Question

I am drawing a polygon in a square window. When I resize the window, for instance by fullscreening, the aspect ratio is disturbed. From a reference I found one way of preserving the aspect ratio. Here is the code:

    void reshape (int width, int height) {
    float cx, halfWidth = width*0.5f;
    float aspect = (float)width/(float)height; 
    glViewport (0, 0, (GLsizei) width, (GLsizei) height);
    glMatrixMode(GL_PROJECTION); 
    glLoadIdentity(); 
    glFrustum(cx-halfWidth*aspect, cx+halfWidth*aspect, bottom, top, zNear, zFar);
    glMatrixMode (GL_MODELVIEW);
}

Here, cx is the eye space center of the zNear plane in X. I request if someone could please explain how could I calculate this. I believe this should be the average of the initial first two arguments to glFrustum(). Am I right? Any help will be greatly appreciated.

Answer 1

It looks like what you want to do is maintain the field of view or angle of view when the aspect ratio changes. See the section titled 9.085 How can I make a call to glFrustum() that matches my call to gluPerspective()? of the OpenGL FAQ for details on how to do that. Here’s the short version:

fov*0.5 = arctan ((top-bottom)*0.5 / near)
top = tan(fov*0.5) * near
bottom = -top
left = aspect * bottom
right = aspect * top

See the link for details.