code: func (captcha *CaptchaImage) DrawHollowLine() *CaptchaImage {
  	if captcha.Error != nil {
  		return captcha
  	}
  	first := captcha.width / 20
  	end := first * 19
  
  	lineColor := color.RGBA{R: 245, G: 250, B: 251, A: 255}
  
  	x1 := float64(r.Intn(first))
  
  	x2 := float64(r.Intn(first) + end)
  
  	multiple := float64(r.Intn(5)+3) / float64(5)
  	if int(multiple*10)%3 == 0 {
  		multiple = multiple * -1.0
  	}
  
  	w := captcha.height / 20
  
  	for ; x1 < x2; x1++ {
  
  		y := math.Sin(x1*math.Pi*multiple/float64(captcha.width)) * float64(captcha.height/3)
  
  		if multiple < 0 {
  			y = y + float64(captcha.height/2)
  		}
  		captcha.nrgba.Set(int(x1), int(y), lineColor)
  
  		for i := 0; i <= w; i++ {
  			captcha.nrgba.Set(int(x1), int(y)+i, lineColor)
  		}
  	}
  
  	return captcha
  }
docs: //添加一个较粗的空白直线