GTK - Python Getting Difference Between Pixbuf Images

I need to get the difference between two GTK 3 pixbuf images. Screenshot is captured in order to get images. get_byte_length gives the same value for different images. How could I get the difference?

Here is the code:

    window = Gdk.get_default_root_window()
    x, y, width, height = window.get_geometry()

    pb = Gdk.pixbuf_get_from_window(window, x, y, width, height)
    length1 = pb.get_byte_length()


Python 3.8, GTK 3.

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