cv::Mat img = cv::imread("img.png");
GLuint tex_in;
glGenTextures(1, &tex_in);
glBindTexture(GL_TEXTURE_2D, tex_in);

/*By practice, the internalParam: 
*   GL_RGB, GL_RGBA, GL_RGB8, GL_RGBA8 work
*   GL_RGB8UI, GL_RGBA8UI don't work
*/
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB8, img.cols, img.rows, 0, GL_BGR, GL_UNSIGNED_BYTE, img.data);  // copy data to texture object

cv::Mat test(img.rows, img.cols, CV_8UC3);
glGetTexImage(GL_TEXTURE_2D, 0, GL_BGR, GL_UNSIGNED_BYTE, (void*)test.data);  // read the image out of texture object

cv::imshow("test", test);