def prependBlack( self, video_input, black_s=1.0 ):
        """ Add black_s seconds of black to the start of the video.
            use ffmpeg for efficiency.
        """
        if black_s is None:
            black_s = 1.0

        # intermediate
        video_timescaled = tmp_filename('_video_timescaled.mp4')
        video_black = tmp_filename('_black_video.mp4')
        video_list = tmp_filename('_video_list.txt')
        # output
        video_with_black = ('_video_with_black.mp4')

        try:
            t1 = time.time()

            # ffmpeg -i fullvideo.mp4 -vf trim=0:3,geq=0:128:128 -af atrim=0:3,volume=0 -video_track_timescale 600 3sec.mp4
            args_create_black = ['ffmpeg', '-y', '-i', video_input,
                                 '-vf', 'trim=0:{},geq=0:128:128'.format(float(black_s)),
                                 '-af', 'atrim=0:1,volume=0',
                                 '-video_track_timescale', '600', '-strict', '-2',
                                 '-threads', '0', video_black]
            subprocess.run(args_create_black, check=True)

            t2 = time.time()
            print('Elapsed time creating black:', t2 - t1)

            # ffmpeg -i fullvideo.mp4 -c copy -video_track_timescale 600 full600.mp4
            args_ensure_speed = ['ffmpeg', '-y', '-i', video_input, '-c', 'copy',
                                 '-video_track_timescale', '600', '-threads', '0',
                                 video_timescaled]
            subprocess.run(args_ensure_speed, check=True)

            t3 = time.time()
            print('Elapsed time for timescale conversion:', t3 - t2)

            # create a list of concatencated videos
            with open(video_list, 'w') as f:
                f.write('file ' + video_black + '\n' +
                        'file ' + video_timescaled + '\n')

            # ffmpeg -f concat -i list.txt -c copy merged.mp4
            args_concat = ['ffmpeg', '-y', '-f', 'concat', '-safe', '0',
                           '-i', video_list, '-c', 'copy', '-threads', '0',
                           video_with_black]
            subprocess.run(args_concat, check=True)

            t4 = time.time()
            print('Elapsed time concatenating videos:', t4 - t3)

        # in case ffmpeg does not support the input video feature, use other backend
        except subprocess.CalledProcessError as e:
            print('Error:', e)
            print('Fallback to moviepy library!')
            t_fallback_1 = time.time()

            # Add 1.0 seconds of black to the start of the video
            video = VideoFileClip(video_input)
            black_clip = ColorClip(video.size, color=(0, 0, 0),
                                   duration=1.0)
            final_clip = concatenate_videoclips([black_clip, video])
            final_clip.write_videofile(video_with_black, fps=video.fps)

            t_fallback_2 = time.time()
            print('Elapsed time for fallback adding black to video with moviepy:',
                  t_fallback_2 - t_fallback_1)

        return video_with_black