Allow loading obj meshes

unity/Editor/Importer/MjImporterWithAssets.cs

@@ -153,20 +153,21 @@ public class MjImporterWithAssets : MjcfImporter {
     var assetReferenceName = MjEngineTool.Sanitize(unsanitizedAssetReferenceName);
     var sourceFilePath = Path.Combine(_sourceMeshesDir, fileName);
 
-    if (Path.GetExtension(sourceFilePath) == ".obj") {
-      throw new NotImplementedException("OBJ mesh file loading is not yet implemented. " +
-                                        "Please convert to binary STL. " +
+    if (Path.GetExtension(sourceFilePath) != ".obj" && Path.GetExtension(sourceFilePath) != ".stl") {
+      throw new NotImplementedException("Type of mesh file not yet supported. " +
+                                        "Please convert to binary STL or OBJ. " +
                                         $"Attempted to load: {sourceFilePath}");
     }
 
-    var targetFilePath = Path.Combine(_targetMeshesDir, assetReferenceName + ".stl");
+    var targetFilePath = Path.Combine(_targetMeshesDir, assetReferenceName 
+                                                        + Path.GetExtension(sourceFilePath));
     if (File.Exists(targetFilePath)) {
       File.Delete(targetFilePath);
     }
     var scale = MjEngineTool.UnityVector3(
         parentNode.GetVector3Attribute("scale", defaultValue: Vector3.one));
     CopyMeshAndRescale(sourceFilePath, targetFilePath, scale);
-    var assetPath = Path.Combine(_targetAssetDir, assetReferenceName + ".stl");
+    var assetPath = Path.Combine(_targetAssetDir, assetReferenceName + Path.GetExtension(sourceFilePath));
     // This asset path should be available because the MuJoCo compiler guarantees element names
     // are unique, but check for completeness (and in case sanitizing the name broke uniqueness):
     if (AssetDatabase.LoadMainAssetAtPath(assetPath) != null) {
@@ -174,7 +175,7 @@ public class MjImporterWithAssets : MjcfImporter {
         $"Trying to import mesh {unsanitizedAssetReferenceName} but {assetPath} already exists.");
     }
     AssetDatabase.ImportAsset(assetPath);
-    var copiedMesh = AssetDatabase.LoadMainAssetAtPath(assetPath) as Mesh;
+    var copiedMesh = AssetDatabase.LoadAssetAtPath<Mesh>(assetPath);
     if (copiedMesh == null) {
       throw new Exception($"Mesh {assetPath} was not imported.");
     }
@@ -186,9 +187,16 @@ public class MjImporterWithAssets : MjcfImporter {
   private void CopyMeshAndRescale(
       string sourceFilePath, string targetFilePath, Vector3 scale) {
     var originalMeshBytes = File.ReadAllBytes(sourceFilePath);
-    var mesh = StlMeshParser.ParseBinary(originalMeshBytes, scale);
-    var rescaledMeshBytes = StlMeshParser.SerializeBinary(mesh);
-    File.WriteAllBytes(targetFilePath, rescaledMeshBytes);
+    if (Path.GetExtension(sourceFilePath) == ".stl") {
+      var mesh = StlMeshParser.ParseBinary(originalMeshBytes, scale);
+      var rescaledMeshBytes = StlMeshParser.SerializeBinary(mesh);
+      File.WriteAllBytes(targetFilePath, rescaledMeshBytes);
+    } else if (Path.GetExtension(sourceFilePath) == ".obj") {
+      ObjMeshImportUtility.CopyAndScaleOBJFile(sourceFilePath, targetFilePath, scale);
+    } else {
+      throw new NotImplementedException($"Extension {Path.GetExtension(sourceFilePath)} " +
+                                        $"not yet supported for MuJoCo mesh asset.");
+    }
   }
 
