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多媒体文件格式(二):FLV 格式

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在网络的直播与点播场景中,FLV也是一种常见的格式,FLV是Adobe发布的一种可以作为直播也可以作为点播的封装格式,其封装格式非常简单,均以FLVTAG的形式存在,并且每一个TAG都是独立存在的,接下来就详细介绍一下FLV标准。

一、FLV 格式标准介绍

FLV包括文件头(File Header)和文件体(File Body)两部分,其中文件体由一系列的Tag组成。FLV文件的结构如下图:

技术图片

1. 文件头 Header

Header 部分记录了FLV的类型、版本等信息,是FLV的开头。一般差不多占9bytes。具体格式如下:

1. 文件标识(3B):总是为”FLV”, 0x46 0x4c 0x56

2. 版本(1B):目前为0x01

3. 流信息(1B):文件的标志位说明。前5位保留,必须为0;第6位为音频Tag:1表示有音频;第七位保留,为0; 第8位为视频Tag:1表示有视频

4. Header长度(4B):整个Header的长度,一般为9(版本为0x01时);大于9表示下面还有扩展信息。即0x00000009。

下图是使用工具FlvAnalyzer获取到的FLV的Header的详细信息:

 技术图片

2. 文件体 FLV Body

文件体由一系列的Tag组成。

其中,每个Tag前面还包含了Previous Tag Size字段,表示前面一个Tag的大小。Tag的类型可以是视频、音频和Script,每个Tag只能包含以上三种类型的数据中的一种。

下图是使用FlvAnalyzer获取到的Body信息:

技术图片

3. Tag

每个Tag由也是由两部分组成的:Tag Header和Tag Data。Tag Header里存放的是当前Tag的类型、数据区(Tag Data)长度等信息,具体如下:

Tag类型(1):0x08:音频;     0x09:视频;      0x12:脚本;     其他:保留

数据区长度(3):数据区的长度

时间戳(3):整数,单位是毫秒。对于脚本型的tag总是0 (CTS)

时间戳扩展(1):将时间戳扩展为4bytes,代表高8位。很少用到

StreamsID(3):总是0

数据区(由数据区长度决定):数据实体

下面是三个Tag类型说明:

  • Audio Tag Data结构(音频类型) :音频Tag Data区域开始的第一个字节包含了音频数据的参数信息,从第二个字节开始为音频流数据。

  • video Tag Data结构(视频类型):视频Tag Data开始的第一个字节包含视频数据的参数信息,从第二个字节开始为视频流数据。
  • Script Tag Data结构(脚本类型、帧类型):该类型Tag又被称为MetaData Tag,存放一些关于FLV视频和音频的元信息,比如:duration、width、height等。通常该类型Tag会作为FLV文件的第一个tag,并且只有一个,跟在File Header后。

二、FLV 分析工具

在上节的内容中,我们介绍了FLV的格式信息,同时也提到了FlvAnalyzer工具,下面我们就介绍两个工具,帮助大家整理和学习FLV相关知识:

1. FlvAnalyzer

通过FlvAnalyzer可以很清晰的看到FLV文件的基本结构,这样能够结合上面了解的FLV的知识,更清晰的查看FLV的格式及结构。

工具地址:https://github.com/renhui/Thinking-in-AV/blob/master/多媒体格式/FLV/FlvAnalyzer.exe

工具使用如图:

技术图片

左侧树状结构显示flv的信息,可以清楚了解flv文件的结构;

点击左侧节点,右侧显示对应hex与ascii信息,这样就不必打开二进制编辑器了;

通过此工具可以查看audio tag与video tag各个字节(精确到bit)的详细信息,了解每个tag是如何构造的,同时右下角黑色输出框显示某个值的意义;

2. FLV Format Analysis 工具

此工具是雷霄骅整理flvparse的开源代码,制作的flvformatanalysis工具,此工具可以用来帮助学习FLV封装格式结构。此外它还支持分离FLV中的视频流和音频流。

工具地址:https://github.com/renhui/Thinking-in-AV/blob/master/多媒体格式/FLV/SpecialFFLV.exe

工具使用如图:

