def transform_framerate(self, in_fps, out_fps):
        """
        Rescale all timestamps by ratio of in_fps/out_fps.

        Can be used to fix files converted from frame-based to time-based
        with wrongly assumed framerate.

        Arguments:
            in_fps (float)
            out_fps (float)

        Raises:
            ValueError: Non-positive framerate given.

        """
        if in_fps <= 0 or out_fps <= 0:
            raise ValueError("Framerates must be positive, cannot transform %f -> %f" % (in_fps, out_fps))

        ratio = in_fps / out_fps
        for line in self:
            line.start = int(round(line.start * ratio))
            line.end = int(round(line.end * ratio))
Rescale all timestamps by ratio of in_fps/out_fps.

        Can be used to fix files converted from frame-based to time-based
        with wrongly assumed framerate.

        Arguments:
            in_fps (float)
            out_fps (float)

        Raises:
            ValueError: Non-positive framerate given.