Mesh manipulation very slow

Question

My program receives a Mesh at runtime. The mesh can be made of over 200k vertices. I need to reverse it (inside-out). I reverse the indices, but I also need to invert the normals. So far, I used this loop:

Vector3[] newnormals = new Vector3[mesh.normals.Length];
for (int i=0;i<mesh.normals.Length;i++)
{
    newnormals[i] = -mesh.normals[i];
}

revMesh.normals = newnormals;

where "mesh" is the original Mesh, and of course "revMesh" is the reversed one. I don't know why, but this loop is incredibly slow. it takes many seconds on my i7. If I substitute it with

revMesh.RecalculateNormals();

the execution time goes down to under 100 ms.…

Why is my routine so inefficient? Is there some way to speed it up?

Answer 1

Maybe getting mesh.normals[i] in each iteration makes it slow, try:

Vector3[] newnormals= revMesh.normals;
for (int i=0;i<newnormals.Length;i++)
    newnormals[i] = -newnormals[i];
revMesh.normals = newnormals;

Note: To make changes to the normals it is important to copy the normals from the Mesh. Once the normals have been copied and changed the normals can be reassigned back to the Mesh.

https://docs.unity3d.com/ScriptReference/Mesh-normals.html

You could also do: (didn't test but i think it works)

using System.Linq;
revMesh.triangles = revMesh.triangles.Reverse().ToArray();