private void setGuiBounding() {
        float sx = width * viewPortLeft;
        float ex = width * viewPortRight;
        float sy = height * viewPortBottom;
        float ey = height * viewPortTop;
        float xExtent = Math.max(0f, (ex - sx) / 2f);
        float yExtent = Math.max(0f, (ey - sy) / 2f);
        guiBounding.setCenter(sx + xExtent, sy + yExtent, 0);
        guiBounding.setXExtent(xExtent);
        guiBounding.setYExtent(yExtent);
        guiBounding.setZExtent(Float.MAX_VALUE);
    }
Called when the viewport has been changed.
jme3-core/src/main/java/com/jme3/renderer/Camera.java