Question

That’s problem: I’m currently in OpenGL eye-space, now I want make a black rectangle to cover all window’s area. How can I determine a exactly X, Y, Z position to do this?

Update
Or can someone tell me how can I determine the X, Y (top-left) of window when we have a Z value?

Answer 1

You can try to calculate a rectangle that fits the camera exactly and go from there. To do that, you would need to take into account the projection matrix and calculate an inverse.

However, there is a simpler method. All you have to do is to change the camera temporarily, so that you know exactly how to draw the rectangle. A simple enough camera is the default orthogonal camera, with simple limits(from -1 to 1). The following does that:

glMatrixMode(GL_PROJECTION);
glPushMatrix();
glLoadIdentity();

glMatrixMode(GL_MODELVIEW);
glPushMatrix();
glLoadIdentity();

glBegin(GL_QUAD);
glVertex2f(-1.f, -1.f);
glVertex2f(1.f, -1.f);
glVertex2f(1.f, 1.f);
glVertex2f(-1.f, 1.f);
glEnd();

glPopMatrix();
glMatrixMode(GL_PROJECTION);
glPopMatrix();
glMatrixMode(GL_MODELVIEW);