Urdf Geometry

URDF Geometry Creation Skill

This skill helps you choose and create the right type of geometry for robot links in URDF files.

When to Use This Skill

Use this skill when:

• Creating new robot links that need visual or collision geometry
• Deciding between basic shapes, OpenSCAD models, or mesh files
• Optimizing existing geometry for better performance or maintainability

Geometry Decision Tree

1. Can you use basic geometry?

Basic geometry types in URDF:

• <box size="x y z"/> - Rectangular boxes
• <cylinder radius="r" length="l"/> - Cylinders
• <sphere radius="r"/> - Spheres

Use basic geometry when:

• The part is roughly rectangular, cylindrical, or spherical
• Visual accuracy is less important than performance
• You want fast preview rendering
• The robot description should be easily portable

Example:

<link name="base_link">
  <visual>
    <geometry>
      <box size="0.5 0.3 0.1"/>
    </geometry>
    <material name="gray">
      <color rgba="0.7 0.7 0.7 1"/>
    </material>
  </visual>
  <collision>
    <geometry>
      <box size="0.5 0.3 0.1"/>
    </geometry>
  </collision>
</link>

2. Do you need a custom shape?

If basic geometry won't work, you have two options:

Option A: Create an OpenSCAD file (Recommended)

Use OpenSCAD when:

• You need a custom shape but don't have a mesh file
• The shape can be defined programmatically
• You want parametric/reusable geometry
• You want to use OpenSCAD libraries (MCAD, BOSL2, etc.)

Advantages:

• Human-readable and versionable (text file)
• Automatic STL conversion by the extension
• Can reuse and modify easily
• Can leverage existing OpenSCAD libraries

Process:

1. Ask user permission: "I can create an OpenSCAD file for this geometry. Would you like me to do that?"
2. Create .scad file in appropriate location (usually meshes/ folder)
3. Reference the STL in URDF (extension auto-converts .scad → .stl)
4. Verify using the preview

Example:

// meshes/custom_bracket.scad
module bracket(width=50, height=30, thickness=5) {
    difference() {
        // Main body
        cube([width, height, thickness]);
        // Mounting holes
        translate([10, height/2, 0])
            cylinder(h=thickness, r=3);
        translate([width-10, height/2, 0])
            cylinder(h=thickness, r=3);
    }
}

bracket(width=60, height=40, thickness=6);

<!-- Reference in URDF -->
<link name="mounting_bracket">
  <visual>
    <geometry>
      <mesh filename="package://my_robot/meshes/custom_bracket.stl"/>
    </geometry>
  </visual>
</link>

Option B: Use existing mesh file

Use existing meshes when:

• You have a CAD-designed part exported as STL/DAE/OBJ
• The geometry is very complex (organic shapes, imported from CAD)
• You're working with standardized parts

Supported formats:

• .stl - Standard Tessellation Language (most common)
• .dae - COLLADA (supports colors and textures)
• .glb/.gltf - GL Transmission Format

Important checks:

1. Verify file exists before referencing
2. Use package:// URI for portability: package://robot_name/meshes/part.stl
3. Consider file size - large meshes slow down preview
4. Scale appropriately - Check units (some CAD exports use mm, URDF typically uses meters)

Example:

<link name="sensor_housing">
  <visual>
    <geometry>
      <mesh filename="package://my_robot/meshes/housing.stl" scale="0.001 0.001 0.001"/>
    </geometry>
  </visual>
</link>

OpenSCAD Library Integration

Before creating custom OpenSCAD code, check for existing libraries:

Common OpenSCAD libraries:

• MCAD - Mechanical CAD library (gears, bearings, motors)
• BOSL2 - Belfry OpenSCAD Library (advanced shapes, threading)
• dotSCAD - Computational geometry library
• Round-Anything - Rounding and filleting library

Check available libraries:

1. Run command: "URDF: Generate OpenSCAD Libraries Documentation"
2. Review available modules and functions
3. Use existing modules instead of writing from scratch

Example using MCAD:

include <MCAD/motors.sh>
use <MCAD/gears.scad>

// Use predefined motor mount
motorWidth = 28;
motorHeight = 46;
motorLength = 42;

stepper_motor_mount(28);

Best Practices

Visual vs Collision Geometry

Visual geometry:

• Can be detailed and complex
• Used for display and rendering
• Performance matters less

Collision geometry:

• Should be SIMPLE (use basic shapes when possible)
• Used for physics simulation and collision detection
• Drastically affects simulation performance

Example:

<link name="gripper">
  <!-- Detailed visual -->
  <visual>
    <geometry>
      <mesh filename="package://my_robot/meshes/gripper_detailed.stl"/>
    </geometry>
  </visual>
  
  <!-- Simple collision (box approximation) -->
  <collision>
    <geometry>
      <box size="0.08 0.12 0.05"/>
    </geometry>
  </collision>
</link>

Optimization Tips

1. Start simple: Use basic geometry first, upgrade to meshes only if needed
2. Reuse geometry: Use Xacro macros to avoid duplication
3. OpenSCAD parametrization: Make dimensions variables for easy tweaking
4. File organization: Keep meshes in meshes/ directory, OpenSCAD in scad/ or meshes/
5. Version control friendly: OpenSCAD (.scad) files work better with Git than binary STLs

Common Mistakes to Avoid

❌ Don't:

• Reference mesh files that don't exist
• Use complex meshes for collision geometry
• Forget to scale imported CAD meshes (check units!)
• Mix absolute and package:// paths
• Create separate mesh files for identical parts

✅ Do:

• Verify all file references before committing
• Use basic geometry for collision when possible
• Document the expected units in comments
• Use package:// URIs for portability
• Use Xacro macros for repeated geometry

Workflow Example

User request: "I need a wheel for my robot"

Decision process:

1. Is a cylinder sufficient? → Yes for basic visualization
2. Need treads or details? → No → Use cylinder
3. Need realistic model? → Yes → Offer OpenSCAD option

Response:

For a basic wheel, I'll use a cylinder:

```xml
<link name="wheel">
  <visual>
    <geometry>
      <cylinder radius="0.1" length="0.05"/>
    </geometry>
    <material name="black">
      <color rgba="0.1 0.1 0.1 1"/>
    </material>
  </visual>
  <collision>
    <geometry>
      <cylinder radius="0.1" length="0.05"/>
    </geometry>
  </collision>
</link>

If you need a more detailed wheel with treads, I can create an OpenSCAD file. Would you like me to do that?

## Extension Integration

This skill works with the URDF editor's features:
- **Auto-conversion**: .scad files automatically convert to .stl
- **Preview**: See geometry in 3D immediately
- **Library discovery**: Access to configured OpenSCAD libraries
- **Validation**: Schema validation ensures correct URDF syntax

## Summary

**Decision flow:**
1. Basic geometry (box/cylinder/sphere)? → Use it
2. Need custom shape? → Ask user → Create OpenSCAD file
3. Have existing mesh? → Verify existence → Reference it
4. Always use simple collision geometry
5. Test with preview command

---
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