How can I prevent deformation when rotating about the line-of-sight in OpenGL?

Question

I've drawn an ellipse in the XZ plane, and set my perspective slightly up on the Y-axis and back on the Z, looking at the center of ellipse from a 45-degree angle, using gluPerspective() to set my viewing frustrum.

Unrotated, the major axis of the ellipse spans the width of my viewport. When I rotate 90-degrees about my line-of-sight, the major axis of the ellipse now spans the height of my viewport, thus deforming the ellipse (in this case, making it appear less eccentric).

What do I need to do to prevent this deformation (or at least account for it), so rotation about the line-of-sight preserves the perceived major axis of the ellipse (in this case, causing it to go beyond the viewport)?

Answer 1

It looks like you're using 1.0 as the aspect when you call gluPerspective(). You should use width/height. For example, if your viewport is 640x480, you would use 1.33333 as the aspect argument.

Answer 2

According to the OpenGL Spec:

void gluPerspective( GLdouble fovy,
                     GLdouble aspect,
                     GLdouble zNear,
                   GLdouble zFar )

Aspect should be a function of your window width and height. Specifically width divided by height (but watch out for division by zero).

Perhaps you are using 1 as the aspect which is not accurate unless your window is a square.

Answer 3

It looks like the aspect parameter on your gluPerspective call need tweaking. See The Man Page. If your window were physically square, the aspect ratio would be 1 and your problem would go away. However, your window is rectangular, so the viewing frustum needs to be non-square.

Set the aspect ratio to window_width / window_height, and your ellipse should look correct. Note that you'll need to update this whenever the window resizes; if you're using GLUT set a glutReshapeFunc and recalculate the projection matrix in there.