def triangles
      tris = Array.new(tri_indices.size)
      i = 0
      tri_indices.each do |tri_index|
        tris[i] =  self.triangle(i)
        i += 1
      end
      return tris
    end
def triangles
      tris = Array.new(tri_indices.size)
      i = 0
      tri_indices.each do |tri_index|
        tris[i] =  self.triangle(i)
        i += 1
      end
      return tris
    end
[Output]
  return Array of Triangle.