Fix vertex reordering when reading file

src/lib.rs

@@ -388,6 +388,12 @@ pub struct Mesh {
     /// Optional material id associated with this mesh. The material id indexes
     /// into the Vec of Materials loaded from the associated `MTL` file
     pub material_id: Option<usize>,
+    /// Faces from the obj file are reindexed when reading them
+    /// For example if the first two faces are [4,2,6] and [4,3,7]
+    /// they will be stored inside the mesh.indices as [0,1,2] and [0,2,3]
+    /// To obtain back the original index we use this face_reindex which maps from final_index->original_index
+    /// Therefore faces_original_index[0]=4 and faces_original_index[1]=2
+    pub faces_original_index: Vec<u32>,
 }
 
 /// Options for processing the mesh during loading.
@@ -955,6 +961,28 @@ fn export_faces(
         mesh.face_arities = Vec::new();
     }
 
+    //store inside the mesh the faces_original_index
+    let max_final_idx = index_map.values().copied().max().unwrap_or(0) as usize;
+
+    if max_final_idx != 0 {
+        mesh.faces_original_index.resize(max_final_idx + 1, 0);
+        for (original_indidces, final_index) in index_map {
+            let final_index = final_index as usize;
+
+            //sanity check that the index we are going to write is the same as the one that was written by a different face
+            if mesh.faces_original_index[final_index] != 0 {
+                //we already have a original index associated with this one but we make sure it's the same
+                let old = mesh.faces_original_index[final_index];
+                assert!(
+                original_indidces.v as u32 == old,
+                "Something is wrong when creating the face_orignal_index. The old index doesn't correspond to the new one."
+            );
+            }
+
+            mesh.faces_original_index[final_index] = original_indidces.v as u32;
+        }
+    }
+
     Ok(mesh)
 }