code: func (captcha *CaptchaImage) drawBeeline(point1 Point, point2 Point, lineColor color.RGBA) *CaptchaImage {
  	if captcha.Error != nil {
  		return captcha
  	}
  	dx := math.Abs(float64(point1.X - point2.X))
  
  	dy := math.Abs(float64(point2.Y - point1.Y))
  	sx, sy := 1, 1
  	if point1.X >= point2.X {
  		sx = -1
  	}
  	if point1.Y >= point2.Y {
  		sy = -1
  	}
  	err := dx - dy
  	//循环的画点直到到达结束坐标停止.
  	for {
  		captcha.nrgba.Set(point1.X, point1.Y, lineColor)
  		captcha.nrgba.Set(point1.X+1, point1.Y, lineColor)
  		captcha.nrgba.Set(point1.X-1, point1.Y, lineColor)
  		captcha.nrgba.Set(point1.X+2, point1.Y, lineColor)
  		captcha.nrgba.Set(point1.X-2, point1.Y, lineColor)
  		if point1.X == point2.X && point1.Y == point2.Y {
  			return captcha
  		}
  		e2 := err * 2
  		if e2 > -dy {
  			err -= dy
  			point1.X += sx
  		}
  		if e2 < dx {
  			err += dx
  			point1.Y += sy
  		}
  	}
  	return captcha
  }
docs: //画直线.