public ViewPort createPreView(String viewName, Camera cam) {
        ViewPort vp = new ViewPort(viewName, cam);
        preViewPorts.add(vp);
        return vp;
    }
Creates a new pre ViewPort, to display the given camera's content.

<p>The view will be processed before the main and post viewports.

@param viewName the desired viewport name
@param cam the Camera to use for rendering (alias created)
@return a new instance
jme3-core/src/main/java/com/jme3/renderer/RenderManager.java