将 GIF 与音乐节奏同步导致持续时间比预期的要短

Question

我正在尝试将 gif 同步到 Spotify 上播放的音乐节拍，但在执行此操作时遇到了速度问题。我一定要疯了，因为我找不到为什么这不起作用的原因。以下是我的方法：

• 获取初始 BPM（例如：150）并找到 Beats/Second (BPS)
  • BPS = BPM / 60
• 从 Beats/Second (BPS) 中找到 Seconds/Beat (SPB)
  • SPB = 1 / BPS
• 通过乘以 .gif 的节拍/循环 (BPL) 的数量来查找秒/循环 (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，四舍五入到自 .gif 以来最接近的偶数帧时间仅精确到整毫秒
  • frame_time = MSPL / num_frames
• 将总帧时间 (actual_duration) 相加并循环帧加或减 1 毫秒，直到 actual_duration 与 ceil(MSPL) 匹配（始终优先考虑较长的实际持续时间而不是较短的持续时间）

  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，如果校正值不为 0，则 .gif 始终以比预期更快的速度播放。我注意到，当使用提供相同“将 gif 同步到音乐”功能的其他在线服务时，情况也是如此；所以我认为不仅仅是我发疯了。

以下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 (Pillow) 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)

我在这里做错了什么吗？我似乎无法找出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。提供此功能的其他网站/服务最终得到相同的结果让我感觉稍微好一些，但同时我觉得这绝对是可能的。

Answer 1

解决了！我不知道这是 .gif 格式的限制，还是 PIL 中 Image 模块的限制，但似乎帧时间只能精确到 10 毫秒的倍数。在检查修改后图像的实际帧时间后，它们被限制到最接近 10 的倍数，导致整体播放速度比预期的要快。

为了解决这个问题，我修改了代码以选择以 10 为增量的帧时间（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