OpenGL: How to optimize 2d rendering with multiple layers overlapping each other in 3d mode?

I know how to speed up rendering in 3d by simply re开发者_如何学运维ndering the nearest planes first.

But how do i get advantage of this type of method in 2d mode? I cant use depth testing because they are all in the same z-level.

So i was thinking if it could be speed up when i dont need to render the invisible parts of the layers "below". Is this possible?

Note that i am rendering in 3d mode, there may be 3d objects and 2d objects at the same time. So i cant switch to 2d render only, i always use 3d coordinates for everything. And i may rotate the camera as i wish, so camera-specific tricks arent acceptable.

Edit: i tried the method Ville suggested:

( http://img815.imageshack.us/img815/7857/zfighting.png )

but as you see, it will result in z-fighting.

The code i used for rendering that is here:

glDepthFunc(GL_LESS);
glEnable(GL_DEPTH_TEST);
glDisable(GL_TEXTURE_2D);
glDisable(GL_ALPHA_TEST);
glDisable(GL_POLYGON_OFFSET_FILL);

glColor4f(1,0,0,1);
DrawQuad(0, 0, 10, 10);

glColor4f(0,0,1,1);
DrawQuad(5, 5, 15, 15);

glDepthFunc(GL_LEQUAL);

It sounds like you are rendering all your "2D" objects on the same plane. You could render your 2D parts into an off-screen framebuffer with an orthographic projection and give them different Z values as datenwolf suggested. Then render the framebuffer texture into your main 3D scene.

What do you understand by 2D mode? Do you mean orthographic projection? Then I have good news: Depth testing works there perfectly as well. gluOrtho2D is basically the same like glOrtho(..., -1, 1); i.e. you have the Z range -1 ... 1 to spend.

EDIT due to comment:

It is perfectly possible to combine rendering several projections in one single frame:

void render_perspective_scene(void);

void render_ortho_scene(void);

void render_HUD();

void display()
{
    float const aspect = (float)win_width/(float)win_height;

    glViewport(0,0,win_width,win_height);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glFrustum(-aspect*near/lens, aspect*near/lens, -near/lens, near/lens, near, far);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    render_perspective_scene();

    // just clear the depth buffer, so that everything that's
    // drawn next will overlay the previously rendered scene.
    glClear(GL_DEPTH_BUFFER_BIT);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(-aspect*scale, aspect*scale, -scale, scale, 0, 1);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    render_ortho_scene();

    // Same for the HUD, only that we render
    // that one in pixel coordinates.
    glClear(GL_DEPTH_BUFFER_BIT);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(0, win_width, 0, win_height, 0, 1);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    render_HUD();
}

Of course if you've fallen for those bad tutorials that place the projection matrix setup in the reshape handler you're of course mind blocked, to see that obvious solution.