def optical_distortion(img, k=0, dx=0, dy=0, interpolation=cv2.INTER_LINEAR, border_mode=cv2.BORDER_REFLECT_101,
                       value=None):
    """Barrel / pincushion distortion. Unconventional augment.

    Reference:
        |  https://stackoverflow.com/questions/6199636/formulas-for-barrel-pincushion-distortion
        |  https://stackoverflow.com/questions/10364201/image-transformation-in-opencv
        |  https://stackoverflow.com/questions/2477774/correcting-fisheye-distortion-programmatically
        |  http://www.coldvision.io/2017/03/02/advanced-lane-finding-using-opencv/
    """
    height, width = img.shape[:2]

    fx = width
    fy = width

    cx = width * 0.5 + dx
    cy = height * 0.5 + dy

    camera_matrix = np.array([[fx, 0, cx],
                              [0, fy, cy],
                              [0, 0, 1]], dtype=np.float32)

    distortion = np.array([k, k, 0, 0, 0], dtype=np.float32)
    map1, map2 = cv2.initUndistortRectifyMap(camera_matrix, distortion, None, None, (width, height), cv2.CV_32FC1)
    img = cv2.remap(img, map1, map2, interpolation=interpolation, borderMode=border_mode, borderValue=value)
    return img