for visual QA. ## Resources - **Engine source**: [source/geniesim_ros/src/ros_ws/src/genie_sim_engine/scripts/assemble_robot.py](../../src/ros_ws/src/genie_sim_engine/scripts/assemble_robot.py) → `_parse_material_override_blocks` / `_apply_material_overrides` - **Reference usage**: [source/geniesim_ros/src/ros_ws/src/genie_sim_robot_model/robots/agilex/aloha/aloha.urdf.xacro](../../src/ros_ws/src/genie_sim_robot_model/robots/agilex/aloha/aloha.urdf.xacro) - **Engine AGENTS.md** → "": [source/geniesim_ros/src/ros_ws/src/genie_sim_engine/AGENTS.md](../../src/ros_ws/src/genie_sim_engine/AGENTS.md) - **Related skills**: `add-robot`, `launch-scene`