Question

我正在尝试将gif同步到Spotify上播放的音乐的节拍中，但与此同时遇到速度问题。我一定要疯了，因为我找不到为什么它不起作用的原因。以下是我的方法：

进行初始BPM（例如：150）并找到Beats / Second（BPS）
- BPS = BPM / 60
从节拍/秒（SPB）中找到秒/节拍（BPS）
- SPB = 1 / BPS
通过乘以.gif的节拍数/循环数（SPL）来找到秒数/循环数（BPL）。
- SPL = SPB * BPL
将秒/循环（SPL）转换为毫秒/循环（MSPL）
- MSPL = SPL * 1000
将.gif中的帧数（MSPL除以毫秒/循环（num_frames），以找到一帧（frame_time）所需的时间，四舍五入为最接近的偶数，因为.gif帧时间仅精确到整毫秒
- frame_time = MSPL / num_frames

合计总帧时间（actual_duration）并循环遍历所有帧，增加或减少1毫秒，直到actual_duration与ceil(MSPL)匹配（始终将较长的实际持续时间优先于较短的持续时间）

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，当我以指定的帧时间保存.gif时，如果校正值不为0，则.gif总是以比预期更快的速度播放。我注意到在使用其他提供相同的“将gif同步到音乐”功能的在线服务时，情况也是如此。所以我想不仅仅是疯了。

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

我通过使用PIL（枕头）的“图像”模块来保存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)

我在这里做错什么了吗？我似乎无法找出为什么这行不通，以及为什么gif的实际持续时间比指定的帧时间短得多。提供该功能的其他站点/服务最终获得相同的结果使我感觉稍微好一点，但与此同时，我感觉这绝对应该可行。

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，但是看来帧时间最多只能精确到10毫秒的倍数。在检查了修改后的图像的实际帧时间后，它们被下限到最接近的10的倍数，从而导致整体回放速度比预期的快。

要解决此问题，我修改了代码，以10为增量选择帧时间（如果需要，可以再次设置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

将gif同步到音乐的节奏比预期的持续时间短

1 个答案: