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 :).

Answer 2

You must draw your quad in the other order. By default, OpenGL use counterclockwise front facing polygons. That means that you don't see your polygon because you see only its back face.

You might take a look at glFrontFace.

EDIT:

Also, if that doesn't work, you could try to disable the following states:

glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(-100, 100, -100, 100);
glDisable(GL_DEPTH_TEST);
glDisable(GL_CULL_FACE);
glDisable(GL_BLENDING);
glDisable(GL_TEXTURE_2D);
glDisable(GL_LIGHTING);

You might want use glPushAttrib and glPopAttrib in order not to mess your state.

Answer 3

glDisable(GL_DEPTH_TEST);
glDisable(GL_CULL_FACE);
glDisable(GL_TEXTURE_2D);
glDisable(GL_LIGHTING);

glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluOrtho2D(-100, 100, -100, 100);

glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glColor3f(1, 1, 1);
glBegin(GL_QUADS); 
    glVertex3f(20.0f, 20.0f, 0.0f); 
    glVertex3f(20.0f, -20.0f, 0.0f); 
    glVertex3f(-20.0f, -20.0f, 0.0f); 
    glVertex3f(-20.0f, 20.0f, 0.0f); 
glEnd();   
/// Now swap buffers

Answer 4

In addition, I also use a separate FBO for these kind of things. Usually the overlay doesn't have to be redrawn all the time, so render it on demand to a FBO and just render it as a fullscreen quad each frame. It wastes some fillrate but in general I find it is usually faster anyway and makes the code so much cleaner.