public void onViewPortChange() {
        if (!(viewPortBottom < viewPortTop)) {
            throw new IllegalArgumentException(
                    "Viewport must have bottom < top");
        }
        if (!(viewPortLeft < viewPortRight)) {
            throw new IllegalArgumentException(
                    "Viewport must have left < right");
        }

        viewportChanged = true;
        setGuiBounding();
    }
Called when the viewport has been changed.
jme3-core/src/main/java/com/jme3/renderer/Camera.java