OpenGL: perspective view centered not in the middle of the view?

Question

I have a main scene centered on the center of the viewport
in addition to that I want another small object to be displayed on the corner of the viewport. The trouble is that when I draw the small object, it is transformed by the main projection transformation and appears slanted. I want the small object to have its own vanishing point centered in its center.
Is this possible which just transformations?

Answer 1

You want your main scene to be projected in one way and your corner object to be projected in another way. This directly leads you to the solution:

void render() {
    glMatrixMode(GL_PROJECTION);
    setUpMainProjection();
    glMatrixMode(GL_MODELVIEW);
    drawMainObject();

    glMatrixMode(GL_PROJECTION);
    setUpCornerProjection();
    glMatrixMode(GL_MODELVIEW);
    drawCornerObject();
}

Perhaps you're wondering how to implement setUpCornerProjection. It would look something like this:

// let's say r is a rect, which, in eye space, contains the corner object and is
// centered on it
glFrustum(r.left, r.right, r.bottom, r.top, nearVal, farVal);
// let's say p is the rect in screen-space where you want to
// place the corner object
glViewport(p.x, p.y, p.width, p.height);

And then in setUpMainProjection() you'd need to also call glFrustum and glViewport.