@Override
    public void postQueue(RenderQueue rq) {
        // lay depth first
        rm.setForcedMaterial(preDepth);
        rq.renderQueue(RenderQueue.Bucket.Opaque, rm, vp.getCamera(), false);
        rm.setForcedMaterial(null);

        rm.setForcedRenderState(forcedRS);
    }
Processor that lays depth first, this can improve performance in complex
scenes.
jme3-core/src/main/java/com/jme3/post/PreDepthProcessor.java