技术图片

三、FLV格式 与 FFmpeg 实战

1. 使用FFmpeg生成带关键索引信息的FLV

在网络视频点播文件为FLV格式文件时,人们经常用工具先对FLV文件进行一次转换,主要是将FLV文件中的关键帧建立一个索引,并将索引写到Metadata头中,这个步骤用FFmpeg同样也可以实现,使用参数add_keyframe_index即可:

ffmpeg -i 好汉歌.mp4 -c copy -f flv -flvflags add_keyframe_index out.flv 

生成FLV包含了关键帧索引信息,这些关键帧索引信息并不是FLV的标准字段,但是由于其被广泛使用,已经成为常用的字段,所以FFmpeg也同样支持了这个功能。

2. 使用ffprobe查看FLV关键帧索引相关信息

除了在第二节介绍的两个工具,我们也可以使用ffprobe来解析FLV文件,并且还能将关键帧索引的相关信息打印出来,命令如下:

ffprobe -v trace -i out.flv 

输出如下:

[NULL @ 0x7fc669002a00] Opening out.flv for reading
[file @ 0x7fc667f00480] Setting default whitelist file,crypto
Probing flv score:100 size:2048
Probing mp3 score:1 size:2048
[flv @ 0x7fc669002a00] Format flv probed with size=2048 and score=100
[flv @ 0x7fc669002a00] Before avformat_find_stream_info() pos: 13 bytes read:32768 seeks:0 nb_streams:0
[flv @ 0x7fc669002a00] type:18, size:1184, last:-1, dts:0 pos:21
[flv @ 0x7fc669002a00] keyframe stream hasnt been created
[flv @ 0x7fc669002a00] type:9, size:45, last:-1, dts:0 pos:1220
[flv @ 0x7fc669002a00] keyframe filepositions = 1296 times = 0
[flv @ 0x7fc669002a00] keyframe filepositions = 159283 times = 3000
[flv @ 0x7fc669002a00] keyframe filepositions = 258004 times = 4000
[flv @ 0x7fc669002a00] keyframe filepositions = 272776 times = 4000
[flv @ 0x7fc669002a00] keyframe filepositions = 405340 times = 6000
[flv @ 0x7fc669002a00] keyframe filepositions = 1215104 times = 16000
[flv @ 0x7fc669002a00] keyframe filepositions = 2529035 times = 26000
[flv @ 0x7fc669002a00] keyframe filepositions = 3198814 times = 36000
[flv @ 0x7fc669002a00] keyframe filepositions = 3623757 times = 41000
[flv @ 0x7fc669002a00] keyframe filepositions = 4882191 times = 51000
[flv @ 0x7fc669002a00] keyframe filepositions = 5951597 times = 61000
[flv @ 0x7fc669002a00] keyframe filepositions = 6256906 times = 63000
[flv @ 0x7fc669002a00] keyframe filepositions = 7235927 times = 73000
[flv @ 0x7fc669002a00] keyframe filepositions = 8175324 times = 83000
[flv @ 0x7fc669002a00] keyframe filepositions = 9203399 times = 93000
[flv @ 0x7fc669002a00] keyframe filepositions = 9936528 times = 103000
[flv @ 0x7fc669002a00] keyframe filepositions = 11056393 times = 113000
[flv @ 0x7fc669002a00] keyframe filepositions = 12183978 times = 123000
[flv @ 0x7fc669002a00] keyframe filepositions = 13014068 times = 133000
[flv @ 0x7fc669002a00] keyframe filepositions = 13610750 times = 143000
[flv @ 0x7fc669002a00] keyframe filepositions = 14628601 times = 153000
[flv @ 0x7fc669002a00] keyframe filepositions = 15873046 times = 163000
[flv @ 0x7fc669002a00] keyframe filepositions = 17112198 times = 173000
[flv @ 0x7fc669002a00] keyframe filepositions = 18301365 times = 182000
[flv @ 0x7fc669002a00] keyframe filepositions = 18604436 times = 186000
[flv @ 0x7fc669002a00] 0 17 0 
[flv @ 0x7fc669002a00] type:8, size:9, last:-1, dts:0 pos:1280
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:2117, last:-1, dts:0 pos:1304
[flv @ 0x7fc669002a00] 0 17 0 
[NULL @ 0x7fc668809e00] nal_unit_type: 7, nal_ref_idc: 3
[NULL @ 0x7fc668809e00] nal_unit_type: 8, nal_ref_idc: 3
[NULL @ 0x7fc668809e00] user data:"x264 - core 142 r2 dd79a61 - H.264/MPEG-4 AVC codec - Copyleft 2003-2014 - http://www.videolan.org/x264.html - options: cabac=1 ref=8 deblock=1:-1:-1 analyse=0x1:0x131 me=umh subme=9 psy=1 psy_rd=1.00:0.15 mixed_ref=1 me_range=24 chroma_me=1 trellis=2 8x8dct=0 cqm=0 deadzone=21,11 fast_pskip=0 chroma_qp_offset=-3 threads=24 lookahead_threads=2 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 stitchable=1 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=2 b_bias=0 direct=3 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=60 rc=2pass mbtree=1 bitrate=680 ratetol=1.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 cplxblur=20.0 qblur=0.5 ip_ratio=1.40 aq=1:1.00"
[h264 @ 0x7fc668809e00] nal_unit_type: 7, nal_ref_idc: 3
[h264 @ 0x7fc668809e00] nal_unit_type: 8, nal_ref_idc: 3
[h264 @ 0x7fc668809e00] nal_unit_type: 6, nal_ref_idc: 0
[h264 @ 0x7fc668809e00] nal_unit_type: 5, nal_ref_idc: 3
