Three.js Exporters and STEP Format Support

Based on the available Three.js documentation, there is currently no built-in exporter for STEP (.stp) format. The official exporters provided by Three.js include:

• DRACOExporter
• EXRExporter
• GLTFExporter
• OBJExporter
• PLYExporter
• STLExporter

STEP format is commonly used in CAD applications and is more complex than the formats Three.js natively supports, as it contains parametric information and precise geometry definitions that go beyond the mesh-based approach of Three.js.

Possible approaches:

1. Intermediate Format: Export your scene to a format like STL or OBJ (which you've already implemented), then use a third-party converter to transform it to STEP format.

2. Custom Exporter: Develop a custom exporter that converts Three.js geometry data to STEP format. This would be complex as STEP is a sophisticated CAD format that requires understanding of the ISO 10303 standard.

3. Third-party Libraries: Look for JavaScript libraries that can handle STEP conversion. Some CAD-focused libraries might offer this functionality, though they may not integrate directly with Three.js.

4. Server-side Conversion: Implement a server-side solution where you export to a simpler format from Three.js, then use more powerful CAD libraries (like OpenCascade) on the server to convert to STEP.

5. WebAssembly Solutions: Explore WebAssembly ports of CAD libraries that might support STEP export directly from the browser.

Note that converting from mesh-based formats to parametric CAD formats like STEP often results in loss of parametric information, as the original design intent is not preserved in mesh formats.