def __init__(self):
        # symbol key : QRect(...) coordinates where symbol can be found in atlas.
        # note that the coordinate list will always be the same list object as
        # long as the symbol is in the atlas, but the coordinates may
        # change if the atlas is rebuilt.
        # weak value; if all external refs to this list disappear,
        # the symbol will be forgotten.
        self.symbolMap = weakref.WeakValueDictionary()

        self.atlasData = None # numpy array of atlas image
        self.atlas = None     # atlas as QPixmap
        self.atlasValid = False
        self.max_width=0