Compute model data with current parameters

        Returns
        -------
        qpi: qpimage.QPImage
            Modeled phase data

        Notes
        -----
        The model image might deviate from the fitted image
        because of interpolation during the fitting process.

def compute_qpi(self):
        """Compute model data with current parameters

        Returns
        -------
        qpi: qpimage.QPImage
            Modeled phase data

        Notes
        -----
        The model image might deviate from the fitted image
        because of interpolation during the fitting process.
        """
        kwargs = self.model_kwargs.copy()
        kwargs["radius"] = self.radius
        kwargs["sphere_index"] = self.sphere_index
        kwargs["center"] = [self.posx_offset, self.posy_offset]
        qpi = self.sphere_method(**kwargs)
        # apply phase offset
        bg_data = np.ones(qpi.shape) * -self.pha_offset
        qpi.set_bg_data(bg_data=bg_data, which_data="phase")
        return qpi