def homothétie c1, c2, c3=1.0
      if c1 == nil or c2 == nil
        Error.call "Coefficients are invalids"
      end

      s = Matrix.new [[c1.to_f, 0, 0], [0, c2.to_f, 0], [0, 0, c3.to_f]]
      @matrix_op = s
      cpy = self
      cpy.z = 1.0

      puts "homothétie de rapports #{c1.to_f}, #{c2.to_f}" if @verbose

      return (s * cpy.to_matrix).to_vector
    end
def homothétie c1, c2, c3=1.0
      if c1 == nil or c2 == nil
        Error.call "Coefficients are invalids"
      end

      s = Matrix.new [[c1.to_f, 0, 0], [0, c2.to_f, 0], [0, 0, c3.to_f]]
      @matrix_op = s
      cpy = self
      cpy.z = 1.0

      puts "homothétie de rapports #{c1.to_f}, #{c2.to_f}" if @verbose

      return (s * cpy.to_matrix).to_vector
    end
== Parameters:
 c1,c2,c3::
   c1 and c2 are the coeficiens of the homothetie. c3 is optional