Three.js OBJExporter - export multiple materials?

Question

I have a mesh in my scene I'm trying to export to OBJ using the OBJExporter script that comes with Three.js. My mesh has multiple materials applied to it, meaning each face has a different materialIndex on it.

This is one mesh with multiple materials applied to it using a THREE.MultiMaterial. According to the .obj specification under "Vertex Texture Coordinate Indices", which I don't fully understand, it says:

Optionally, texture coordinate indices can be used to specify texture coordinates when defining a face.

In the format

f v1/vt1 v2/vt2 v3/vt3

In the .obj data exported by the script, that section just looks like an incremental counter:

f 1/1/1 2/2/2 3/3/3
f 4/4/4 5/5/5 6/6/6
f 7/7/7 8/8/8 9/9/9
f 10/10/10 11/11/11 12/12/12

Additionally, when I save the exported string to a .obj file, and then re-import that using the OBJLoader, then apply a MultiMaterial to it, it only shows one material.

I'm trying to untangle the whole puzzle and have a lot of unknowns. I don't care about the material definition itself, I just care which face has a material index on it. Can the .obj format alone hold a material index for a multi-textured object per face?

Andy Ray Andy Ray · Accepted Answer · 2016-07-07T21:44:42

Yes, the .obj format can support multiple materials on the same object. The easiest way is to add a usemtl call for a group of faces. Something like:

usemtl 1
f 41/153/11 40/154/13 36/155/20 38/156/5
f 38/157/5 36/158/20 44/159/14 37/160/8
f 24/161/9 29/162/1 27/163/21 23/164/23
f 29/165/1 24/166/9 22/167/10 28/168/4
usemtl 3
f 55/185/2 54/186/5 51/187/8 56/188/3
f 51/189/8 53/190/7 50/191/4 56/192/3
f 54/193/5 49/194/6 53/195/7 51/196/8
f 55/197/2 52/198/1 49/199/6 54/200/5
f 49/201/6 52/202/1 50/203/4 53/204/7

In this case 1 and 3 are arbitrary identifiers I made up. They can be any string name.

To actually give some visual definition to the materials, you need to include a .mtl file along with the .obj file, like this example does. But to simply get faces to have their own material indices, you don't need to include that file.

Three.js OBJExporter - export multiple materials?

1 Answers