   private void ParseMaterial(XmlElement parentNode) {

unity/Editor/Importer/ObjMeshImportUtility.cs

@@ -0,0 +1,94 @@
+// Copyright 2019 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+using System;
+using System.Globalization;
+using System.IO;
+using System.Linq;
+using System.Text;
+using UnityEngine;
+
+namespace Mujoco {
+
+  /// <summary>
+  /// Scale vertex data manually line by line. We skip normals. Parameter vertex points
+  /// (`vp`) was unclear to me how to properly scale them, if you use them and notice an
+  /// issue please report it.
+  /// </summary>
+  public static class ObjMeshImportUtility {
+    private static Vector3 ToXZY(float x, float y, float z) => new Vector3(x, z, y);
+
+    public static void CopyAndScaleOBJFile(string sourceFilePath, string targetFilePath,
+        Vector3 scale, bool flipFaces=false) {
+      // OBJ files are human readable
+      string[] lines = File.ReadAllLines(sourceFilePath);
+      StringBuilder outputBuilder = new StringBuilder();
+      // Culture info for consistent decimal point handling
+      CultureInfo invariantCulture = CultureInfo.InvariantCulture;
+
+      scale = ToXZY(scale.x, scale.y, scale.z);
+      foreach (string line in lines) {
+        if (line.StartsWith("v ")) // Vertex line
+        {
+          // Split the line into components
+          string[] parts = line.Split(' ');
+          if (parts.Length >= 4) {
+            // Scale the vertex
+            float x = -float.Parse(parts[1], invariantCulture) * scale.x;
+            float y = float.Parse(parts[2], invariantCulture) * scale.y;
+            float z = float.Parse(parts[3], invariantCulture) * scale.z;
+
+            var swizzled = ToXZY(x, y, z);
+            outputBuilder.AppendLine(
+                $"v {swizzled.x.ToString(invariantCulture)} {swizzled.y.ToString(invariantCulture)} {swizzled.z.ToString(invariantCulture)}");
+          }
+        } else if (line.StartsWith("vn ")) {
+          string[] parts = line.Split(' ');
+          if (parts.Length >= 4) {
+            float x = -float.Parse(parts[1], invariantCulture);
+            float y = float.Parse(parts[2], invariantCulture);
+            float z = float.Parse(parts[3], invariantCulture);
+
+            var swizzled = ToXZY(x, y, z);
+            outputBuilder.AppendLine(
+                $"vn {swizzled.x.ToString(invariantCulture)} {swizzled.y.ToString(invariantCulture)} {swizzled.z.ToString(invariantCulture)}");
+          }
+        } else if (line.StartsWith("f ") && flipFaces) {
+          string[] parts = line.Split(' ');
+          if (parts.Length >= 4) {
+            outputBuilder.Append(parts[0] + " ");
+
+            // Apply same vertex order as STL parser: [0,2,1]
+            var face = parts.Skip(1).ToArray();
+            if (face.Length >= 3) {
+              outputBuilder.Append(face[0] + " ");  // vertex 0
+              outputBuilder.Append(face[2] + " ");  // vertex 2
+              outputBuilder.Append(face[1]);        // vertex 1
+
+              // Append any remaining vertices in original order
+              for (int i = 3; i < face.Length; i++) {
+                outputBuilder.Append(" " + face[i]);
+              }
+            }
+            outputBuilder.AppendLine();
+          }
+        } else {
+          // Copy non-vertex lines as-is
+          outputBuilder.AppendLine(line);
+        }
+      }
+      // Write the scaled OBJ to the target file
+      File.WriteAllText(targetFilePath, outputBuilder.ToString());
+    }
+  }
+}

unity/Runtime/Components/Shapes/MjMeshShape.cs

@@ -36,7 +36,7 @@ public class MjMeshShape : IMjShape {
     var assetName = MjEngineTool.Sanitize(
         mjcf.GetStringAttribute("mesh", defaultValue: string.Empty));
     if (!string.IsNullOrEmpty(assetName)) {
-      Mesh = (Mesh)Resources.Load(assetName);
+      Mesh = Resources.Load<Mesh>(assetName);
     }
   }