@SuppressWarnings("fallthrough")
    @Override
    public void postQueue(RenderQueue rq) {
        lightReceivers.clear();
        skipPostPass = false;
        if ( !checkCulling(viewPort.getCamera()) ) {
            skipPostPass = true;
            return;
        }

        updateShadowCams(viewPort.getCamera());

        Renderer r = renderManager.getRenderer();
        renderManager.setForcedMaterial(preshadowMat);
        renderManager.setForcedTechnique("PreShadow");

        for (int shadowMapIndex = 0; shadowMapIndex < nbShadowMaps; shadowMapIndex++) {

                if (debugfrustums) {
                    doDisplayFrustumDebug(shadowMapIndex);
                }
                renderShadowMap(shadowMapIndex);

            }

        debugfrustums = false;

        //restore setting for future rendering
        r.setFrameBuffer(viewPort.getOutputFrameBuffer());
        renderManager.setForcedMaterial(null);
        renderManager.setForcedTechnique(null);
        renderManager.setCamera(viewPort.getCamera(), false);
    }
responsible for displaying the frustum of the shadow cam for debug
purpose

@param shadowMapIndex
jme3-vr/src/main/java/com/jme3/shadow/AbstractShadowRendererVR.java