在使用 Ffmpeg 进行视频抽帧时，发现 -vsync 参数的意思理解的不是很清楚。本文通过不同的场景 使用结果对比来辨析 -vsync 选项的枚举参数值的区别。

以下是 Ffmpeg 官网上关于 -vsync 的全部说明：

-vsync parameter

    Video sync method. For compatibility reasons old values can be specified as numbers. Newly added values will have to be specified as strings always.

    0, passthrough
        Each frame is passed with its timestamp from the demuxer to the muxer.
    1, cfr
        Frames will be duplicated and dropped to achieve exactly the requested constant frame rate.
    2, vfr
        Frames are passed through with their timestamp or dropped so as to prevent 2 frames from having the same timestamp.
    drop
        As passthrough but destroys all timestamps, making the muxer generate fresh timestamps based on frame-rate.
    -1, auto
        Chooses between 1 and 2 depending on muxer capabilities. This is the default method.

    Note that the timestamps may be further modified by the muxer, after this. For example, in the case that the format option avoid_negative_ts is enabled.

    With -map you can select from which stream the timestamps should be taken. You can leave either video or audio unchanged and sync the remaining stream(s) to the unchanged one.

🔗字面理解

简单翻译一下官方说明：

• passthrough 每一帧从解码器到编码器，时间戳保持不变
• cfr 如果指定了输出帧率，输入帧会按照需要进行复制（如果输出帧率大于输入帧率）或丢弃（如果输出帧率小于输入帧率）
• vfr 输入帧从解码器到编码器，时间戳保持不变；如果出现相同时间戳的帧，则丢弃之
• drop 同 passthrough，但将所有帧的时间戳清空

🔗Case 分析

使用 Blender - Big Buck Bunny 网站提供的视频进行 case 分析，其 ffprobe 输出信息如下：

{
  "streams": [
    {
      "index": 0,
      "codec_name": "h264",
      "codec_long_name": "H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10",
      "profile": "High",
      "codec_type": "video",
      "codec_time_base": "1/60",
      "codec_tag_string": "avc1",
      "codec_tag": "0x31637661",
      "width": 1920,
      "height": 1080,
      "coded_width": 1920,
      "coded_height": 1088,
      "has_b_frames": 2,
      "sample_aspect_ratio": "1:1",
      "display_aspect_ratio": "16:9",
      "pix_fmt": "yuv420p",
      "level": 40,
      "chroma_location": "left",
      "refs": 1,
      "is_avc": "true",
      "nal_length_size": "4",
      "r_frame_rate": "30/1",
      "avg_frame_rate": "30/1",
      "time_base": "1/15360",
      "start_pts": 0,
      "start_time": "0.000000",
      "duration_ts": 9747456,
      "duration": "634.600000",
      "bit_rate": "2736521",
      "bits_per_raw_sample": "8",
      "nb_frames": "19038",
      "disposition": {
        "default": 1,
        "dub": 0,
        "original": 0,
        "comment": 0,
        "lyrics": 0,
        "karaoke": 0,
        "forced": 0,
        "hearing_impaired": 0,
        "visual_impaired": 0,
        "clean_effects": 0,
        "attached_pic": 0,
        "timed_thumbnails": 0
      },
      "tags": {
        "language": "und",
        "handler_name": "GPAC ISO Video Handler"
      }
    },
    {
      "index": 1,
      "codec_name": "ac3",
      "codec_long_name": "ATSC A/52A (AC-3)",
      "codec_type": "audio",
      "codec_time_base": "1/48000",
      "codec_tag_string": "ac-3",
      "codec_tag": "0x332d6361",
      "sample_fmt": "fltp",
      "sample_rate": "48000",
      "channels": 6,
      "channel_layout": "5.1(side)",
      "bits_per_sample": 0,
      "dmix_mode": "-1",
      "ltrt_cmixlev": "-1.000000",
      "ltrt_surmixlev": "-1.000000",
      "loro_cmixlev": "-1.000000",
      "loro_surmixlev": "-1.000000",
      "r_frame_rate": "0/0",
      "avg_frame_rate": "0/0",
      "time_base": "1/48000",
      "start_pts": 0,
      "start_time": "0.000000",
      "duration_ts": 30438912,
      "duration": "634.144000",
      "bit_rate": "320000",
      "nb_frames": "19556",
      "disposition": {
        "default": 1,
        "dub": 0,
        "original": 0,
        "comment": 0,
        "lyrics": 0,
        "karaoke": 0,
        "forced": 0,
        "hearing_impaired": 0,
        "visual_impaired": 0,
        "clean_effects": 0,
        "attached_pic": 0,
        "timed_thumbnails": 0
      },
      "tags": {
        "language": "und",
        "handler_name": "GPAC ISO Audio Handler"
      },
      "side_data_list": [
        {
          "side_data_type": "Audio Service Type"
        }
      ]
    }
  ],
  "format": {
    "filename": "big_buck_bunny.1920x1080.30fps.mp4",
    "nb_streams": 2,
    "nb_programs": 0,
    "format_name": "mov,mp4,m4a,3gp,3g2,mj2",
    "format_long_name": "QuickTime / MOV",
    "start_time": "0.000000",
    "duration": "634.600000",
    "size": "242901742",
    "bit_rate": "3062108",
    "probe_score": 100,
    "tags": {
      "major_brand": "isom",
      "minor_version": "512",
      "compatible_brands": "isomiso2avc1mp41",
      "title": "Big Buck Bunny, Sunflower version",
      "artist": "Blender Foundation 2008, Janus Bager Kristensen 2013",
      "composer": "Sacha Goedegebure",
      "encoder": "Lavf58.20.100",
      "comment": "Creative Commons Attribution 3.0 - http://bbb3d.renderfarming.net",
      "genre": "Animation"
    }
  }
}

🔗应用于转码

对输入视频进行转码，为 4 个 -vsync 枚举值分别生成两个输出视频，其中一个输出视频的帧率大于输入帧率，另一个小于。

即对于输入帧率为 30/1，分别按以下参数进行转码：

• -vsync passthrough -r 24
• -vsync passthrough -r 45
• -vsync cfr -r 24
• -vsync cfr -r 45
• -vsync vfr -r 24
• -vsync vfr -r 45
• -vsync drop -r 24
• -vsync drop -r 45

详见 转码脚本 和 辅助脚本。

执行结果如下：

对于 drop 模式，命令执行失败。

🔗应用于抽帧

对输入视频进行抽帧，为 4 个 -vsync 枚举值分别生成两组输出，其中一个输出的帧率大于输入帧率，另一个小于。

即对于输入帧率为 30/1，分别按以下参数进行转码：

• -vsync passthrough -r 24 -frame_pts true
• -vsync passthrough -r 45 -frame_pts true
• -vsync cfr -r 24 -frame_pts true
• -vsync cfr -r 45 -frame_pts true
• -vsync vfr -r 24 -frame_pts true
• -vsync vfr -r 45 -frame_pts true
• -vsync drop -r 24 -frame_pts true
• -vsync drop -r 45 -frame_pts true

添加了 -frame_pts 选项以从输出图片的文件名中观察对应帧的 PTS。

详见 抽帧脚本。

执行结果如下：

生成的对应图片文件名和大小如下：

🔗（不负责任的）结论

• cfr 可以和 -r 输出选项搭配使用，使输出视频的帧率符合指定值
• vfr passthrough drop 都不应该和 -r 输出选项搭配使用
• vfr 常用于按类型过滤帧等输入帧被主动丢弃的场景
• passthrough drop 不清楚应用场景

参见官方文档中关于 -vsync 的说明文档。

To be continued

以上。