Convert GLB to OBJ

The scale changes by a thousand

glTF and GLB are defined in metres: one unit is one metre. Most other software treats a unit as a millimetre, and slicers certainly do. A 0.06 unit bracket in a GLB is 60 mm in real life, and if the number goes across untouched the OBJ arrives as a shape six hundredths of a millimetre wide.

So the first unit box starts at metres, which is what the specification says, and the second is yours to set. Millimetres for anything going to a printer. The measurements next to the 3D view show the result before you download.

Colours come out as a material file

A GLB stores a material on each mesh. OBJ stores materials in a separate .mtl file that the .obj refers to by name. Where the GLB has anything to say about a surface, both files are written and you get a .zip holding them, together with any pictures they use.

The base colour and the normal map carry over, because an .mtl has a slot for each. Roughness becomes shininess, which is the same idea measured from the other end and is close rather than exact. Metalness and occlusion have no slot at all and are left out rather than faked.

The pictures come out with it

A GLB usually has its texture images packed inside it, which is the main reason people like the format. Those images are unpacked here, written out as PNG files and named in the .mtl, so the download holds the .obj, the .mtl and one picture for each texture. Put all of it in one folder and open the .obj — every modelling program finds the rest by itself.

The texture coordinates go across in full, seams and all, so the unwrap survives, and the unwrap is the part that took somebody a long afternoon. One thing does change on the way. glTF measures a picture downwards from its top edge and OBJ measures it upwards from the bottom, so every image is turned over as it is written. That is why the texture lands the right way up in Blender instead of mirrored.

One material per mesh, which is not how a GLB always works

An OBJ assigns materials by writing a `usemtl` line and having every face after it use that material. A glTF assigns them per primitive: a single mesh can be split into several primitives, each with its own material, all sharing one vertex buffer.

Those map onto each other cleanly in most files, because most meshes have one material. Where a mesh has several, each primitive becomes its own `usemtl` run in the .obj, which is the correct translation and means the face order in the OBJ may not match the order the GLB stored them in. Nothing is lost; the grouping is simply expressed the way OBJ expresses it.

The material names come from the glTF where it named them. Where it did not — and plenty of exporters leave materials unnamed — they are numbered instead, because an .mtl entry with no name is not a valid .mtl entry.