C++ OpenGL - Overlay example - c++

Question

I've an OpenGL application (maze style) that I need to work on as soon as possible. My problem at the moment is the following:
I've 3 subwindows on my main window and they are all working fine. Aparently I should be only using one subwindow and the left side subwindows (smaller ones) should be displayed as an overlay. My actual app has the following window display:
And I would like to go for something like this:
I've searched the internet and so far I've found nothing about this subject. Is there anywhere I can read about on how to solve this?
Thank you very much.

Answer 1

You could render the overlays to a texture, and then render this wherever you want on the screen. Look into the gl*FrameBuffer functions. It might look something like this:
// Create a texture to render to
glGenTextures(1, &overlay_tex);
glBindTexture(GL_TEXTURE_2D, overlay_tex);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
// NULL means reserve texture memory
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, width, height, 0, GL_BGRA, GL_UNSIGNED_BYTE, NULL);
glGenFramebuffers(1, &fb);
glBindFramebuffer(GL_FRAMEBUFFER, fb);
// Attach the texture to the framebuffer
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, overlay_tex, 0);
glGenRenderbuffers(1, &depth_rb);
glBindRenderbuffer(GL_RENDERBUFFER, depth_rb);
glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT24, width, height);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depth_rb);
glBindFramebuffer(GL_FRAMEBUFFER, fb);
// Render your overlay here
glBindFramebuffer(GL_FRAMEBUFFER, 0);
// Render to the backbuffer again