def blend(self, other):
        """Alpha blend *other* on top of the current image."""
        raise NotImplementedError("This method has not be implemented for "
                                  "xarray support.")

        if self.mode != "RGBA" or other.mode != "RGBA":
            raise ValueError("Images must be in RGBA")
        src = other
        dst = self
        outa = src.channels[3] + dst.channels[3] * (1 - src.channels[3])
        for i in range(3):
            dst.channels[i] = (src.channels[i] * src.channels[3] +
                               dst.channels[i] * dst.channels[3] *
                               (1 - src.channels[3])) / outa
            dst.channels[i][outa == 0] = 0
        dst.channels[3] = outa
Alpha blend *other* on top of the current image.