Get visible triangles of a mesh

In order to get the visible triangles of a mesh you need to:

1) Derive the Mesh class and override its DrawForSelection method in order to draw the triangles in false colors:

public class MyMesh : Mesh
{
   public MyMesh(Mesh other, ViewportLayout vp) : base(other)
   {
       this.vp = vp;
   }

   public bool GetAllVisibleTriangles { get; set; }

   private ViewportLayout vp;

   protected override void DrawForSelection(DrawParams data)
   {
       if (GetAllVisibleTriangles)
       {
           // Draws the quads with the color-coding needed for visibility computation 
           for (int i = 0; i < Triangles.Length; i++)
           {
               vp.SetColorDrawForSelection(i);

               IndexTriangle tri = Triangles[i];
               data.RenderContext.DrawTriangles(new Point3D[]
               {
                   Vertices[tri.V1],
                   Vertices[tri.V2],
                   Vertices[tri.V3],
               },  new Vector3D[]
                   {
                       Vector3D.AxisZ,
                       Vector3D.AxisZ,
                       Vector3D.AxisZ,
                   });
           }
       }
       else
       {
           base.DrawForSelection(data);
       }
   }

   public int[] SelectedTriangles;
   
   protected override void Draw(DrawParams data)
   {
       // Just for Debug
       if (SelectedTriangles != null)
       {
           for (int i = 0; i < SelectedTriangles.Length; i++)
           {
               SmoothTriangle tri = (SmoothTriangle) Triangles[SelectedTriangles[i]];
               data.RenderContext.DrawTriangles(new Point3D[]
               {
                   Vertices[tri.V1],
                   Vertices[tri.V2],
                   Vertices[tri.V3],
               },  new Vector3D[]
                   {
                      Normals[tri.N1],
                      Normals[tri.N2],
                      Normals[tri.N3],
                   });
           }
       }
       else
           base.Draw(data);
    }
}

2) Derive the ViewportLayout and expose a method that gets the visible triangles:

public class MyViewportLayout : ViewportLayout
{
   public int[] GetVisibleTriangles()
   {
       Form1.MyMesh mymesh = null;

       // Set Selectable = false to all the entities different from the MyMesh, so they don't interfere (only one MyMesh is supported)
       for (int i = 0; i < Entities.Count; i++)
       {
           if (!(Entities[i] is Form1.MyMesh))
           {
               Entities[i].Selectable = false;
           }
           else
           {
               mymesh = ((Form1.MyMesh) Entities[i]);
           }
       }

       mymesh.GetAllVisibleTriangles = true;

       int[] triangles = GetAllVisibleEntities(new Rectangle(0, 0, Size.Width, Size.Height));

       mymesh.GetAllVisibleTriangles = false;

       for (int i = 0; i < Entities.Count; i++)
       {
           Entities[i].Selectable = true;
       }

       mymesh.SelectedTriangles = triangles; // For debug

       return triangles;
   }
}

3) Create the MyMesh object and call the exposed method to get its visible triangles: 

viewportLayout1.Entities.Add(new Form1.MyMesh(Mesh.CreateTorus(20, 5, 10, 20), viewportLayout1), 0, Color.Red);

...

int[] visibleTriangles = viewportLayout1.GetVisibleTriangles();

On the left the original mesh, on the right the same mesh with just the visible triangles drawn after the view has been rotated a bit: