Assimp
A C++ library that reads 40 plus 3D formats into one common scene structure. A developer dependency, not an application.
Format conversion and interoperability
Almost nothing in a 3D pipeline stays in the format it was authored in. A model gets exported to be printed, converted to be opened in CAD, re-exported to be viewed in a browser, and at each of those steps the file is rewritten by a program that has to decide what to keep. This section is about those decisions: which of them are lossless, which quietly discard something, and how to tell before the loss reaches the end of the pipeline.
What this covers
Converting between mesh formats (STL, OBJ, PLY, GLB, FBX, 3MF) and between mesh and solid formats (STEP), plus the interoperability failures that follow: wrong scale, missing materials, split or merged parts. It does not cover repairing a mesh that is already broken, which is its own section, or choosing between the programs that do the converting, which is what the tool directory is for.
Choosing between two formats
- STEP or STL: which one should this model be in? Guide STEP to STL is routine. STL to STEP is not a conversion at all, it is a rebuild. Here is what each format stores, what the converters actually output, and the tool limits nobody mentions.
- OBJ or STL: what does OBJ store that STL cannot? Guide STL does not share vertices and OBJ does, which decides file size, what survives conversion, and why an STL converted to OBJ arrives shattered. With measured figures.
- FBX or OBJ: which survives a move between engines? Guide FBX carries rigs and animation that OBJ cannot. On PBR materials the ranking flips, and the reason is built into the FBX SDK. What each format stores, and how to convert without losing textures.
- 3MF or STL for 3D printing? Guide 3MF carries units, colour, objects and print settings that STL cannot. It also overwrites your slicer profile, breaks between vendors, and has a real security history. All four, with sources.
- What is a GLB file and when is it the right choice? Guide A GLB is a whole 3D scene in one binary file: geometry, materials and textures together. Here is how to open one without installing anything, what is actually inside, and when to convert it.
What a conversion discards
- What does a GLB to STL conversion throw away? Guide Two named parts, a texture, UVs, a PBR material and vertex colours go in. One unnamed blob of triangles comes out, in a file nearly twice the size. Measured field by field.
- Can an STL be converted back into a STEP solid? Guide A mesh from CAD and a mesh from a scan need opposite algorithms, and using the wrong one fails predictably. Exact menu paths for FreeCAD, Fusion 360 and SolidWorks, plus which methods are paywalled.
Why the model arrives wrong
- Why does a model import at the wrong size? Guide Measured across five formats: STL, OBJ, PLY and GLB write no unit of length into the file. Only 3MF does. That one missing field is why a model lands 1000 times too small or too big.
- Which formats actually record a unit of length? Measured data The same 100 mm cube written to GLB, STL, OBJ, PLY and 3MF, then opened as raw bytes. Only one of the five states a unit anywhere in the file.
Tools you can use here, in the browser
Format compatibility table Which tool in the directory imports and exports which extension, generated from the tool entries. GLB to STL converter Converts in the browser and reports what the conversion dropped. The file never leaves your machine. Online GLB viewer Open a GLB and read its triangle count, units, shell count and UV status before converting anything.
Software for this work
CAD Exchanger
Desktop app and SDK that read, heal and convert over 30 CAD and mesh formats. A translator, not an authoring tool.
Autodesk Fusion
Cloud-connected CAD, CAM, CAE and PCB in one subscription, strongest on integrated manufacturing and weakest on air-gapped or very large assemblies.
FreeCAD
Open source parametric CAD built on the OpenCASCADE B-Rep kernel, with a full Python API and workbenches for mechanical design, BIM, FEM and CAM.
Google model-viewer
A web component that puts an interactive glTF model and an AR button on a page with one HTML tag. Reads glTF and GLB only.
MeshLab
Open source system for processing and editing 3D triangular meshes, widely used for scan cleanup, repair and format conversion.