def _default_merge(mosaic):
    """The default merge strategy -- just average all 4 pixels"""
    return (mosaic[::2, ::2] / 4. +
            mosaic[1::2, ::2] / 4. +
            mosaic[::2, 1::2] / 4. +
            mosaic[1::2, 1::2] / 4.).astype(mosaic.dtype)