The HUD is drawn but the OpenGL 3D scene disappers

Question

I've been through a number of tutorials but still haven't got the hang of how to draw a 2D HUD with a 3D scene without some sort of a disaster happening. I've gathered some sample code from here and here and worked out the order in which matrices have to be treated in a certain way (see here) to achieve exactly what I've set out for. I've worked out a rendering code as shown below:

void render()
{
    //Clear screen
    glClearColor(0.2f, 0.0f, 0.2f, 1.0f); // Clear the background of our window to red  
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glMatrixMode(GL_MODELVIEW); //Switch to the drawing perspective
    glLoadIdentity(); //Reset the drawing perspective

    // Movement as response to input:
    glTranslatef( cam.getEye().getX(), cam.getEye().getY(), cam.getEye().getZ() ); // Translate our object along the y axis  
    glRotatef( cam.getTheta(), 0.0f, 1.0f, 0.0f ); // Rotate our object around the y axis 
    // No pushing/popping a matrix and no gluLookAt() used here.

    // ==================== Drawing scene here: ======================
    // ...
    // =================================== HUD: ==================================

    glMatrixMode(GL_PROJECTION);

    glPushMatrix();
    //glPopMatrix();

    glLoadIdentity();
    gluOrtho2D(0, winW, 0, winH); //left,right,bottom,top
    glMatrixMode(GL_MODELVIEW);

    //glPushMatrix();
    glPopMatrix();
    glLoadIdentity();

    // drawing the HUD rect here:
    glBegin(GL_QUADS);
        // ...
    glEnd();

    glPopMatrix();
    glPopMatrix();
    //glPushMatrix();
    //glPushMatrix();

    // ===============================================================================
    glutSwapBuffers(); //Send scene to the screen to be shown
}

... but for some reason it only displays my dark-cyan HUD and the 3D scene simply disappeared. What am I doing wrong? What have I missed? I know I'm supposed to push the projection matrix, then the model-view matrix, then do glOrtho2D(), then pop the two matrices off of the stack. That didn't work. I'm tired of fiddling with pushing and popping matrices in a random order.

Answer 1

If your HUD is an overlay as they usually are, you'll probably want to Disable the depth test & potentially depth writes before drawing the HUD Content:

glDisable(GL_DEPTH_TEST);
glDepthMask(GL_FALSE);

This way the Z value of the HUD is irrelevant. Don't forget to re-enable before drawing the scene.

As others have observed, you have matrix stack issues as well. Separate stacks are associated with the projection & modelview matrices. Right now you are pushing projection matrices and popping modelview matrices, so you're overflowing one and underflowing the other. You need to push & pop in the same mode symmetrically. Given that you're starting with a glLoadIdentity() in each mode, you may not need to use the matrix stacks at all.

EDIT: A more direct answer

Here's what your render code should probably look like:

void render()
{
  //Clear screen
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

  //*************************************************************************
  //* Prepare to render main scene:
  //*************************************************************************
  glEnable(GL_DEPTH_TEST);
  glDepthMask(GL_TRUE);

  glMatrixMode(GL_PROJECTION);
  glLoadIdentity();
  gluPerspective(...);

  glMatrixMode(GL_MODELVIEW);
  glLoadIdentity();
  glTranslatef( cam.getEye().getX(), cam.getEye().getY(), cam.getEye().getZ() );
  glRotatef( cam.getTheta(), 0.0f, 1.0f, 0.0f );

  //*************************************************************************
  //* Draw Main Scene Here:
  //*************************************************************************

  // ... 


  //*************************************************************************
  //* Prepare to render HUD overlay:
  //*************************************************************************
  glDisable(GL_DEPTH_TEST);
  glDepthMask(GL_FALSE);

  glMatrixMode(GL_PROJECTION);
  glLoadIdentity();
  gluOrtho2D(...);

  glMatrixMode(GL_MODELVIEW);
  glLoadIdentity();

  //*************************************************************************
  //* Draw HUD here:
  //*************************************************************************

  // ...


  // Finish the frame:
  glutSwapBuffers(); //Send scene to the screen to be shown
}

Now, since you'll be adjusting the projection matrix twice during each render call, you should remove gluPerspective() from your resizeWindow() function. Also, I removed the glClearColor() call from the render function--better to set that once at the beginning of the program and forget it. State changes are expensive so you want to avoid them as much as possible.