c - Opengl drawing a 2d overlay on a 3d scene problem

Question

I have a moving 3d scene set up, and I want to make a stationary 2d GUI overlay that is always on top, when I try making 2d shapes I don't see anything. When I call: glMatrixMode(GL_PROJECTION); my 3d scene disappears and I'm left with a blank window...

here is the code I'm using for the overlay

EDIT: updated code

    glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(-100, 100, -100, 100);
glDisable(GL_DEPTH_TEST);
glDisable(GL_CULL_FACE);
glDisable(GL_TEXTURE_2D);
glDisable(GL_LIGHTING);
glColor3f(1, 1, 1);
glPushMatrix();
glBegin(GL_QUADS); 
glVertex3f(-5.0f, 5.0f, 0.0f); 
glVertex3f(-5.0f, -5.0f, 0.0f); 
glVertex3f(5.0f, -5.0f, 0.0f); 
glVertex3f(5.0f, 5.0f, 0.0f); 
glEnd();   
glPopMatrix();
glEnable(GL_DEPTH_TEST);
glutSwapBuffers();

Answer 1

Hmm... Basing on the fragment of code you posted, I believe that your scene disappears because of what you're doing with your matrices - looks a bit chaotic to me. The approach should look like this:

• clean the screen
• 3D:
  • enable lighting, z-test, etc
  • set active matrix mode to projection
  • load identity and establish a perspective projection
  • set active matrix mode back to modelview
  • draw everything 3D
• 2D:
  • disable lighting, z-test, etc
  • set active matrix mode to projection
  • load identity and establish an ortogonal projection
  • set active matrix mode back to modelview
  • draw everything 2D
• swap buffers

Also, consider switching to shaders (and to a modern OpenGL version in general) if you want to make your life even easier :).