How do I keep an object always in front of everything else in OpenGL?

Question

I have this function which draws a small 3D axis coordinate system on the bottom left corner of the screen but depending on what I have in front of me, it may get clipped.

For instance, I have drawn a plain terrain on the ground, on the XZ plane at Y = 0. The camera is positioned on Y = 1.75 (to simulate an average person's height). If I'm looking up, it works fine, if I'm looking down, it gets clipped by the ground plane.

Looking up: http://i.stack.imgur.com/Q0i6g.png
Looking down: http://i.stack.imgur.com/D5LIx.png

The function I call to draw the axis system on the corner is this:

void Axis3D::DrawCameraAxisSystem(float radius, float height, const Vector3D rotation) {
    if(vpHeight == 0) vpHeight = 1;

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glViewport(0, 0, vpWidth, vpHeight);
    gluPerspective(45.0f, 1.0 * vpWidth / vpHeight, 1.0f, 5.0f);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    glTranslatef(0.0f, 0.0f, -3.0f);

    glRotatef(-rotation.x, 1.0f, 0.0f, 0.0f);
    glRotatef(-rotation.y, 0.0f, 1.0f, 0.0f);

    DrawAxisSystem(radius, height);
}

An now a couple of main functions I think are relevant to the problem:

glutDisplayFunc(renderScene);
glutReshapeFunc(changeSize);

void changeSize(int width, int height) {
    if(height == 0) height = 1;

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();

    glViewport(0, 0, width, height);

    gluPerspective(60.0f, 1.0 * width / height, 1.0f, 1000.0f);

    glMatrixMode(GL_MODELVIEW);
}

void renderScene(void) {
    glClearColor(0.7f, 0.7f, 0.7f, 0.0f);

    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    changeSize(glutGet(GLUT_WINDOW_WIDTH), glutGet(GLUT_WINDOW_HEIGHT));

    glLoadIdentity();

    SceneCamera.Move(CameraDirection, elapsedTime);
    SceneCamera.LookAt();

    SceneAxis.DrawCameraAxisSystem(0.03f, 0.8f, SceneCamera.GetRotationAngles());

    glutSwapBuffers();
}

Suggestions?

Answer 1

Rather than disable depth testing, you can just glClear(GL_DEPTH_BUFFER_BIT); before you render your overlay. Then whatever depth information was there is gone, but the pixels are all still there. However, your overlay will still render appropriately.