fascat 0.6.0

Fascat is a Python library and CLI for converting STEP, IGES, OpenCASCADE BREP, and JT CAD into realtime-ready OpenUSD, glTF, OBJ, STL, and FBX assets.

flowchart TD
    A["STEP / IGES / BREP / JT CAD"] --> B["Imported assembly"]
    B --> C["Tessellated meshes"]
    C --> D["Repaired meshes"]
    D --> E["Staged materials & UVs"]
    E --> F["Optimized LODs"]
    F --> G["OpenUSD / glTF / OBJ / STL / FBX"]

Installation

pip / pipx

pipx install fascat
# or
pip install fascat

The base install does not require a C++ compiler. To enable optional glTF EXT_meshopt_compression export, install the meshopt backend separately:

pip install "fascat[meshopt]"

Quick Start

# Show help
fascat --help
fascat help convert

# Print version
fascat version

# Inspect a CAD assembly
fascat inspect motor.step
fascat --json inspect motor.step

# Convert STEP to binary OpenUSD
fascat convert motor.step
fascat convert motor.step motor.usdc --profile realtime-desktop

# Convert STEP to binary glTF for VR/runtime engines
fascat convert motor.step motor.glb --profile virtual-reality
fascat convert motor.step motor.glb --profile mixed-reality

# Convert JT 8.x/9.x/10.x tessellation to GLB and import all stored JT LODs
fascat convert assembly.jt assembly.glb --lod-selection all

# Tune tessellation, UVs, optimization, and LODs
fascat convert motor.step motor.usdc \
  --sag 0.1 \
  --angle 15 \
  --max-edge-length 25 \
  --target-triangles 500000 \
  --materials display \
  --uv1 box \
  --lods 0.5,0.25,0.1

# Keep valid stored LODs and generate only missing or invalid fallbacks
fascat convert assembly.jt assembly.glb --lod-selection all --lod-source auto

# Preview a conversion without writing files
fascat convert motor.step motor.usdc --dry-run

# Emit a debuggable ASCII USD and report
fascat convert motor.step motor.usda --debug --report report.json

# Validate generated output
fascat validate motor.usdc
fascat validate motor.glb

# Render multi-angle turntable previews of the result
fascat validate motor.glb --turntable-dir views/

Commands

Fascat follows standard CLI stream conventions: primary output and JSON go to stdout, while errors and per-stage conversion progress go to stderr. Conversion validates the generated asset before reporting success. File arguments accept - for stdin/stdout where meaningful.

Python API

import fascat as fc

asset = fc.read_step("motor.step")          # or read_iges(...) / read_brep(...) / read_jt(...)

asset = (
    asset.tessellate(sag=0.1, angle=15.0)
    .repair(tolerance=0.05)
    .stage(materials="cad", uv0="box")
    .optimize(target_triangles=500_000)
    .lods([0.5, 0.25, 0.1])
)

asset.write_usd("motor.usdc")
asset.write_gltf("motor.glb")

Keyword arguments mirror each operation's *Options dataclass; pass a prebuilt options object instead when you prefer (asset.repair(fc.RepairOptions(tolerance=0.05))).

Agentic Skill

skills/cad-to-rt3d is an agentic skill that automates the whole CAD → RT3D tuning loop: it converts with a target profile, validates geometry quality, renders the result from multiple turntable angles, compares each view against a high-fidelity reference conversion, and adjusts fascat flags iteration by iteration until the asset passes both deterministic gates (topology, triangle and file-size budgets — checked by scripts/gates.py) and a visual inspection of the renders.

Open this repository in your coding agent (or copy the skill folder into your own project) and ask, for example:

Use the cad-to-rt3d skill to convert motor.step for realtime-web.

Every iteration's settings, reports, and renders are kept in a <output>.fascat-work/ directory, so the tuning history stays auditable. The turntable rendering it relies on is plain fascat validate --turntable-dir — see the CLI reference for the flags.

Docs

Full documentation at pavelsimo.github.io/fascat.

License

MIT