Exports Unity meshes to an obj file

ObjExporter.cs

using UnityEngine;
using UnityEditor;
using System.Collections;
using System.IO;
using System.Text;

public class ObjExporterScript
{
	private static int StartIndex = 0;
	
	public static void Start()
	{
		StartIndex = 0;
	}
	public static void End()
	{
		StartIndex = 0;
	}
	
	
	public static string MeshToString(MeshFilter mf, Transform t) 
	{	
		Vector3 s 		= t.localScale;
		Vector3 p 		= t.localPosition;
		Quaternion r 	= t.localRotation;
		
		
		int numVertices = 0;
		Mesh m = mf.sharedMesh;
		if (!m)
		{
			return "####Error####";
		}
		Material[] mats = mf.GetComponent<Renderer>().sharedMaterials;
		
		StringBuilder sb = new StringBuilder();
		
		foreach(Vector3 vv in m.vertices)
		{
			Vector3 v = t.TransformPoint(vv);
			numVertices++;
			sb.Append(string.Format("v {0} {1} {2}\n",v.x,v.y,-v.z));
		}
		sb.Append("\n");
		foreach(Vector3 nn in m.normals) 
		{
			Vector3 v = r * nn;
			sb.Append(string.Format("vn {0} {1} {2}\n",-v.x,-v.y,v.z));
		}
		sb.Append("\n");
		foreach(Vector3 v in m.uv) 
		{
			sb.Append(string.Format("vt {0} {1}\n",v.x,v.y));
		}
		for (int material=0; material < m.subMeshCount; material ++) 
		{
			sb.Append("\n");
			sb.Append("usemtl ").Append(mats[material].name).Append("\n");
			sb.Append("usemap ").Append(mats[material].name).Append("\n");
			
			int[] triangles = m.GetTriangles(material);
			for (int i=0;i<triangles.Length;i+=3) {
				sb.Append(string.Format("f {0}/{0}/{0} {1}/{1}/{1} {2}/{2}/{2}\n", 
				                        triangles[i]+1+StartIndex, triangles[i+1]+1+StartIndex, triangles[i+2]+1+StartIndex));
			}
		}
		
		StartIndex += numVertices;
		return sb.ToString();
	}
}

public class ObjExporter : ScriptableObject
{
	[MenuItem ("File/Export/Wavefront OBJ")]
	static void DoExportWSubmeshes()
	{
		DoExport(true);
	}
	
	[MenuItem ("File/Export/Wavefront OBJ (No Submeshes)")]
	static void DoExportWOSubmeshes()
	{
		DoExport(false);
	}
	
	
	static void DoExport(bool makeSubmeshes)
	{
		if (Selection.gameObjects.Length == 0)
		{
			Debug.Log("Didn't Export Any Meshes; Nothing was selected!");
			return;
		}
		
		string meshName = Selection.gameObjects[0].name;
		string fileName = EditorUtility.SaveFilePanel("Export .obj file", "", meshName, "obj");
		
		ObjExporterScript.Start();
		
		StringBuilder meshString = new StringBuilder();
		
		meshString.Append("#" + meshName + ".obj"
		                  + "\n#" + System.DateTime.Now.ToLongDateString() 
		                  + "\n#" + System.DateTime.Now.ToLongTimeString()
		                  + "\n#-------" 
		                  + "\n\n");
		
		Transform t = Selection.gameObjects[0].transform;
		
		Vector3 originalPosition = t.position;
		t.position = Vector3.zero;
		
		if (!makeSubmeshes)
		{
			meshString.Append("g ").Append(t.name).Append("\n");
		}
		meshString.Append(ProcessTransform(t, makeSubmeshes));
		
		WriteToFile(meshString.ToString(),fileName);
		
		t.position = originalPosition;
		
		ObjExporterScript.End();
		Debug.Log("Exported Mesh: " + fileName);
	}
	
	static string ProcessTransform(Transform t, bool makeSubmeshes)
	{
		StringBuilder meshString = new StringBuilder();
		
		meshString.Append("#" + t.name
		                  + "\n#-------" 
		                  + "\n");
		
		if (makeSubmeshes)
		{
			meshString.Append("g ").Append(t.name).Append("\n");
		}
		
		MeshFilter mf = t.GetComponent<MeshFilter>();
		if (mf != null)
		{
			meshString.Append(ObjExporterScript.MeshToString(mf, t));
		}
		
		for(int i = 0; i < t.childCount; i++)
		{
			meshString.Append(ProcessTransform(t.GetChild(i), makeSubmeshes));
		}
		
		return meshString.ToString();
	}
	
	static void WriteToFile(string s, string filename)
	{
		using (StreamWriter sw = new StreamWriter(filename)) 
		{
			sw.Write(s);
		}
	}
}

for anyone who gets here via Google -- just fyi, Unity won't import OBJ submeshes separately unless there's a .MTL file, among other possible problems... see https://forum.unity.com/threads/importing-obj-file-with-submeshes.724640/#post-8220573