@Override
    protected void preFrame(float tpf) {
        shadowRenderer.preFrame(tpf);
        material.setMatrix4("ViewProjectionMatrixInverse", viewPort.getCamera().getViewProjectionMatrix().invert());
        Matrix4f m = viewPort.getCamera().getViewProjectionMatrix();
        material.setVector4("ViewProjectionMatrixRow2", tmpv.set(m.m20, m.m21, m.m22, m.m23));

    }
Get the {@link Material material} used by this filter.
@return the {@link Material material} used by this filter.
jme3-vr/src/main/java/com/jme3/shadow/AbstractShadowFilterVR.java