protected FrameBuffer createOffScreenFrameBuffer(int mapSize, ViewPort offView) {
        // create offscreen framebuffer
        final FrameBuffer offBuffer = new FrameBuffer(mapSize, mapSize, 1);
        offBuffer.setDepthBuffer(Image.Format.Depth);
        offView.setOutputFrameBuffer(offBuffer);
        return offBuffer;
    }
create an offscreen frame buffer.

@param mapSize the desired size (pixels per side)
@param offView the off-screen viewport to be used (alias created)
@return a new instance
jme3-core/src/main/java/com/jme3/environment/EnvironmentCamera.java