OpenGL offscreen rendering and reading the pixel values with a HIDDEN window using glfw

I need to render a 3d model into many views and the image size is much larger than screen resolution, so I can not open a so large window and I do not need to open a window to see the rendering result, too. I only want to render the 3d model into many images with different viewpoints.

I find this tutorial, http://www.opengl-tutorial.org/intermediate-tutorials/tutorial-14-render-to-texture/, and I try to use glGetTexImage and glReadPixels to read pixel values from fbo.

The problem is if the window is not hidden, it is OK. If I set the window hidden using

glfwWindowHint(GLFW_VISIBLE, GL_FALSE);

I can only read the background color from fbo using glGetTexImage or glReadPixels. And the model drawed using glDrawElements disappears.

Could anyone help me or show me an example? Thanks!

This my code of reading fbo: (It work well if the window is not hidden, the remaining part of my code is same as http://www.opengl-tutorial.org/intermediate-tutorials/tutorial-14-render-to-texture/)

unsigned char *image = new unsigned char[windowWidth * windowHeight * 3];
glBindFramebuffer(GL_FRAMEBUFFER, FramebufferName);
glBindTexture(GL_TEXTURE_2D, renderedTexture);
glGetTexImage(GL_TEXTURE_2D, 0, GL_BGR, GL_UNSIGNED_BYTE, image);

cv::Mat img;
img = cv::Mat::zeros(windowHeight, windowWidth, CV_8UC3);
for (int i = 0; i < windowHeight; i ++) {
    for (int j = 0; j < windowWidth; j ++) {
        int index = (i * windowWidth + j) * 3;
        cv::Vec3b px;
        px.val[0] = image[index + 0];
        px.val[1] = image[index + 1];
        px.val[2] = image[index + 2];
        img.at<cv::Vec3b>(i, j) = px;
    }
}
cv::imwrite("test.png", img);