def add_px_err(isoel, col1, col2, px_um, inplace=False):
        """Undo pixelation correction

        Isoelasticity lines are already corrected for pixelation
        effects as described in

        Mapping of Deformation to Apparent Young's Modulus
        in Real-Time Deformability Cytometry
        Christoph Herold, arXiv:1704.00572 [cond-mat.soft] (2017)
        https://arxiv.org/abs/1704.00572.

        If the isoealsticity lines are displayed with deformation data
        that are not corrected, then the lines must be "un"-corrected,
        i.e. the pixelation error must be added to the lines to match
        the experimental data.

        Parameters
        ----------
        isoel: list of 2d ndarrays of shape (N, 3)
            Each item in the list corresponds to one isoelasticity
            line. The first column is defined by `col1`, the second
            by `col2`, and the third column is the emodulus.
        col1, col2: str
            Define the fist to columns of each isoelasticity line.
            One of ["area_um", "circ", "deform"]
        px_um: float
            Pixel size [µm]
        """
        Isoelastics.check_col12(col1, col2)
        if "deform" in [col1, col2]:
            # add error for deformation
            sign = +1
        else:
            # subtract error for circularity
            sign = -1
        if col1 == "area_um":
            area_ax = 0
            deci_ax = 1
        else:
            area_ax = 1
            deci_ax = 0

        new_isoel = []
        for iso in isoel:
            iso = np.array(iso, copy=not inplace)
            ddeci = feat_emod.corrpix_deform_delta(area_um=iso[:, area_ax],
                                                   px_um=px_um)
            iso[:, deci_ax] += sign * ddeci
            new_isoel.append(iso)
        return new_isoel
Undo pixelation correction

        Isoelasticity lines are already corrected for pixelation
        effects as described in

        Mapping of Deformation to Apparent Young's Modulus
        in Real-Time Deformability Cytometry
        Christoph Herold, arXiv:1704.00572 [cond-mat.soft] (2017)
        https://arxiv.org/abs/1704.00572.

        If the isoealsticity lines are displayed with deformation data
        that are not corrected, then the lines must be "un"-corrected,
        i.e. the pixelation error must be added to the lines to match
        the experimental data.

        Parameters
        ----------
        isoel: list of 2d ndarrays of shape (N, 3)
            Each item in the list corresponds to one isoelasticity
            line. The first column is defined by `col1`, the second
            by `col2`, and the third column is the emodulus.
        col1, col2: str
            Define the fist to columns of each isoelasticity line.
            One of ["area_um", "circ", "deform"]
        px_um: float
            Pixel size [µm]