SDF

Provenance: maintained in earthtojake/text-to-cad. Use the installed local skill files as the runtime source of truth; the repository link is only for provenance and release review.

Use this skill when the deliverable is an SDFormat document. SDFormat describes simulator and world behavior: models, worlds, frames, poses, links, joints, inertials, visuals, collisions, sensors, lights, physics, plugins, includes, and simulator metadata.

This skill is for SDFormat, not signed-distance-field geometry.

The .sdf file is the source of truth: author and edit the XML directly. There is no gen_sdf() contract.

Core rules

1. Author .sdf XML directly and validate every created or modified file with scripts/validate before reporting completion.
2. Identify the target consumer before editing: Gazebo/libsdformat version, another simulator, visualization-only tooling, model package, or world handoff.
3. Decide document kind: model-level SDF, world-level SDF, or model-in-world. Prefer model-level SDF for reusable robot/object exports.
4. Use SI units unless the target explicitly requires otherwise: meters, kilograms, seconds, radians.
5. Prefer version="1.12" for new outputs unless the target consumer constrains the version.
6. Establish the design ledger before writing poses, frames, joint axes, mesh scales, inertials, sensors, or plugins, and keep it as a comment block at the top of the .sdf. Use references/design-ledger.md and references/llm-guardrails.md.
7. Write relative_to / expressed_in explicitly on every nontrivial pose and axis. Implicit frame defaults are the top SDF failure mode. See references/frame-semantics.md.
8. Do not infer spatial transforms from visual impression alone. Derive poses, axes, scale, mass, inertia, and frame names from upstream source data, drawings, simulator documentation, measured values, or explicit assumptions. Never freehand computed numbers — use formulas or a throwaway helper script (inertia tensors, unit conversions).
9. When the robot already has a URDF, derive the SDF from it instead of re-authoring geometry; see references/interoperability.md.
10. Regenerate upstream geometry, mesh, robot-description, render, topology, or package assets with their owning workflows before editing SDF that references them.
11. After authoring, run available checks: bundled validation, optional gz sdf --check, simulator load, joint motion, and plugin/sensor startup.
12. Report assumptions, skipped checks, unresolved resource paths, and target-specific compatibility risks.

Scope

Use this skill for SDFormat outputs. Do not use it for signed-distance-field modeling, raw geometry generation, planning semantics, or to paper over incorrect upstream robot/source data unless the task is explicitly simulator-only.

CAD Viewer Handoff

After completing SDF work that creates or modifies a .sdf, you must ALWAYS hand the explicit file path to $cad-viewer when that skill is installed. $cad-viewer must start CAD Viewer if it is not already running and return link(s) to the relevant created or updated file(s); if $cad-viewer is unavailable or startup fails, report that instead of silently omitting the handoff.

Workflow

1. Locate the target .sdf and its consumers.
2. Read or create the design ledger comment block.
3. Read references/frame-semantics.md before editing any <pose>, <frame>, joint axis, relative_to, expressed_in, nested scope, sensor frame, or plugin frame.
4. Author the XML directly, following the worked examples in references/examples.md.
5. Validate the explicit target with scripts/validate; treat bundled validation as a guardrail, not simulator proof.
6. Run target-consumer smoke tests when available (references/smoke-tests.md).
7. Hand the file to $cad-viewer. Static rendering does not execute SDF plugins or read file-authored motion metadata.
8. Report checks run, checks skipped, and assumptions.

Commands

Run with the project or workspace Python environment. Treat python in examples as an interpreter placeholder; if bare python is unavailable, substitute python3, a project virtualenv interpreter, or the configured interpreter path. The validator uses only the Python standard library.

python scripts/validate path/to/model.sdf
python scripts/validate path/to/a.sdf path/to/b.sdf
python scripts/validate path/to/model.sdf --strict

The validator checks document shape, name scopes, pose/frame graphs, joints, geometry, mesh URIs, inertials, sensors, and plugins, and prints per-file findings plus a summary. --strict treats warnings as failures. It exits nonzero if any target fails.

Optional external checking:

python scripts/validate path/to/model.sdf --gz-check auto
python scripts/validate path/to/model.sdf --gz-check required
python scripts/validate path/to/model.sdf --gz-check never

gz sdf --check is optional target-consumer validation. It should be reported as skipped when unavailable unless explicitly required.

Required report shape

When finishing an SDF task, include a compact report:

Validated: path/to/model.sdf
Checks run:
- bundled SDF validation: passed
- gz sdf --check: skipped, gz not installed
- simulator load: skipped, target simulator unavailable
- viewer handoff: `$cad-viewer` link returned
Assumptions:
- Assumed mesh units are meters.
- Assumed lidar frame is coincident with lidar_link.
Risks:
- Camera plugin filename was not verified in the target simulator environment.

Snapshot Tool

scripts/snapshot renders the robot to a PNG still or an orbit GIF, using the same shared CLI and headless browser runtime every rendering skill uses — so a snapshot matches what the CAD Viewer shows.

python scripts/snapshot --input path/to/robot.sdf --output review.png
python scripts/snapshot --input path/to/robot.sdf --output turntable.gif --mode orbit

It accepts .sdf only. Pose the robot with the job field "jointValues" (joint name to degrees, defaulting to the rest pose) rather than --params, which is STEP-only; robots are authored in metres and are framed on the robot scene scale automatically.

Theme settings live under one --theme, mirroring the viewer's Theme tab. The default theme is snapshot — Workbench Light with the ground grid, origin axis and shadows removed, because in a still image those read as geometry. There is no --display: display settings (mode, clip, exploded, edges) are CAD topology settings, and a robot carries none.

Link meshes are resolved relative to the description, so they must be present: an unhydrated Git LFS pointer fails as "No link mesh loaded for robot". Run git lfs checkout <mesh dir> first.

Use python scripts/snapshot --help for the complete current command interface.

References

• SDF workflow: references/sdf-workflow.md
• Worked examples (golden skeletons): references/examples.md
• LLM guardrails: references/llm-guardrails.md
• Design ledger: references/design-ledger.md
• Frame semantics: references/frame-semantics.md
• Validation scope: references/validation.md
• Smoke tests: references/smoke-tests.md
• Interoperability notes (URDF-derived SDF, meshes, Gazebo): references/interoperability.md