Flipping mesh for performance

Hello,

We compose the enemies of our game by constructing them out of a small subset of meshes and materials. I recently noticed in the frame debugguer that Unity doesn't like transforms with negative scales. So I started doing a script to generate meshes with flipped vertices to replace those and avoid those warnings. Coupled with instancing, it greatly reduces our draw call count!

Unfortunately, I seem to be missing something as the result isn't exactly what I expected it to be.

Code (CSharp):

1.     Mesh FlipMesh(Mesh mesh, Vector3 scale) // scale = -1 or 1 per component, will also be applied to transform
2.     {
3.         vertices.Clear();
4.         normals.Clear();
5.         mesh.GetVertices(vertices);
6.         mesh.GetNormals(normals);
7.  
8.         for (int i = 0; i < vertices.Count; i++)
9.             vertices[i] = Vector3.Scale(vertices[i], scale);
10.         for (int i = 0; i < normals.Count; i++)
11.             normals[i] = Vector3.Scale(normals[i], scale);
12.  
13.         Mesh fixedMesh = new Mesh();
14.  
15.         fixedMesh.vertices = vertices.ToArray();
16.         fixedMesh.normals = normals.ToArray();
17.  
18.         fixedMesh.uv = mesh.uv;
19.         if (mesh.uv2 != null)
20.             fixedMesh.uv2 = mesh.uv2;
21.         if (mesh.colors != null)
22.             fixedMesh.colors = mesh.colors;
23.         else if (mesh.colors32 != null)
24.             fixedMesh.colors32 = mesh.colors32;
25.  
26.         fixedMesh.subMeshCount = mesh.subMeshCount;
27.         for (int i = 0; i < mesh.subMeshCount; i++)
28.             fixedMesh.SetIndices(mesh.GetIndices(i), mesh.GetTopology(i), i);
29.  
30.         fixedMesh.RecalculateTangents();
31.         fixedMesh.RecalculateBounds();
32.  
33.         return fixedMesh;
34.     }

It works on some enemies but I sometimes get weird results, like this :


I noticed on this one that if I manually flip the Y scale on the transform, I get something back that is close to the original but not quite and darker.


Any idea of what I would be missing ?

 

Well, while making a gif, I found the issue: I also needed to change the winding order. Here's the updated code for anyone who would care!

Code (CSharp):

1.     Mesh FlipMesh(Mesh mesh, Vector3 scale) // scale = -1 or 1 per component, will also be applied to transform
2.     {
3.         vertices.Clear();
4.         normals.Clear();
5.         mesh.GetVertices(vertices);
6.         mesh.GetNormals(normals);
7.  
8.         for (int i = 0; i < vertices.Count; i++)
9.             vertices[i] = Vector3.Scale(vertices[i], scale);
10.         for (int i = 0; i < normals.Count; i++)
11.             normals[i] = Vector3.Scale(normals[i], scale);
12.  
13.         Mesh fixedMesh = new Mesh();
14.  
15.         fixedMesh.vertices = vertices.ToArray();
16.         fixedMesh.normals = normals.ToArray();
17.  
18.         fixedMesh.uv = mesh.uv;
19.         if (mesh.uv2 != null)
20.             fixedMesh.uv2 = mesh.uv2;
21.         if (mesh.colors != null)
22.             fixedMesh.colors = mesh.colors;
23.         else if (mesh.colors32 != null)
24.             fixedMesh.colors32 = mesh.colors32;
25.  
26.         fixedMesh.subMeshCount = mesh.subMeshCount;
27.         for (int i = 0; i < mesh.subMeshCount; i++)
28.         {
29.             indices.Clear();
30.             mesh.GetIndices(indices, i);
31.             MeshTopology topology = mesh.GetTopology(i);
32.             switch (topology)
33.             {
34.                 case MeshTopology.Triangles:
35.                 {
36.                     for (int j = 0; j < indices.Count; j += 3)
37.                     {
38.                         int s = indices[j];
39.                         indices[j] = indices[j + 2];
40.                         indices[j + 2] = s;
41.                     }
42.                     break;
43.                 }
44.  
45.                 default:
46.                 {
47.                     Debug.LogError("Unsupported topology: " + topology);
48.                     break;
49.                 }
50.             }
51.             fixedMesh.SetIndices(indices.ToArray(), topology, i);
52.         }
53.  
54.         fixedMesh.RecalculateTangents();
55.         fixedMesh.RecalculateBounds();
56.  
57.         return fixedMesh;
58.     }