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.
How do you get a model from Blender into Unity without breaking its materials? Guide We sent ten Blender materials into Unity 6 URP by FBX and by GLB. FBX kept roughness and lost metallic, transparency and roughness maps. GLB kept everything, with one catch.
How do you get a Blender model into Godot with its materials and animations? Guide We sent the same Blender scenes into Godot 4.7 as .blend, GLB and FBX. GLB and .blend kept every material; FBX lost roughness and metal, and turned one idle into twelve animations.
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.
What does that loss look like measured, field by field? Measured data A two-part textured GLB scene inspected before conversion, after a GLB round trip, and after export to STL. Six of eight properties do not survive.
What happens to a 3MF project file when it becomes an STL? Guide We sent ten test files through nine 3MF to STL converters. Simple files mostly survive. A Bambu Studio project file did not survive any of them intact, Bambu Studio included.
Which 3MF to STL converters handle units, plates, modifiers and negative parts? Measured data Ten 3MF files with known answers through nine converters. Units, mirroring, plates, modifiers and negative parts, cell by cell. No converter turned a two plate Bambu project into a clean STL.
How do you edit a downloaded 3MF without losing its plates and settings? Guide We opened and re-saved one Bambu Studio project in four programs. Only Bambu kept everything; two refused it. Plus what Replace with STL keeps, from its source code.
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.
Why does a converted model arrive rotated, or lying on its side? Measured data The same 10 x 20 x 30 mm box written to GLB, STL, OBJ, PLY and 3MF, then opened as raw bytes. None of the five states an up axis, and the two specifications that decide one disagree.
Why did the textures disappear when the model moved between programs? Guide A textured OBJ is three files, not one. We measured the chain: two hops, both by bare filename, and zero image bytes in the model file. Nothing warns you when it breaks.
Why does the same PLY open in one program and not another? Guide A PLY header is a schema, so two files with the same extension can have nothing in common. Here is how to read the header, and what each kind of PLY actually is.
Why does a watertight model become non-watertight after conversion? Measured data One closed sphere whose triangles do not share corners, sent through STL, OBJ, PLY, GLB and 3MF. Four come back closed. The format that preserves the file most faithfully arrives in 1280 pieces.