def zoom(params, factor):
    """
    Applies a zoom on the current parameters.

    Computes the top-left plane-space coordinates from the Mandelbrot-space coordinates.

    :param params: Current application parameters.
    :param factor: Zoom factor by which the zoom ratio is divided (bigger factor, more zoom)
    """
    params.zoom /= factor
    n_x = params.mb_cx / params.zoom
    n_y = params.mb_cy / params.zoom
    params.plane_x0 = int((n_x + 1.0) * params.plane_w / (2.0 * params.
        plane_ratio)) - params.plane_w // 2
    params.plane_y0 = int((n_y + 1.0) * params.plane_h / 2.0
        ) - params.plane_h // 2
