Does CombineMesh always remove unused vertecies or not?

Hi all,

I wonder if this behavior is a bug or not: Using Mesh.CombineMesh, all vertices not used by triangles are dismissed (and the sum of vertices of the combined mesh may be lower than the sum of individual meshes). I found out that CombineMesh usually removes all unused vertices but I can create Meshes where it does not and I don't see a reason for that.

I have code to reproduce it (I am using 2018.2.14f1):

 using System.Collections.Generic;
 using UnityEngine;
 
 public class EmptyMeshCreator : MonoBehaviour
 {
     MeshFilter filter;
     MeshRenderer meshRenderer;
 
     private void Start()
     {
         filter = gameObject.GetComponent<MeshFilter>();
         meshRenderer = gameObject.GetComponent<MeshRenderer>();
         //Create(true, false);
         //Create(true, true);
         Create(false, true);
     }
 
     private void Create(bool startOpen, bool endOpen)
     {
         int count;
         Mesh mesh = CreateMesh(Vector3.zero, Vector3.right, out count, startOpen, endOpen);
         CombineInstance ci = new CombineInstance { mesh = mesh};
         CombineInstance[] cis = new CombineInstance[] { ci };
         Mesh combinedMesh = new Mesh();
         combinedMesh.CombineMeshes(cis, true, false);
         filter.sharedMesh = combinedMesh;
         Debug.Log(combinedMesh.vertexCount + " vs " + count);
     }
 
     public static Mesh CreateMesh(Vector3 startPoint, Vector3 endPoint, out int trueVertexCount, bool startOpen, bool endOpen)
     {
         Vector3 side = Vector3.forward * 0.2f;
         Vector3 startConnectionPointLeft = startPoint + side;
         Vector3 startConnectionPointRight = startPoint - side;
         Vector3 endConnectionPointLeft = endPoint + side;
         Vector3 endConnectionPointRight = endPoint - side;
 
         //Create World Positions:
         Vector3 levelHeight = new Vector3(0, 1, 0);
 
         Vector3 bottomStartLeft = startConnectionPointLeft;         //  0
         Vector3 bottomStartCenter = startPoint;                     //  1
         Vector3 bottomStartRight = startConnectionPointRight;       //  2
         Vector3 bottomEndLeft = endConnectionPointLeft;             //  3
         Vector3 bottomEndCenter = endPoint;                         //  4
         Vector3 bottomEndRight = endConnectionPointRight;           //  5
 
         Vector3 topStartLeft = bottomStartLeft + levelHeight;           //  6
         Vector3 topStartCenter = bottomStartCenter + levelHeight;       //  7
         Vector3 topStartRight = bottomStartRight + levelHeight;         //  8
         Vector3 topEndLeft = bottomEndLeft + levelHeight;               //  9
         Vector3 topEndCenter = bottomEndCenter + levelHeight;           //  10
         Vector3 topEndRight = bottomEndRight + levelHeight;             //  11
 
         //Create Mesh
         List<int> triangles = new List<int>();
         Vector3[] vertices = new Vector3[] {
                 bottomStartLeft , bottomStartCenter , bottomStartRight,         //Start
                 bottomEndLeft, bottomEndCenter, bottomEndRight,                 //End
                 topStartLeft, topStartCenter, topStartRight,                    //TopFront
                 topEndLeft, topEndCenter, topEndRight,                          //TopBack
                 bottomStartLeft, bottomEndLeft, topStartLeft, topEndLeft,       //Front
                 bottomStartRight, bottomEndRight, topStartRight, topEndRight};  //Back
 
         int unusedVertecies = 0;
         if (endOpen)
             unusedVertecies += 3;
         if (startOpen)
             unusedVertecies += 3;
 
         //1. End-Right
         if (!endOpen)
             AddGroundTopFace(triangles, 6, 5, 4);
         //2. End-Left
         if (!endOpen)
             AddGroundTopFace(triangles, 6, 4, 3);
         //3. Start-Right
         if (!startOpen)
             AddGroundTopFace(triangles, 6, 1, 2);
         //4. Start-Left
         if (!startOpen)
             AddGroundTopFace(triangles, 6, 0, 1);
         //5. Top-Right
         AddFace(triangles, 6, 7, 9, 10);
         //6. Top-Left
         AddFace(triangles, 7, 8, 10, 11);
         //7. Front
         AddFace(triangles, 16, 17, 18, 19);
         //8. Back
         AddFace(triangles, 13, 12, 15, 14);
 
         Mesh mesh = new Mesh
         {
             vertices = vertices,
             triangles = triangles.ToArray()
         };
         mesh.RecalculateNormals();
         mesh.RecalculateBounds();
 
 
         trueVertexCount = mesh.vertexCount - unusedVertecies;
 
         return mesh;
     }
 
     //Adds a face (two triangles) from groundA, groundB and topA, topB (which are ground+offset)
     private static void AddGroundTopFace(List<int> triangles, int topOffset, int groundA, int groundB)
     {
         int topA = groundA + topOffset;
         int topB = groundB + topOffset;
 
         AddFace(triangles, groundA, groundB, topA, topB);
     }
 
     private static void AddFace(List<int> triangles, int a, int b, int c, int d)
     {
         //Add Triangle 1
         triangles.Add(a);
         triangles.Add(c);
         triangles.Add(b);
 
         //Add Triangle 2
         triangles.Add(b);
         triangles.Add(c);
         triangles.Add(d);
     }
 
 }
 

Add it to a GameObject with MeshFilter and MeshRenderer. My output is: 20 vs 17 Hence. all 20 vertices are still present in the combined mesh, even though no. 3, 4 and 5 are never used. Using any other Create-Method call (there are two more given in a comment) the number of vertices is correctly reduced in the combined mesh.

If you want to visualize it better, here is a simple script to iterate through its vertices (attach to same game object):

 using UnityEngine;
 
 public class VertexView : MonoBehaviour {
 
     MeshFilter filter;
     public int vertex = 4;
 
     void Start () {
         filter = GetComponent<MeshFilter>();
     }
 
     private void OnDrawGizmos()
     {
         if (!filter || !filter.mesh)
             return;
         Gizmos.DrawSphere(filter.mesh.vertices[vertex], 0.1f);
     }
 }

So. Is this a bug or somehow a desired behavior?

Best, Tobi