STL, OBJ, and 3MF: what survives a mesh repair?

STL: triangles without a unit declaration

STL describes a triangle surface in ASCII text or binary form. It does not provide a standard unit declaration or preserve a CAD feature history. The same coordinates can be interpreted as millimeters or inches by different workflows, so check scale when importing into your slicer.

Fix3DFile's STL input cap is 500 MB and 5 million triangles, subject to a separate working-memory check. Large models can be refused below those caps when analysis is estimated to exceed the browser's memory budget. Keeping the browser alive matters more than accepting a file that cannot be processed safely.

OBJ: experimental polygon-mesh import

Choose or drop an OBJ polygon mesh; the format is detected automatically, with no extra checkbox. OBJ support is still experimental. Triangle faces and supported planar polygons become triangles for the existing inspection and repair tools. Positive and negative face indices are supported. OBJ does not give this importer a dependable unit declaration, so dimensions remain model units.

This is geometry-only support. Texture coordinates, normals from the file, vertex colors, material assignments, and external MTL or image files are not preserved. External resources are never fetched. Curves, free-form surfaces, and unsupported polygons are rejected rather than silently converted into an incomplete model.

3MF: experimental core geometry and assemblies

A 3MF is a ZIP package containing an XML model and potentially other resources. The importer reads core triangle meshes and local component assemblies referenced by the build list. It applies instance transforms and converts declared lengths to millimeters. It is not a full 3MF project editor.

Materials, textures, colors, support settings, printer profiles, and slicer-project metadata do not round-trip. Unsupported required extensions and external model references are rejected. Some multi-part or vendor-specific 3MF projects therefore need to be exported as a standard mesh from the originating application first.

Use a copy and expect an STL download

STL, OBJ, and 3MF share the same file caps: 500 MB per file and 5,000,000 generated triangles. These are input limits, not a guarantee that a browser can repair a file that large. Memory checks can reject smaller files; 3MF also has expanded-archive and XML complexity limits. OBJ and 3MF support remains experimental, and a valid file in another application may still contain an unsupported feature here.

All three formats are processed locally in the browser. A repaired download is STL, not a rewritten OBJ or 3MF project. STL export drops material information, assembly structure, and unit metadata. For a 3MF input, import the resulting STL as millimeters. Keep the untouched source project whenever those extra features matter.

• Keep an untouched copy of your model.
• Read experimental-import warnings before starting repair.
• Check instance positions, dimensions, and component count.
• Confirm millimeter scale for STL exported from a 3MF input.