Welding vertices at runtime

Well you have to do it "manually". Just follow those steps:

• First find all duplicate vertices in the vertices array. This could be done by using an List and a int[] array with one element for each vertex. Use a double for loop and iterate trough the vertices. Compare the current vertex with each referenced vertex in the int-list. If no match was found, add the current index to the list and set the index in the int array to the current index. If a duplicate was found in the list, don't add the current index to the list. Instead just add the index of the found duplicate to the int array of the current vertex.

• Now you have two things: A list that contains indices of all unique vertices and an array that contains the vertex mapping for every vertex.

• The list can be used to create the new vertices array. So you only add the vertices which are referenced from the List.

• The array can be used to fix the indices in the triangle list of the mesh. So just use the indices of the triangle list as index into the int array. This gives you the new index.

Something like this (untested):

 public static void WeldVertices(Mesh aMesh, float aMaxDelta = 0.001f)
 {
     var verts = aMesh.vertices;
     var normals = aMesh.normals;
     var uvs = aMesh.uv;

     List<int> newVerts = new List<int>();
     int[] map = new int[verts.Length];

     // create mapping and filter duplicates.
     for(int i = 0; i < verts.Length; i++)
     {
         var p = verts[i];
         var n = normals[i];
         var uv = uvs[i];
         bool duplicate = false;
         for(int i2 = 0; i2 < newVerts.Count; i2++)
         {
             int a = newVerts[i2];
             if (
                 (verts[a] - p).sqrMagnitude <= aMaxDelta && // compare position
                 Vector3.Angle(normals[a], n) <= aMaxDelta && // compare normal
                 (uvs[a] - uv).sqrMagnitude <= aMaxDelta // compare first uv coordinate
                 )
             {
                 map[i] = i2;
                 duplicate = true;
                 break;
             }
         }
         if (!duplicate)
         {
             map[i] = newVerts.Count;
             newVerts.Add(i);
         }
     }

     // create new vertices
     var verts2 = new Vector3[newVerts.Count];
     var normals2 = new Vector3[newVerts.Count];
     var uvs2 = new Vector2[newVerts.Count];
     for(int i = 0; i < newVerts.Count; i++)
     {
         int a = newVerts[i];
         verts2[i] = verts[a];
         normals2[i] = normals[a];
         uvs2[i] = uvs[a];
     }

     // map the triangle to the new vertices
     var tris = aMesh.triangles;
     for(int i = 0; i < tris.Length; i++)
     {
         tris[i] = map[tris[i]];
     }

     aMesh.vertices = verts2;
     aMesh.normals = normals2;
     aMesh.uv = uvs2;
     aMesh.triangles = tris;
 }

Note that this specific implementation only works when the mesh has normals and uv coordinates. It does not compare the second or third uv channel nor if the vertex colors nor the tangents are equal. Creating a method that dynamically only checks and copies the vertex attributes that are available is very tricky. The "if statement" that determines if two vertices are equal would need to be extended to the other attributes. Also when you copy the old vertices into the new arrays it would also need to adapt automatically which data is actually present.

At the moment i only use a single maxDelta value for comparison, however comparing positions or angles might require different maxdelta values. Same for UV coordinates those would need a better resolution than the position.

edit
I just created a "small" helper class to "weld" vertices manually. I wanted to put it on the Unity wiki, but it seems it's currently down. Anyways here's a dropbox link. The class should dynamically compare and assign only those vertex attributes that are actually present in the mesh. It also has seperate max delta values. Note that i haven't really tested the class on any meshes, so feel free to give feedback if it works or if there are any errors.

 var welder = new MeshWelder(someMesh);
 
 // change any setting if necessary
 
 welder.Weld();

For those who are not familiar with namespaces, you need to use

 using B83.MeshHelper;

at the top in order to use the class.