FFmpeg為視頻加字幕

為視頻添加字幕的方式有很多種输虱，大概可以分為：
1、將字幕編碼進視頻流中昂勉。
2浪册、在封裝容器中加入字幕流。
其中第一種方式與視頻加水印基本相似岗照，第二種方式則需要封裝容器支持加入字幕流村象。

1、ASS字幕流寫入視頻流

需要FFmpeg在編譯時支持ass攒至，--enable-libass厚者，字幕文件內(nèi)容格式大致如下：

[Script Info]

[V4+ Styles]
Format: Name, Fontname, Fontsize, PrimaryColour, SecondaryColour, OutlineColour, BackColour, Bold, Italic, Underline, StrikeOut, ScaleX, ScaleY, Spacing, Angle, BorderStyle, Outline, Shadow, Alignment, MarginL, MarginR, MarginV, Encoding
Style: Default,Arial Narrow,16,&H00FFFFFF,&H0000FFFF,&H00000000,&H80000000,-1,0,0,0,100,100,0,0,1,2,1,2,20,20,3,1

[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:05.58,0:00:07.20,Default,,0000,0000,0000,,{\fnArial Narrow\fs20}名字是一個人的代號{\r}\NName is aword by which a person is spoken of.
Dialogue: 0,0:00:07.87,0:00:09.04,Default,,0000,0000,0000,,{\fnArial Narrow\fs20}但當(dāng)一個人的職業(yè){\r}\NWhen a person's profession
Dialogue: 0,0:00:09.20,0:00:11.16,Default,,0000,0000,0000,,{\fnArial Narrow\fs20}可以變成他的代號的時候{\r}\Nbecomes his nick name,{\r}
Dialogue: 0,0:00:11.87,0:00:14.29,Default,,0000,0000,0000,,{\fnArial Narrow\fs20}往往人們就會忘記了他本來的名字{\r}\Npeople will forget his original name.

命令行如下：

ffmpeg -i 111.mp4 -vf ass=t1.ass output.mp4

輸入輸出信息如下：

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '111.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 0
    compatible_brands: mp41avc1qt  
    creation_time   : 2017-02-04T01:33:49.000000Z
    playback_requirements: QuickTime 6.0 or greater
    playback_requirements-eng: QuickTime 6.0 or greater
    encoder         : vlc 2.1.5 stream output
    encoder-eng     : vlc 2.1.5 stream output
  Duration: 00:01:21.89, start: 0.000000, bitrate: 2037 kb/s
    Stream #0:0(eng): Audio: aac (LC) (mp4a / 0x6134706D), 16000 Hz, mono, fltp, 23 kb/s (default)
    Metadata:
      creation_time   : 2017-02-04T01:33:49.000000Z
      handler_name    : SoundHandler
    Stream #0:1(eng): Video: h264 (Main) (avc1 / 0x31637661), yuvj420p(pc, bt709), 1920x1080, 2009 kb/s, 25 fps, 25 tbr, 1000k tbn, 50 tbc (default)
    Metadata:
      creation_time   : 2017-02-04T01:33:49.000000Z
      handler_name    : VideoHandler
Stream mapping:
  Stream #0:1 -> #0:0 (h264 (native) -> h264 (libx264))
  Stream #0:0 -> #0:1 (aac (native) -> aac (native))
Output #0, mp4, to 'output.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 0
    compatible_brands: mp41avc1qt  
    encoder         : Lavf58.20.100
    playback_requirements: QuickTime 6.0 or greater
    playback_requirements-eng: QuickTime 6.0 or greater
    Stream #0:0(eng): Video: h264 (libx264) (avc1 / 0x31637661), yuvj420p(pc), 1920x1080, q=-1--1, 25 fps, 12800 tbn, 25 tbc (default)
    Metadata:
      creation_time   : 2017-02-04T01:33:49.000000Z
      handler_name    : VideoHandler
      encoder         : Lavc58.35.100 libx264
    Side data:
      cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: -1
    Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 16000 Hz, mono, fltp, 69 kb/s (default)
    Metadata:
      creation_time   : 2017-02-04T01:33:49.000000Z
      handler_name    : SoundHandler
      encoder         : Lavc58.35.100 aac

