def normals_for_each_vertices
      normals_map = Hash.new(nil)
      triangles.each_with_index do | tri, tri_idx |
        tri.vertices.each_with_index do | vertex, ver_idx |
          normals_map[vertex] = Vector3.new() if( !normals_map.key?(vertex) )
          normals_map[vertex] += tri.normal*tri.angle(ver_idx)
        end
      end
      normals_map.each do |vertex, normal|
        normals_map[vertex] = normal.normalize
      end
      return  normals_map
    end
def normals_for_each_vertices
      normals_map = Hash.new(nil)
      triangles.each_with_index do | tri, tri_idx |
        tri.vertices.each_with_index do | vertex, ver_idx |
          normals_map[vertex] = Vector3.new() if( !normals_map.key?(vertex) )
          normals_map[vertex] += tri.normal*tri.angle(ver_idx)
        end
      end
      normals_map.each do |vertex, normal|
        normals_map[vertex] = normal.normalize
      end
      return  normals_map
    end
[Output]
  return normal vector for each vertex as Hash{ Vector3 vertex => Vector3 normal_vector}.