code: public static BufferedImage channelImage(Object srcIm, int channel) {
          BufferedImage srcImage = read(srcIm);
          BufferedImage rcImage = new BufferedImage(srcImage.getWidth(),
                                                    srcImage.getHeight(),
                                                    srcImage.getType());
          for (int i = 0; i < srcImage.getWidth(); i++) {
              for (int j = 0; j < srcImage.getHeight(); j++) {
                  int r = Colors.getRGB(srcImage, i, j)[channel];
                  rcImage.setRGB(i, j, new Color(r, r, r).getRGB());
              }
          }
          return rcImage;
      }
docs: 获取三原色通道图片
  
  @param srcIm
  源图片
  @param channel
  通道编号，0:red 1:green 2:blue
  @return 单一通道图片