Save only visible part of mesh

I am trying to save only visible parts of my cloth mesh as obj file. So far I have the visible vertices and their corresponding vertex indices (triangles). But when I save my mesh, the visible part is removed and the rest is saved. i want to reach exactly the opposite but changing the if statement destroys the whole mesh. Please tell me where I go wrong.

  void Update()
     {
          clothMesh = Instantiate(cloth.gameObject.GetComponent<MeshFilter>().sharedMesh);
 
         Vector3[] vertices = clothMesh.vertices;
         int[] triangles = clothMesh.triangles;
 
             
         List<Vector3> visibleVertices = new List<Vector3>();
         List<int> visibleTriIndecies = new List<int>();
 
         //--find out the visible vertices
         Vector3 visibleVertex = new Vector3();
 
         for(int i=0; i < vertices.Length;i++)
         {
             visibleVertex = cam.WorldToScreenPoint(cloth.transform.TransformPoint( vertices[i]));
 
             //---if vertex is visible
             if(visibleVertex.x <= cam.pixelWidth && visibleVertex.x >= 0 && visibleVertex.y >= 0 && visibleVertex.y <= cam.pixelHeight)
             {
                 visibleVertices.Add(vertices[i]);
                 
             }
         }
 
         //---find out the vertex index of visible vertices
         for (int i = 0; i < triangles.Length; i+=3)
         {
             foreach(Vector3 vpoint in visibleVertices)
             {
                 if(vpoint == vertices[triangles[i]])
                 {
                     visibleTriIndecies.Add(triangles[i]);
                     visibleTriIndecies.Add(triangles[i+1]);
                     visibleTriIndecies.Add(triangles[i+2]);
                 }
             }
         }
 
         Mesh mesh =  clothMesh;
         int[] newtris = mesh.triangles;
 
         for(int i = 0; i < visibleTriIndecies.Count; i+=3)
         {
             for(int j = 0; j < newtris.Length; j+=3)
             {
                 if(newtris[j] == visibleTriIndecies[i] && newtris[j + 1] == visibleTriIndecies[i + 1] && newtris[j + 2] == visibleTriIndecies[i + 2])
                 {
                                 newtris[j] = 0;
                                 newtris[j + 1] = 0;
                                 newtris[j + 2] = 0;
                 }
             }
         }
 
      
         mesh.triangles = newtris;
 
         Debug.Log("Vertex count: " + visibleVertices.Count);
 
 
 
         SaveMeshAsObj(mesh);
         Destroy(mesh);
 
     }