從以上信息中可以看出，輸入與輸出的封裝容器格式基本相同迫吐，均為一個視頻流和一個音頻流库菲，并未包含字幕流，因為字幕流是通過ASS容器將文字寫入視頻流中的志膀。

2熙宇、ASS字幕流寫入封裝容器

除了以上將字幕流加入到視頻編碼中的方式外，還可以在視頻封裝容器中增加字幕流梧却，只要封裝容器格式支持字幕流即可奇颠。
如下，使用FFmpeg將ASS字幕流寫入MKV封裝容器中：

ffmpeg -i 111.mp4 -i t1.ass -acodec copy -vcodec copy -scodec copy output.mkv

輸入輸出信息如下：

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '111.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 0
    compatible_brands: mp41avc1qt  
    creation_time   : 2017-02-04T01:33:49.000000Z
    playback_requirements: QuickTime 6.0 or greater
    playback_requirements-eng: QuickTime 6.0 or greater
    encoder         : vlc 2.1.5 stream output
    encoder-eng     : vlc 2.1.5 stream output
  Duration: 00:01:21.89, start: 0.000000, bitrate: 2037 kb/s
    Stream #0:0(eng): Audio: aac (LC) (mp4a / 0x6134706D), 16000 Hz, mono, fltp, 23 kb/s (default)
    Metadata:
      creation_time   : 2017-02-04T01:33:49.000000Z
      handler_name    : SoundHandler
    Stream #0:1(eng): Video: h264 (Main) (avc1 / 0x31637661), yuvj420p(pc, bt709), 1920x1080, 2009 kb/s, 25 fps, 25 tbr, 1000k tbn, 50 tbc (default)
    Metadata:
      creation_time   : 2017-02-04T01:33:49.000000Z
      handler_name    : VideoHandler
Input #1, ass, from 't1.ass':
  Duration: N/A, bitrate: N/A
    Stream #1:0: Subtitle: ass
Output #0, matroska, to 'output.mkv':
  Metadata:
    major_brand     : isom
    minor_version   : 0
    compatible_brands: mp41avc1qt  
    encoder         : Lavf58.20.100
    playback_requirements: QuickTime 6.0 or greater
    playback_requirements-eng: QuickTime 6.0 or greater
    Stream #0:0(eng): Video: h264 (Main) (avc1 / 0x31637661), yuvj420p(pc, bt709), 1920x1080, q=2-31, 2009 kb/s, 25 fps, 25 tbr, 1k tbn, 1000k tbc (default)
    Metadata:
      creation_time   : 2017-02-04T01:33:49.000000Z
      handler_name    : VideoHandler
    Stream #0:1(eng): Audio: aac (LC) ([255][0][0][0] / 0x00FF), 16000 Hz, mono, fltp, 23 kb/s (default)
    Metadata:
      creation_time   : 2017-02-04T01:33:49.000000Z
      handler_name    : SoundHandler
    Stream #0:2: Subtitle: ass
Stream mapping:
  Stream #0:1 -> #0:0 (copy)
  Stream #0:0 -> #0:1 (copy)
  Stream #1:0 -> #0:2 (copy)
Press [q] to stop, [?] for help
frame= 2048 fps=0.0 q=-1.0 Lsize=   20343kB time=00:01:21.88 bitrate=2035.1kbits/s speed= 623x

命令行執(zhí)行之后放航，會將111.mp4中的音頻流烈拒、視頻流、t1.ass中的字幕流在不改變編碼的情況下封裝到output.mkv文件中广鳍，因此output.mkv文件將會包含三個流荆几，分別為視頻流、音頻流赊时、字幕流吨铸。
如果111.mp4中或輸入的視頻文件中原本帶有字幕流，并希望使用t1.ass字幕流時祖秒，可通過map功能將對應(yīng)的字幕流指定封裝到output.mkv中：

ffmpeg -i input.mp4 -i t1.ass -map 0:0 -map 0:1 -map 1:0 -acodec copy -vcodec copy -scodec copy output1.mkv

命令行執(zhí)行之后生成的文件信息如下：

Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'input.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf58.20.100
  Duration: 00:01:21.88, start: 0.000000, bitrate: 2598 kb/s
    Stream #0:0(eng): Video: h264 (High) (avc1 / 0x31637661), yuvj420p(pc), 1920x1080, 2530 kb/s, 25 fps, 25 tbr, 12800 tbn, 50 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 16000 Hz, mono, fltp, 62 kb/s (default)
    Metadata:
      handler_name    : SoundHandler
Input #1, ass, from 't1.ass':
  Duration: N/A, bitrate: N/A
    Stream #1:0: Subtitle: ass
Output #0, matroska, to 'output1.mkv':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf58.20.100
    Stream #0:0(eng): Video: h264 (High) (avc1 / 0x31637661), yuvj420p(pc), 1920x1080, q=2-31, 2530 kb/s, 25 fps, 25 tbr, 1k tbn, 12800 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(eng): Audio: aac (LC) ([255][0][0][0] / 0x00FF), 16000 Hz, mono, fltp, 62 kb/s (default)
    Metadata:
      handler_name    : SoundHandler
    Stream #0:2: Subtitle: ass
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
  Stream #0:1 -> #0:1 (copy)
  Stream #1:0 -> #0:2 (copy)