[h264 @ 0x7fc668809e00] user data:"x264 - core 142 r2 dd79a61 - H.264/MPEG-4 AVC codec - Copyleft 2003-2014 - http://www.videolan.org/x264.html - options: cabac=1 ref=8 deblock=1:-1:-1 analyse=0x1:0x131 me=umh subme=9 psy=1 psy_rd=1.00:0.15 mixed_ref=1 me_range=24 chroma_me=1 trellis=2 8x8dct=0 cqm=0 deadzone=21,11 fast_pskip=0 chroma_qp_offset=-3 threads=24 lookahead_threads=2 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 stitchable=1 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=2 b_bias=0 direct=3 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=60 rc=2pass mbtree=1 bitrate=680 ratetol=1.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 cplxblur=20.0 qblur=0.5 ip_ratio=1.40 aq=1:1.00"
[h264 @ 0x7fc668809e00] Reinit context to 576x432, pix_fmt: yuv420p
[h264 @ 0x7fc668809e00] no picture 
[flv @ 0x7fc669002a00] type:9, size:1653, last:-1, dts:40 pos:3436
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:9, size:323, last:-1, dts:80 pos:5104
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:41, last:-1, dts:80 pos:5442
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:103, last:-1, dts:120 pos:5498
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:314, last:-1, dts:126 pos:5616
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:156, last:-1, dts:160 pos:5945
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:634, last:-1, dts:173 pos:6116
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:751, last:-1, dts:200 pos:6765
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:636, last:-1, dts:219 pos:7531
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:208, last:-1, dts:240 pos:8182
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:625, last:-1, dts:266 pos:8405
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:134, last:-1, dts:280 pos:9045
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:607, last:-1, dts:312 pos:9194
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:73, last:-1, dts:320 pos:9816
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:599, last:-1, dts:359 pos:9904
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:358, last:-1, dts:360 pos:10518
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:9, size:108, last:-1, dts:400 pos:10891
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:628, last:-1, dts:405 pos:11014
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:61, last:-1, dts:440 pos:11657
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:614, last:-1, dts:452 pos:11733
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:53, last:-1, dts:480 pos:12362
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:630, last:-1, dts:498 pos:12430
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:287, last:-1, dts:520 pos:13075
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:625, last:-1, dts:544 pos:13377
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:47, last:-1, dts:560 pos:14017
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:622, last:-1, dts:591 pos:14079
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:79, last:-1, dts:600 pos:14716
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:625, last:-1, dts:637 pos:14810
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:324, last:-1, dts:640 pos:15450
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:9, size:55, last:-1, dts:680 pos:15789
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:605, last:-1, dts:684 pos:15859
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:67, last:-1, dts:720 pos:16479
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:613, last:-1, dts:730 pos:16561
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:350, last:-1, dts:760 pos:17189
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:604, last:-1, dts:777 pos:17554
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:89, last:-1, dts:800 pos:18173
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:620, last:-1, dts:823 pos:18277
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:68, last:-1, dts:840 pos:18912
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:601, last:-1, dts:869 pos:18995
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:58, last:-1, dts:880 pos:19611
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:603, last:-1, dts:916 pos:19684
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:307, last:-1, dts:920 pos:20302
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:9, size:89, last:-1, dts:960 pos:20624
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:596, last:-1, dts:962 pos:20728
