ubuntu（双系统、虚拟机）+opencv打开摄像头，出现以下等错误：

error: (-2:Unspecified error) The function is not implemented. Rebuild the library with Windows, GTK+ 2.x or Carbon support. If you are on Ubuntu or Debian, install libgtk2.0-dev and pkg-config, then re-run cmake or configure script in function ‘cvNamedWindow’

OpenCV Error: Unspecified error (The function is not implemented. Rebuild the library with Windows, GTK+ 2.x or Carbon support. If you are on Ubuntu or Debian, install libgtk2.0-dev and pkg-config, then re-run cmake or configure script) in cvShowImage

cv2.error: /io/opencv/modules/highgui/src/window.cpp:583: error: (-2) The function is not implemented. Rebuild the library with Windows, GTK+ 2.x or Carbon support. If you are on Ubuntu or Debian, install libgtk2.0-dev and pkg-config, then re-run cmake or configure script in function cvShowImage

主要是GTK+ 2.x ，libgtk2.0-dev ，pkg-config这串关键字
解决办法：
终端输入：

sudo apt-get install libgtk2.0-dev pkg-config

安装完毕之后，将opencv卸载后重新编译。
ubuntu16.04 + opencv3.4.0
ubuntu18.04 + opencv4.1.0
例如上方两种环境下的不同版本的opencv编译，其中都存在

那么我们回到 build 这个目录文件下，打开终端输入：

make clean
cd ..
rm build -r

完毕，接着就是下图同样的操作，重新编译一次
具体教程可以点开上面的链接，最后运行示例代码验证：
如果识别不了USB设备，那么解决办法

最后大功告成！！！！