3D model converter

STL stores triangles and nothing else — no colour, no texture, no material, not even a unit. It is what 3D printing has standardised on precisely because a printer needs a watertight shape and nothing more, and it is why converting a textured model to STL loses everything except the form.

OBJ adds materials and texture coordinates. GLB and glTF go further and are built for the web and for game engines: one file carrying geometry, materials, textures and animation together. Converting up from STL cannot invent the information STL never held, so a converted STL is a shape and stays a shape.

STL has no unit. The numbers in the file are just numbers, and whether they mean millimetres or inches is a convention between the tool that wrote it and the tool that reads it. This is why a model sometimes arrives in a slicer at twenty-five times the intended size, and it is a property of the format rather than of the conversion.

If a converted model comes out wrong by a suspiciously round factor, that is almost always what happened, and the fix is in your slicer rather than here.

STL and OBJ describe geometry: triangles and, in OBJ’s case, texture coordinates. GLB, GLTF and FBX describe scenes — geometry plus materials, textures, cameras, lights, node hierarchies and animation. Converting from a scene format to a mesh format necessarily discards everything that is not shape.

That is the decision to make before converting rather than after. If the destination is a 3D printer, geometry is all that matters and STL is correct. If it is a game engine, a web viewer or an animation tool, converting to STL throws away the work.

An STL file is a list of triangles and nothing else. There is no material, no texture, no colour, and — importantly — no declared unit. A model authored in millimetres and one authored in inches look identical in the file, and the reader guesses.

That is the origin of the classic problem of a print coming out twenty-five times too large or too small. It is not a conversion error; the information was never in the file. Checking the dimensions in the slicer before printing is the only reliable defence.

GLTF is a JSON file that references its textures and geometry as separate files sitting beside it. GLB packs all of them into one binary file. The content is the same and the difference is entirely about whether the model travels as one file or as a folder.

For anything being sent, uploaded or embedded on a web page, GLB is almost always the right choice — a GLTF whose textures went missing renders as untextured grey, and separating them is easy to do by accident. Keep GLTF where the assets are being edited and referenced by other things.

Every triangle has a facing direction, derived from the order its vertices are listed and from stored normal vectors. Where those disagree or are missing, a renderer draws parts of the model inside out — a surface that vanishes when viewed from one side.

Slicers are more forgiving than renderers because they care about enclosed volume rather than appearance, which is why a model can print correctly and look broken in a viewer. If a converted model has holes that are not holes, inverted normals are the usual cause and most 3D tools can recalculate them.