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:64, last:-1, dts:1000 pos:21339
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:625, last:-1, dts:1009 pos:21418
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:58, last:-1, dts:1040 pos:22058
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:604, last:-1, dts:1055 pos:22131
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:333, last:-1, dts:1080 pos:22750
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:598, last:-1, dts:1102 pos:23098
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:94, last:-1, dts:1120 pos:23711
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:605, last:-1, dts:1148 pos:23820
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:58, last:-1, dts:1160 pos:24440
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:613, last:-1, dts:1195 pos:24513
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:58, last:-1, dts:1200 pos:25141
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:9, size:352, last:-1, dts:1240 pos:25214
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:607, last:-1, dts:1241 pos:25581
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:129, last:-1, dts:1280 pos:26203
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:620, last:-1, dts:1287 pos:26347
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:81, last:-1, dts:1320 pos:26982
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:604, last:-1, dts:1334 pos:27078
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:57, last:-1, dts:1360 pos:27697
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:618, last:-1, dts:1380 pos:27769
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:350, last:-1, dts:1400 pos:28402
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:609, last:-1, dts:1427 pos:28767
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:83, last:-1, dts:1440 pos:29391
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:626, last:-1, dts:1473 pos:29489
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:44, last:-1, dts:1480 pos:30130
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:9, size:86, last:-1, dts:1520 pos:30189
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:608, last:-1, dts:1520 pos:30290
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:298, last:-1, dts:1560 pos:30913
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] type:8, size:629, last:-1, dts:1566 pos:31226
[flv @ 0x7fc669002a00] 1 AF 0 
[flv @ 0x7fc669002a00] type:9, size:88, last:-1, dts:1600 pos:31870
[flv @ 0x7fc669002a00] 0 27 0 
[flv @ 0x7fc669002a00] All info found
[flv @ 0x7fc669002a00] stream 0: start_time: 0.080 duration: -9223372036854776.000
[flv @ 0x7fc669002a00] stream 1: start_time: 0.080 duration: -9223372036854776.000
[flv @ 0x7fc669002a00] format: start_time: 0.080 duration: 189.440 bitrate=787 kb/s
[flv @ 0x7fc669002a00] After avformat_find_stream_info() pos: 31965 bytes read:32768 seeks:0 frames:74
Input #0, flv, from out.flv:
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    artist          : yinyuetai.com
    album           : Yinyuetai
    date            : 04/01/15 15:51:32
    comment         : Yinyuetai-1TR1026
    encoder         : Lavf57.83.100
    hasVideo        : true
    hasKeyframes    : true
    hasAudio        : true
    hasMetadata     : true
    canSeekToEnd    : true
    datasize        : 18639072
    videosize       : 16303552
    audiosize       : 2335015
    lasttimestamp   : 189
    lastkeyframetimestamp: 187
    lastkeyframelocation: 18603951
  Duration: 00:03:09.44, start: 0.080000, bitrate: 787 kb/s
    Stream #0:0, 41, 1/1000: Video: h264 (Main), 1 reference frame, yuv420p(progressive, left), 576x432, 0/1, 684 kb/s, 25 fps, 25 tbr, 1k tbn, 50 tbc
    Stream #0:1, 33, 1/1000: Audio: aac (HE-AAC), 44100 Hz, stereo, fltp, 95 kb/s
[h264 @ 0x7fc668824400] nal_unit_type: 7, nal_ref_idc: 3
[h264 @ 0x7fc668824400] nal_unit_type: 8, nal_ref_idc: 3
[AVIOContext @ 0x7fc667f005c0] Statistics: 32768 bytes read, 0 seeks

从以上内容可以看到,输出信息包含了keyframe关键帧存储在文件中的偏移位置及时间戳。

多媒体文件格式(二):FLV 格式

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原文地址:https://www.cnblogs.com/renhui/p/10348629.html

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