code: func (captcha *CaptchaImage) DrawNoise(complex CaptchaComplex) *CaptchaImage {
  	if captcha.Error != nil {
  		return captcha
  	}
  	density := 18
  	if complex == CaptchaComplexLower {
  		density = 28
  	} else if complex == CaptchaComplexMedium {
  		density = 18
  	} else if complex == CaptchaComplexHigh {
  		density = 8
  	}
  	maxSize := (captcha.height * captcha.width) / density
  
  	for i := 0; i < maxSize; i++ {
  
  		rw := r.Intn(captcha.width)
  		rh := r.Intn(captcha.height)
  
  		captcha.nrgba.Set(rw, rh, randColor())
  		size := r.Intn(maxSize)
  		if size%3 == 0 {
  			captcha.nrgba.Set(rw+1, rh+1, randColor())
  		}
  	}
  	return captcha
  }
docs: //画噪点.