How to recalculate mesh triangles

I am trying to get only visible vertices from a cloth mesh. In order to do so I am using Cam.WorldToViewportPoint to see weather the vertex is in the viewport or not and then I save it in a new mesh and then extract it as an .ObJ files. However I don't know how to calculate new triangles for my new mesh. Can someone please point me to the right direction?

 void Update()
     {
         cloth.gameObject.AddComponent<MeshCollider>();
         clothMesh = cloth.GetComponent<MeshCollider>().sharedMesh = cloth.clothMesh;
         
 
         Vector3[] vertices = clothMesh.vertices;
         int[] triangles = clothMesh.triangles;
 
         Destroy(cloth.GetComponent<MeshCollider>());
 
         List<Vector3> visibleVertices = new List<Vector3>();
         List<int> newTriangles = new List<int>();
 
         Vector3 visibleVertex = new Vector3();
 
         for(int i=0; i < vertices.Length;i++)
         {
             visibleVertex = cam.WorldToViewportPoint(vertices[i]);
 
             //---if vertex is visible
             if(visibleVertex.x <= 1 && visibleVertex.x >= 0 && visibleVertex.y>= 0 && visibleVertex.y <=1)
             {
                 visibleVertices.Add(vertices[i]);
                // newTriangles.Add(i);
             }
         }
 
         Mesh newMesh = new Mesh();
         Vector3[] visibleVerts = new Vector3[visibleVertices.Count];
         int[] newTris = new int[visibleVerts.Length*6];
 
         for (int i = 0; i< visibleVertices.Count; i++)
         {
             visibleVerts[i] = visibleVertices[i];
            
         }
 
 
         newMesh.vertices = visibleVerts;
       
         //newMesh.RecalculateNormals();
         //newMesh.RecalculateBounds();
         SaveMeshAsObj(newMesh);
 
     }