code: private BufferedImage generateBlankImg() {
          BufferedImage image = new BufferedImage(w, h,
                  BufferedImage.TYPE_INT_RGB);
          Graphics2D g2 = (Graphics2D) image.getGraphics();
          //设置图片背景是白色
          g2.setColor(Color.WHITE);
          g2.fillRect(0, 0, w, h);
          g2.dispose();
          return image;
      }
docs: 生成一张背景为白色的空白图片
  
  @return 返回此图片