将GIF与音乐节奏同步会导致比预期的持续时间更短

Question

我试图将gif与Spotify上播放的音乐节拍同步，但在同步过程中遇到了速度问题。我一定是疯了，因为我找不到这不起作用的理由。以下是我的方法：

• 获取初始BPM（例如：150）并找到每秒的节拍（BPS）
  • BPS = BPM / 60
• 从Beats/Second（BPS）中查找秒/拍（SPB）
  • SPB = 1 / BPS
• 通过乘以.gif的拍数/圈数（BPL），找到秒数/圈数（SPL）
  • SPL = SPB * BPL
• 将秒/循环（SPL）转换为毫秒/循环（MSPL）
  • MSPL = SPL * 1000
• 将毫秒/循环（MSPL）除以.gif中的帧数（num_frames），以找到一帧（frame_time）所需的时间，舍入到最接近的偶数，因为.gif帧时间仅精确到整毫秒
  • frame_time = MSPL / num_frames
• 累计总帧时间（actual_duration），循环帧加或减1毫秒，直到actual_duration匹配ceil(MSPL)（始终优先考虑更长的实际持续时间而不是更短的持续时间）

  difference = MSPL - actual_duration
  if not math.isclose(0, difference):
      # Add the difference and always prioritize longer duration compared to real duration value
      correction = int(math.ceil(difference))
      for i in range(0, abs(correction)):
          # Add/subtract corrections as necessary to get actual duration as close as possible to calculated duration
          frame_times[i % len(frame_times)] += math.copysign(1, correction)
  

现在，gif的实际毫秒/循环应该始终等于MSLP或大于MSLP。但是，当我使用指定的帧时间保存.gif时，如果校正值不是0，则.gif的播放速度总是比预期的快。我注意到，当使用其他在线服务提供相同的“将gif同步到音乐”功能时，情况也是如此；所以我想不仅仅是我疯了

下面是用于获取帧时间的实际代码：

def get_frame_times(tempo: float, beats_per_loop: int, num_frames: int):
    # Calculate the number of seconds per beat in order to get number of milliseconds per loop
    beats_per_sec = tempo / 60
    secs_per_beat = 1 / beats_per_sec
    duration = math.ceil(secs_per_beat * beats_per_loop * 1000)
    frame_times = []
    # Try to make frame times as even as possible by dividing duration by number of frames and rounding
    actual_duration = 0
    for _ in range(0, num_frames):
        # Rounding method: Bankers Rounding (round to the nearest even number)
        frame_time = round(duration / num_frames)
        frame_times.append(frame_time)
        actual_duration += frame_time
    # Add the difference and always prioritize longer duration compared to real duration value
    difference = duration - actual_duration
    if not math.isclose(0, difference):
        correction = int(math.ceil(difference))
        for i in range(0, abs(correction)):
            # Add/subtract corrections as necessary to get actual duration as close as possible to calculated duration
            frame_times[i % len(frame_times)] += math.copysign(1, correction)
    return frame_times

我正在使用PIL（枕头）的图像模块保存gif：

frame_times = get_frame_times(tempo, beats_per_loop, num_frames)
frames = []
for i in range(0, num_frames):
  # Frames are appended to frames list here
# disposal=2 used since the frames may be transparent
frames[0].save(
    output_file, 
    save_all=True, 
    append_images=frames[1:], 
    loop=0, 
    duration=frame_times, 
    disposal=2)

这里有什么我做错的吗？我似乎无法找到为什么这不起作用，以及为什么gif的实际持续时间比指定的帧时间短得多。其他提供此功能的站点/服务最终也会获得相同的结果，这让我感觉稍微好一点，但同时我觉得这肯定是可能的

Answer 1

解决了！我不知道这是对.gif格式的限制，还是对PIL中的图像模块的限制，但帧时间似乎只能精确到10毫秒的倍数。在检查修改后图像的实际帧时间时，它们被调整到最接近10的倍数，从而使整体播放速度比预期的快

为了解决这个问题，我修改了代码，以10为增量选择帧时间（如果需要，再次优先考虑更长的实际持续时间），并在整个列表中尽可能均匀地分散帧时间调整：

def get_frame_times(tempo: float, beats_per_loop: int, num_frames: int):
    # Calculate the number of seconds per beat in order to get number of milliseconds per loop
    beats_per_sec = tempo / 60
    secs_per_beat = 1 / beats_per_sec
    duration = round_tens(secs_per_beat * beats_per_loop * 1000)
    frame_times = []
    # Try to make frame times as even as possible by dividing duration by number of frames
    actual_duration = 0
    for _ in range(0, num_frames):
        frame_time = round_tens(duration / num_frames)
        frame_times.append(frame_time)
        actual_duration += frame_time
    # Adjust frame times to match as closely as possible to the actual duration, rounded to multiple of 10
    # Keep track of which indexes we've added to already and attempt to split corrections as evenly as possible
    # throughout the frame times
    correction = duration - actual_duration
    adjust_val = int(math.copysign(10, correction))
    i = 0
    seen_i = {i}
    while actual_duration != duration:
        frame_times[i % num_frames] += adjust_val
        actual_duration += adjust_val
        if i not in seen_i:
            seen_i.add(i)
        elif len(seen_i) == num_frames:
            seen_i.clear()
            i = 0
        else:
            i += 1
        i += num_frames // abs(correction // 10)
    return frame_times