code: func (captcha *CaptchaImage) DrawLine(num int) *CaptchaImage {
  	if captcha.Error != nil {
  		return captcha
  	}
  	first := captcha.width / 10
  	end := first * 9
  
  	y := captcha.height / 3
  
  	for i := 0; i < num; i++ {
  
  		point1 := Point{X: r.Intn(first), Y: r.Intn(y)}
  		point2 := Point{X: r.Intn(first) + end, Y: r.Intn(y)}
  
  		if i%2 == 0 {
  			point1.Y = r.Intn(y) + y*2
  			point2.Y = r.Intn(y)
  		} else {
  			point1.Y = r.Intn(y) + y*(i%2)
  			point2.Y = r.Intn(y) + y*2
  		}
  
  		captcha.drawBeeline(point1, point2, randDeepColor())
  
  	}
  	return captcha
  }
docs: //画一条直线.